--- /dev/null
+ Dynamic DMA mapping Guide
+ =========================
+
+ David S. Miller <davem@redhat.com>
+ Richard Henderson <rth@cygnus.com>
+ Jakub Jelinek <jakub@redhat.com>
+
+This is a guide to device driver writers on how to use the DMA API
+with example pseudo-code. For a concise description of the API, see
+DMA-API.txt.
+
+Most of the 64bit platforms have special hardware that translates bus
+addresses (DMA addresses) into physical addresses. This is similar to
+how page tables and/or a TLB translates virtual addresses to physical
+addresses on a CPU. This is needed so that e.g. PCI devices can
+access with a Single Address Cycle (32bit DMA address) any page in the
+64bit physical address space. Previously in Linux those 64bit
+platforms had to set artificial limits on the maximum RAM size in the
+system, so that the virt_to_bus() static scheme works (the DMA address
+translation tables were simply filled on bootup to map each bus
+address to the physical page __pa(bus_to_virt())).
+
+So that Linux can use the dynamic DMA mapping, it needs some help from the
+drivers, namely it has to take into account that DMA addresses should be
+mapped only for the time they are actually used and unmapped after the DMA
+transfer.
+
+The following API will work of course even on platforms where no such
+hardware exists.
+
+Note that the DMA API works with any bus independent of the underlying
+microprocessor architecture. You should use the DMA API rather than
+the bus specific DMA API (e.g. pci_dma_*).
+
+First of all, you should make sure
+
+#include <linux/dma-mapping.h>
+
+is in your driver. This file will obtain for you the definition of the
+dma_addr_t (which can hold any valid DMA address for the platform)
+type which should be used everywhere you hold a DMA (bus) address
+returned from the DMA mapping functions.
+
+ What memory is DMA'able?
+
+The first piece of information you must know is what kernel memory can
+be used with the DMA mapping facilities. There has been an unwritten
+set of rules regarding this, and this text is an attempt to finally
+write them down.
+
+If you acquired your memory via the page allocator
+(i.e. __get_free_page*()) or the generic memory allocators
+(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
+that memory using the addresses returned from those routines.
+
+This means specifically that you may _not_ use the memory/addresses
+returned from vmalloc() for DMA. It is possible to DMA to the
+_underlying_ memory mapped into a vmalloc() area, but this requires
+walking page tables to get the physical addresses, and then
+translating each of those pages back to a kernel address using
+something like __va(). [ EDIT: Update this when we integrate
+Gerd Knorr's generic code which does this. ]
+
+This rule also means that you may use neither kernel image addresses
+(items in data/text/bss segments), nor module image addresses, nor
+stack addresses for DMA. These could all be mapped somewhere entirely
+different than the rest of physical memory. Even if those classes of
+memory could physically work with DMA, you'd need to ensure the I/O
+buffers were cacheline-aligned. Without that, you'd see cacheline
+sharing problems (data corruption) on CPUs with DMA-incoherent caches.
+(The CPU could write to one word, DMA would write to a different one
+in the same cache line, and one of them could be overwritten.)
+
+Also, this means that you cannot take the return of a kmap()
+call and DMA to/from that. This is similar to vmalloc().
+
+What about block I/O and networking buffers? The block I/O and
+networking subsystems make sure that the buffers they use are valid
+for you to DMA from/to.
+
+ DMA addressing limitations
+
+Does your device have any DMA addressing limitations? For example, is
+your device only capable of driving the low order 24-bits of address?
+If so, you need to inform the kernel of this fact.
+
+By default, the kernel assumes that your device can address the full
+32-bits. For a 64-bit capable device, this needs to be increased.
+And for a device with limitations, as discussed in the previous
+paragraph, it needs to be decreased.
+
+Special note about PCI: PCI-X specification requires PCI-X devices to
+support 64-bit addressing (DAC) for all transactions. And at least
+one platform (SGI SN2) requires 64-bit consistent allocations to
+operate correctly when the IO bus is in PCI-X mode.
+
+For correct operation, you must interrogate the kernel in your device
+probe routine to see if the DMA controller on the machine can properly
+support the DMA addressing limitation your device has. It is good
+style to do this even if your device holds the default setting,
+because this shows that you did think about these issues wrt. your
+device.
+
+The query is performed via a call to dma_set_mask():
+
+ int dma_set_mask(struct device *dev, u64 mask);
+
+The query for consistent allocations is performed via a call to
+dma_set_coherent_mask():
+
+ int dma_set_coherent_mask(struct device *dev, u64 mask);
+
+Here, dev is a pointer to the device struct of your device, and mask
+is a bit mask describing which bits of an address your device
+supports. It returns zero if your card can perform DMA properly on
+the machine given the address mask you provided. In general, the
+device struct of your device is embedded in the bus specific device
+struct of your device. For example, a pointer to the device struct of
+your PCI device is pdev->dev (pdev is a pointer to the PCI device
+struct of your device).
+
+If it returns non-zero, your device cannot perform DMA properly on
+this platform, and attempting to do so will result in undefined
+behavior. You must either use a different mask, or not use DMA.
+
+This means that in the failure case, you have three options:
+
+1) Use another DMA mask, if possible (see below).
+2) Use some non-DMA mode for data transfer, if possible.
+3) Ignore this device and do not initialize it.
+
+It is recommended that your driver print a kernel KERN_WARNING message
+when you end up performing either #2 or #3. In this manner, if a user
+of your driver reports that performance is bad or that the device is not
+even detected, you can ask them for the kernel messages to find out
+exactly why.
+
+The standard 32-bit addressing device would do something like this:
+
+ if (dma_set_mask(dev, DMA_BIT_MASK(32))) {
+ printk(KERN_WARNING
+ "mydev: No suitable DMA available.\n");
+ goto ignore_this_device;
+ }
+
+Another common scenario is a 64-bit capable device. The approach here
+is to try for 64-bit addressing, but back down to a 32-bit mask that
+should not fail. The kernel may fail the 64-bit mask not because the
+platform is not capable of 64-bit addressing. Rather, it may fail in
+this case simply because 32-bit addressing is done more efficiently
+than 64-bit addressing. For example, Sparc64 PCI SAC addressing is
+more efficient than DAC addressing.
+
+Here is how you would handle a 64-bit capable device which can drive
+all 64-bits when accessing streaming DMA:
+
+ int using_dac;
+
+ if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
+ using_dac = 1;
+ } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
+ using_dac = 0;
+ } else {
+ printk(KERN_WARNING
+ "mydev: No suitable DMA available.\n");
+ goto ignore_this_device;
+ }
+
+If a card is capable of using 64-bit consistent allocations as well,
+the case would look like this:
+
+ int using_dac, consistent_using_dac;
+
+ if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
+ using_dac = 1;
+ consistent_using_dac = 1;
+ dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
+ } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
+ using_dac = 0;
+ consistent_using_dac = 0;
+ dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
+ } else {
+ printk(KERN_WARNING
+ "mydev: No suitable DMA available.\n");
+ goto ignore_this_device;
+ }
+
+dma_set_coherent_mask() will always be able to set the same or a
+smaller mask as dma_set_mask(). However for the rare case that a
+device driver only uses consistent allocations, one would have to
+check the return value from dma_set_coherent_mask().
+
+Finally, if your device can only drive the low 24-bits of
+address you might do something like:
+
+ if (dma_set_mask(dev, DMA_BIT_MASK(24))) {
+ printk(KERN_WARNING
+ "mydev: 24-bit DMA addressing not available.\n");
+ goto ignore_this_device;
+ }
+
+When dma_set_mask() is successful, and returns zero, the kernel saves
+away this mask you have provided. The kernel will use this
+information later when you make DMA mappings.
+
+There is a case which we are aware of at this time, which is worth
+mentioning in this documentation. If your device supports multiple
+functions (for example a sound card provides playback and record
+functions) and the various different functions have _different_
+DMA addressing limitations, you may wish to probe each mask and
+only provide the functionality which the machine can handle. It
+is important that the last call to dma_set_mask() be for the
+most specific mask.
+
+Here is pseudo-code showing how this might be done:
+
+ #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
+ #define RECORD_ADDRESS_BITS DMA_BIT_MASK(24)
+
+ struct my_sound_card *card;
+ struct device *dev;
+
+ ...
+ if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) {
+ card->playback_enabled = 1;
+ } else {
+ card->playback_enabled = 0;
+ printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.\n",
+ card->name);
+ }
+ if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
+ card->record_enabled = 1;
+ } else {
+ card->record_enabled = 0;
+ printk(KERN_WARNING "%s: Record disabled due to DMA limitations.\n",
+ card->name);
+ }
+
+A sound card was used as an example here because this genre of PCI
+devices seems to be littered with ISA chips given a PCI front end,
+and thus retaining the 16MB DMA addressing limitations of ISA.
+
+ Types of DMA mappings
+
+There are two types of DMA mappings:
+
+- Consistent DMA mappings which are usually mapped at driver
+ initialization, unmapped at the end and for which the hardware should
+ guarantee that the device and the CPU can access the data
+ in parallel and will see updates made by each other without any
+ explicit software flushing.
+
+ Think of "consistent" as "synchronous" or "coherent".
+
+ The current default is to return consistent memory in the low 32
+ bits of the bus space. However, for future compatibility you should
+ set the consistent mask even if this default is fine for your
+ driver.
+
+ Good examples of what to use consistent mappings for are:
+
+ - Network card DMA ring descriptors.
+ - SCSI adapter mailbox command data structures.
+ - Device firmware microcode executed out of
+ main memory.
+
+ The invariant these examples all require is that any CPU store
+ to memory is immediately visible to the device, and vice
+ versa. Consistent mappings guarantee this.
+
+ IMPORTANT: Consistent DMA memory does not preclude the usage of
+ proper memory barriers. The CPU may reorder stores to
+ consistent memory just as it may normal memory. Example:
+ if it is important for the device to see the first word
+ of a descriptor updated before the second, you must do
+ something like:
+
+ desc->word0 = address;
+ wmb();
+ desc->word1 = DESC_VALID;
+
+ in order to get correct behavior on all platforms.
+
+ Also, on some platforms your driver may need to flush CPU write
+ buffers in much the same way as it needs to flush write buffers
+ found in PCI bridges (such as by reading a register's value
+ after writing it).
+
+- Streaming DMA mappings which are usually mapped for one DMA
+ transfer, unmapped right after it (unless you use dma_sync_* below)
+ and for which hardware can optimize for sequential accesses.
+
+ This of "streaming" as "asynchronous" or "outside the coherency
+ domain".
+
+ Good examples of what to use streaming mappings for are:
+
+ - Networking buffers transmitted/received by a device.
+ - Filesystem buffers written/read by a SCSI device.
+
+ The interfaces for using this type of mapping were designed in
+ such a way that an implementation can make whatever performance
+ optimizations the hardware allows. To this end, when using
+ such mappings you must be explicit about what you want to happen.
+
+Neither type of DMA mapping has alignment restrictions that come from
+the underlying bus, although some devices may have such restrictions.
+Also, systems with caches that aren't DMA-coherent will work better
+when the underlying buffers don't share cache lines with other data.
+
+
+ Using Consistent DMA mappings.
+
+To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
+you should do:
+
+ dma_addr_t dma_handle;
+
+ cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);
+
+where device is a struct device *. This may be called in interrupt
+context with the GFP_ATOMIC flag.
+
+Size is the length of the region you want to allocate, in bytes.
+
+This routine will allocate RAM for that region, so it acts similarly to
+__get_free_pages (but takes size instead of a page order). If your
+driver needs regions sized smaller than a page, you may prefer using
+the dma_pool interface, described below.
+
+The consistent DMA mapping interfaces, for non-NULL dev, will by
+default return a DMA address which is 32-bit addressable. Even if the
+device indicates (via DMA mask) that it may address the upper 32-bits,
+consistent allocation will only return > 32-bit addresses for DMA if
+the consistent DMA mask has been explicitly changed via
+dma_set_coherent_mask(). This is true of the dma_pool interface as
+well.
+
+dma_alloc_coherent returns two values: the virtual address which you
+can use to access it from the CPU and dma_handle which you pass to the
+card.
+
+The cpu return address and the DMA bus master address are both
+guaranteed to be aligned to the smallest PAGE_SIZE order which
+is greater than or equal to the requested size. This invariant
+exists (for example) to guarantee that if you allocate a chunk
+which is smaller than or equal to 64 kilobytes, the extent of the
+buffer you receive will not cross a 64K boundary.
+
+To unmap and free such a DMA region, you call:
+
+ dma_free_coherent(dev, size, cpu_addr, dma_handle);
+
+where dev, size are the same as in the above call and cpu_addr and
+dma_handle are the values dma_alloc_coherent returned to you.
+This function may not be called in interrupt context.
+
+If your driver needs lots of smaller memory regions, you can write
+custom code to subdivide pages returned by dma_alloc_coherent,
+or you can use the dma_pool API to do that. A dma_pool is like
+a kmem_cache, but it uses dma_alloc_coherent not __get_free_pages.
+Also, it understands common hardware constraints for alignment,
+like queue heads needing to be aligned on N byte boundaries.
+
+Create a dma_pool like this:
+
+ struct dma_pool *pool;
+
+ pool = dma_pool_create(name, dev, size, align, alloc);
+
+The "name" is for diagnostics (like a kmem_cache name); dev and size
+are as above. The device's hardware alignment requirement for this
+type of data is "align" (which is expressed in bytes, and must be a
+power of two). If your device has no boundary crossing restrictions,
+pass 0 for alloc; passing 4096 says memory allocated from this pool
+must not cross 4KByte boundaries (but at that time it may be better to
+go for dma_alloc_coherent directly instead).
+
+Allocate memory from a dma pool like this:
+
+ cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
+
+flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
+holding SMP locks), SLAB_ATOMIC otherwise. Like dma_alloc_coherent,
+this returns two values, cpu_addr and dma_handle.
+
+Free memory that was allocated from a dma_pool like this:
+
+ dma_pool_free(pool, cpu_addr, dma_handle);
+
+where pool is what you passed to dma_pool_alloc, and cpu_addr and
+dma_handle are the values dma_pool_alloc returned. This function
+may be called in interrupt context.
+
+Destroy a dma_pool by calling:
+
+ dma_pool_destroy(pool);
+
+Make sure you've called dma_pool_free for all memory allocated
+from a pool before you destroy the pool. This function may not
+be called in interrupt context.
+
+ DMA Direction
+
+The interfaces described in subsequent portions of this document
+take a DMA direction argument, which is an integer and takes on
+one of the following values:
+
+ DMA_BIDIRECTIONAL
+ DMA_TO_DEVICE
+ DMA_FROM_DEVICE
+ DMA_NONE
+
+One should provide the exact DMA direction if you know it.
+
+DMA_TO_DEVICE means "from main memory to the device"
+DMA_FROM_DEVICE means "from the device to main memory"
+It is the direction in which the data moves during the DMA
+transfer.
+
+You are _strongly_ encouraged to specify this as precisely
+as you possibly can.
+
+If you absolutely cannot know the direction of the DMA transfer,
+specify DMA_BIDIRECTIONAL. It means that the DMA can go in
+either direction. The platform guarantees that you may legally
+specify this, and that it will work, but this may be at the
+cost of performance for example.
+
+The value DMA_NONE is to be used for debugging. One can
+hold this in a data structure before you come to know the
+precise direction, and this will help catch cases where your
+direction tracking logic has failed to set things up properly.
+
+Another advantage of specifying this value precisely (outside of
+potential platform-specific optimizations of such) is for debugging.
+Some platforms actually have a write permission boolean which DMA
+mappings can be marked with, much like page protections in the user
+program address space. Such platforms can and do report errors in the
+kernel logs when the DMA controller hardware detects violation of the
+permission setting.
+
+Only streaming mappings specify a direction, consistent mappings
+implicitly have a direction attribute setting of
+DMA_BIDIRECTIONAL.
+
+The SCSI subsystem tells you the direction to use in the
+'sc_data_direction' member of the SCSI command your driver is
+working on.
+
+For Networking drivers, it's a rather simple affair. For transmit
+packets, map/unmap them with the DMA_TO_DEVICE direction
+specifier. For receive packets, just the opposite, map/unmap them
+with the DMA_FROM_DEVICE direction specifier.
+
+ Using Streaming DMA mappings
+
+The streaming DMA mapping routines can be called from interrupt
+context. There are two versions of each map/unmap, one which will
+map/unmap a single memory region, and one which will map/unmap a
+scatterlist.
+
+To map a single region, you do:
+
+ struct device *dev = &my_dev->dev;
+ dma_addr_t dma_handle;
+ void *addr = buffer->ptr;
+ size_t size = buffer->len;
+
+ dma_handle = dma_map_single(dev, addr, size, direction);
+
+and to unmap it:
+
+ dma_unmap_single(dev, dma_handle, size, direction);
+
+You should call dma_unmap_single when the DMA activity is finished, e.g.
+from the interrupt which told you that the DMA transfer is done.
+
+Using cpu pointers like this for single mappings has a disadvantage,
+you cannot reference HIGHMEM memory in this way. Thus, there is a
+map/unmap interface pair akin to dma_{map,unmap}_single. These
+interfaces deal with page/offset pairs instead of cpu pointers.
+Specifically:
+
+ struct device *dev = &my_dev->dev;
+ dma_addr_t dma_handle;
+ struct page *page = buffer->page;
+ unsigned long offset = buffer->offset;
+ size_t size = buffer->len;
+
+ dma_handle = dma_map_page(dev, page, offset, size, direction);
+
+ ...
+
+ dma_unmap_page(dev, dma_handle, size, direction);
+
+Here, "offset" means byte offset within the given page.
+
+With scatterlists, you map a region gathered from several regions by:
+
+ int i, count = dma_map_sg(dev, sglist, nents, direction);
+ struct scatterlist *sg;
+
+ for_each_sg(sglist, sg, count, i) {
+ hw_address[i] = sg_dma_address(sg);
+ hw_len[i] = sg_dma_len(sg);
+ }
+
+where nents is the number of entries in the sglist.
+
+The implementation is free to merge several consecutive sglist entries
+into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
+consecutive sglist entries can be merged into one provided the first one
+ends and the second one starts on a page boundary - in fact this is a huge
+advantage for cards which either cannot do scatter-gather or have very
+limited number of scatter-gather entries) and returns the actual number
+of sg entries it mapped them to. On failure 0 is returned.
+
+Then you should loop count times (note: this can be less than nents times)
+and use sg_dma_address() and sg_dma_len() macros where you previously
+accessed sg->address and sg->length as shown above.
+
+To unmap a scatterlist, just call:
+
+ dma_unmap_sg(dev, sglist, nents, direction);
+
+Again, make sure DMA activity has already finished.
+
+PLEASE NOTE: The 'nents' argument to the dma_unmap_sg call must be
+ the _same_ one you passed into the dma_map_sg call,
+ it should _NOT_ be the 'count' value _returned_ from the
+ dma_map_sg call.
+
+Every dma_map_{single,sg} call should have its dma_unmap_{single,sg}
+counterpart, because the bus address space is a shared resource (although
+in some ports the mapping is per each BUS so less devices contend for the
+same bus address space) and you could render the machine unusable by eating
+all bus addresses.
+
+If you need to use the same streaming DMA region multiple times and touch
+the data in between the DMA transfers, the buffer needs to be synced
+properly in order for the cpu and device to see the most uptodate and
+correct copy of the DMA buffer.
+
+So, firstly, just map it with dma_map_{single,sg}, and after each DMA
+transfer call either:
+
+ dma_sync_single_for_cpu(dev, dma_handle, size, direction);
+
+or:
+
+ dma_sync_sg_for_cpu(dev, sglist, nents, direction);
+
+as appropriate.
+
+Then, if you wish to let the device get at the DMA area again,
+finish accessing the data with the cpu, and then before actually
+giving the buffer to the hardware call either:
+
+ dma_sync_single_for_device(dev, dma_handle, size, direction);
+
+or:
+
+ dma_sync_sg_for_device(dev, sglist, nents, direction);
+
+as appropriate.
+
+After the last DMA transfer call one of the DMA unmap routines
+dma_unmap_{single,sg}. If you don't touch the data from the first dma_map_*
+call till dma_unmap_*, then you don't have to call the dma_sync_*
+routines at all.
+
+Here is pseudo code which shows a situation in which you would need
+to use the dma_sync_*() interfaces.
+
+ my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
+ {
+ dma_addr_t mapping;
+
+ mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
+
+ cp->rx_buf = buffer;
+ cp->rx_len = len;
+ cp->rx_dma = mapping;
+
+ give_rx_buf_to_card(cp);
+ }
+
+ ...
+
+ my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
+ {
+ struct my_card *cp = devid;
+
+ ...
+ if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
+ struct my_card_header *hp;
+
+ /* Examine the header to see if we wish
+ * to accept the data. But synchronize
+ * the DMA transfer with the CPU first
+ * so that we see updated contents.
+ */
+ dma_sync_single_for_cpu(&cp->dev, cp->rx_dma,
+ cp->rx_len,
+ DMA_FROM_DEVICE);
+
+ /* Now it is safe to examine the buffer. */
+ hp = (struct my_card_header *) cp->rx_buf;
+ if (header_is_ok(hp)) {
+ dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len,
+ DMA_FROM_DEVICE);
+ pass_to_upper_layers(cp->rx_buf);
+ make_and_setup_new_rx_buf(cp);
+ } else {
+ /* Just sync the buffer and give it back
+ * to the card.
+ */
+ dma_sync_single_for_device(&cp->dev,
+ cp->rx_dma,
+ cp->rx_len,
+ DMA_FROM_DEVICE);
+ give_rx_buf_to_card(cp);
+ }
+ }
+ }
+
+Drivers converted fully to this interface should not use virt_to_bus any
+longer, nor should they use bus_to_virt. Some drivers have to be changed a
+little bit, because there is no longer an equivalent to bus_to_virt in the
+dynamic DMA mapping scheme - you have to always store the DMA addresses
+returned by the dma_alloc_coherent, dma_pool_alloc, and dma_map_single
+calls (dma_map_sg stores them in the scatterlist itself if the platform
+supports dynamic DMA mapping in hardware) in your driver structures and/or
+in the card registers.
+
+All drivers should be using these interfaces with no exceptions. It
+is planned to completely remove virt_to_bus() and bus_to_virt() as
+they are entirely deprecated. Some ports already do not provide these
+as it is impossible to correctly support them.
+
+ Optimizing Unmap State Space Consumption
+
+On many platforms, dma_unmap_{single,page}() is simply a nop.
+Therefore, keeping track of the mapping address and length is a waste
+of space. Instead of filling your drivers up with ifdefs and the like
+to "work around" this (which would defeat the whole purpose of a
+portable API) the following facilities are provided.
+
+Actually, instead of describing the macros one by one, we'll
+transform some example code.
+
+1) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures.
+ Example, before:
+
+ struct ring_state {
+ struct sk_buff *skb;
+ dma_addr_t mapping;
+ __u32 len;
+ };
+
+ after:
+
+ struct ring_state {
+ struct sk_buff *skb;
+ DEFINE_DMA_UNMAP_ADDR(mapping);
+ DEFINE_DMA_UNMAP_LEN(len);
+ };
+
+2) Use dma_unmap_{addr,len}_set to set these values.
+ Example, before:
+
+ ringp->mapping = FOO;
+ ringp->len = BAR;
+
+ after:
+
+ dma_unmap_addr_set(ringp, mapping, FOO);
+ dma_unmap_len_set(ringp, len, BAR);
+
+3) Use dma_unmap_{addr,len} to access these values.
+ Example, before:
+
+ dma_unmap_single(dev, ringp->mapping, ringp->len,
+ DMA_FROM_DEVICE);
+
+ after:
+
+ dma_unmap_single(dev,
+ dma_unmap_addr(ringp, mapping),
+ dma_unmap_len(ringp, len),
+ DMA_FROM_DEVICE);
+
+It really should be self-explanatory. We treat the ADDR and LEN
+separately, because it is possible for an implementation to only
+need the address in order to perform the unmap operation.
+
+ Platform Issues
+
+If you are just writing drivers for Linux and do not maintain
+an architecture port for the kernel, you can safely skip down
+to "Closing".
+
+1) Struct scatterlist requirements.
+
+ Struct scatterlist must contain, at a minimum, the following
+ members:
+
+ struct page *page;
+ unsigned int offset;
+ unsigned int length;
+
+ The base address is specified by a "page+offset" pair.
+
+ Previous versions of struct scatterlist contained a "void *address"
+ field that was sometimes used instead of page+offset. As of Linux
+ 2.5., page+offset is always used, and the "address" field has been
+ deleted.
+
+2) More to come...
+
+ Handling Errors
+
+DMA address space is limited on some architectures and an allocation
+failure can be determined by:
+
+- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
+
+- checking the returned dma_addr_t of dma_map_single and dma_map_page
+ by using dma_mapping_error():
+
+ dma_addr_t dma_handle;
+
+ dma_handle = dma_map_single(dev, addr, size, direction);
+ if (dma_mapping_error(dev, dma_handle)) {
+ /*
+ * reduce current DMA mapping usage,
+ * delay and try again later or
+ * reset driver.
+ */
+ }
+
+ Closing
+
+This document, and the API itself, would not be in it's current
+form without the feedback and suggestions from numerous individuals.
+We would like to specifically mention, in no particular order, the
+following people:
+
+ Russell King <rmk@arm.linux.org.uk>
+ Leo Dagum <dagum@barrel.engr.sgi.com>
+ Ralf Baechle <ralf@oss.sgi.com>
+ Grant Grundler <grundler@cup.hp.com>
+ Jay Estabrook <Jay.Estabrook@compaq.com>
+ Thomas Sailer <sailer@ife.ee.ethz.ch>
+ Andrea Arcangeli <andrea@suse.de>
+ Jens Axboe <jens.axboe@oracle.com>
+ David Mosberger-Tang <davidm@hpl.hp.com>
+++ /dev/null
- Dynamic DMA mapping Guide
- =========================
-
- David S. Miller <davem@redhat.com>
- Richard Henderson <rth@cygnus.com>
- Jakub Jelinek <jakub@redhat.com>
-
-This is a guide to device driver writers on how to use the DMA API
-with example pseudo-code. For a concise description of the API, see
-DMA-API.txt.
-
-Most of the 64bit platforms have special hardware that translates bus
-addresses (DMA addresses) into physical addresses. This is similar to
-how page tables and/or a TLB translates virtual addresses to physical
-addresses on a CPU. This is needed so that e.g. PCI devices can
-access with a Single Address Cycle (32bit DMA address) any page in the
-64bit physical address space. Previously in Linux those 64bit
-platforms had to set artificial limits on the maximum RAM size in the
-system, so that the virt_to_bus() static scheme works (the DMA address
-translation tables were simply filled on bootup to map each bus
-address to the physical page __pa(bus_to_virt())).
-
-So that Linux can use the dynamic DMA mapping, it needs some help from the
-drivers, namely it has to take into account that DMA addresses should be
-mapped only for the time they are actually used and unmapped after the DMA
-transfer.
-
-The following API will work of course even on platforms where no such
-hardware exists.
-
-Note that the DMA API works with any bus independent of the underlying
-microprocessor architecture. You should use the DMA API rather than
-the bus specific DMA API (e.g. pci_dma_*).
-
-First of all, you should make sure
-
-#include <linux/dma-mapping.h>
-
-is in your driver. This file will obtain for you the definition of the
-dma_addr_t (which can hold any valid DMA address for the platform)
-type which should be used everywhere you hold a DMA (bus) address
-returned from the DMA mapping functions.
-
- What memory is DMA'able?
-
-The first piece of information you must know is what kernel memory can
-be used with the DMA mapping facilities. There has been an unwritten
-set of rules regarding this, and this text is an attempt to finally
-write them down.
-
-If you acquired your memory via the page allocator
-(i.e. __get_free_page*()) or the generic memory allocators
-(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
-that memory using the addresses returned from those routines.
-
-This means specifically that you may _not_ use the memory/addresses
-returned from vmalloc() for DMA. It is possible to DMA to the
-_underlying_ memory mapped into a vmalloc() area, but this requires
-walking page tables to get the physical addresses, and then
-translating each of those pages back to a kernel address using
-something like __va(). [ EDIT: Update this when we integrate
-Gerd Knorr's generic code which does this. ]
-
-This rule also means that you may use neither kernel image addresses
-(items in data/text/bss segments), nor module image addresses, nor
-stack addresses for DMA. These could all be mapped somewhere entirely
-different than the rest of physical memory. Even if those classes of
-memory could physically work with DMA, you'd need to ensure the I/O
-buffers were cacheline-aligned. Without that, you'd see cacheline
-sharing problems (data corruption) on CPUs with DMA-incoherent caches.
-(The CPU could write to one word, DMA would write to a different one
-in the same cache line, and one of them could be overwritten.)
-
-Also, this means that you cannot take the return of a kmap()
-call and DMA to/from that. This is similar to vmalloc().
-
-What about block I/O and networking buffers? The block I/O and
-networking subsystems make sure that the buffers they use are valid
-for you to DMA from/to.
-
- DMA addressing limitations
-
-Does your device have any DMA addressing limitations? For example, is
-your device only capable of driving the low order 24-bits of address?
-If so, you need to inform the kernel of this fact.
-
-By default, the kernel assumes that your device can address the full
-32-bits. For a 64-bit capable device, this needs to be increased.
-And for a device with limitations, as discussed in the previous
-paragraph, it needs to be decreased.
-
-Special note about PCI: PCI-X specification requires PCI-X devices to
-support 64-bit addressing (DAC) for all transactions. And at least
-one platform (SGI SN2) requires 64-bit consistent allocations to
-operate correctly when the IO bus is in PCI-X mode.
-
-For correct operation, you must interrogate the kernel in your device
-probe routine to see if the DMA controller on the machine can properly
-support the DMA addressing limitation your device has. It is good
-style to do this even if your device holds the default setting,
-because this shows that you did think about these issues wrt. your
-device.
-
-The query is performed via a call to dma_set_mask():
-
- int dma_set_mask(struct device *dev, u64 mask);
-
-The query for consistent allocations is performed via a call to
-dma_set_coherent_mask():
-
- int dma_set_coherent_mask(struct device *dev, u64 mask);
-
-Here, dev is a pointer to the device struct of your device, and mask
-is a bit mask describing which bits of an address your device
-supports. It returns zero if your card can perform DMA properly on
-the machine given the address mask you provided. In general, the
-device struct of your device is embedded in the bus specific device
-struct of your device. For example, a pointer to the device struct of
-your PCI device is pdev->dev (pdev is a pointer to the PCI device
-struct of your device).
-
-If it returns non-zero, your device cannot perform DMA properly on
-this platform, and attempting to do so will result in undefined
-behavior. You must either use a different mask, or not use DMA.
-
-This means that in the failure case, you have three options:
-
-1) Use another DMA mask, if possible (see below).
-2) Use some non-DMA mode for data transfer, if possible.
-3) Ignore this device and do not initialize it.
-
-It is recommended that your driver print a kernel KERN_WARNING message
-when you end up performing either #2 or #3. In this manner, if a user
-of your driver reports that performance is bad or that the device is not
-even detected, you can ask them for the kernel messages to find out
-exactly why.
-
-The standard 32-bit addressing device would do something like this:
-
- if (dma_set_mask(dev, DMA_BIT_MASK(32))) {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
- goto ignore_this_device;
- }
-
-Another common scenario is a 64-bit capable device. The approach here
-is to try for 64-bit addressing, but back down to a 32-bit mask that
-should not fail. The kernel may fail the 64-bit mask not because the
-platform is not capable of 64-bit addressing. Rather, it may fail in
-this case simply because 32-bit addressing is done more efficiently
-than 64-bit addressing. For example, Sparc64 PCI SAC addressing is
-more efficient than DAC addressing.
-
-Here is how you would handle a 64-bit capable device which can drive
-all 64-bits when accessing streaming DMA:
-
- int using_dac;
-
- if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
- using_dac = 1;
- } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
- using_dac = 0;
- } else {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
- goto ignore_this_device;
- }
-
-If a card is capable of using 64-bit consistent allocations as well,
-the case would look like this:
-
- int using_dac, consistent_using_dac;
-
- if (!dma_set_mask(dev, DMA_BIT_MASK(64))) {
- using_dac = 1;
- consistent_using_dac = 1;
- dma_set_coherent_mask(dev, DMA_BIT_MASK(64));
- } else if (!dma_set_mask(dev, DMA_BIT_MASK(32))) {
- using_dac = 0;
- consistent_using_dac = 0;
- dma_set_coherent_mask(dev, DMA_BIT_MASK(32));
- } else {
- printk(KERN_WARNING
- "mydev: No suitable DMA available.\n");
- goto ignore_this_device;
- }
-
-dma_set_coherent_mask() will always be able to set the same or a
-smaller mask as dma_set_mask(). However for the rare case that a
-device driver only uses consistent allocations, one would have to
-check the return value from dma_set_coherent_mask().
-
-Finally, if your device can only drive the low 24-bits of
-address you might do something like:
-
- if (dma_set_mask(dev, DMA_BIT_MASK(24))) {
- printk(KERN_WARNING
- "mydev: 24-bit DMA addressing not available.\n");
- goto ignore_this_device;
- }
-
-When dma_set_mask() is successful, and returns zero, the kernel saves
-away this mask you have provided. The kernel will use this
-information later when you make DMA mappings.
-
-There is a case which we are aware of at this time, which is worth
-mentioning in this documentation. If your device supports multiple
-functions (for example a sound card provides playback and record
-functions) and the various different functions have _different_
-DMA addressing limitations, you may wish to probe each mask and
-only provide the functionality which the machine can handle. It
-is important that the last call to dma_set_mask() be for the
-most specific mask.
-
-Here is pseudo-code showing how this might be done:
-
- #define PLAYBACK_ADDRESS_BITS DMA_BIT_MASK(32)
- #define RECORD_ADDRESS_BITS DMA_BIT_MASK(24)
-
- struct my_sound_card *card;
- struct device *dev;
-
- ...
- if (!dma_set_mask(dev, PLAYBACK_ADDRESS_BITS)) {
- card->playback_enabled = 1;
- } else {
- card->playback_enabled = 0;
- printk(KERN_WARNING "%s: Playback disabled due to DMA limitations.\n",
- card->name);
- }
- if (!dma_set_mask(dev, RECORD_ADDRESS_BITS)) {
- card->record_enabled = 1;
- } else {
- card->record_enabled = 0;
- printk(KERN_WARNING "%s: Record disabled due to DMA limitations.\n",
- card->name);
- }
-
-A sound card was used as an example here because this genre of PCI
-devices seems to be littered with ISA chips given a PCI front end,
-and thus retaining the 16MB DMA addressing limitations of ISA.
-
- Types of DMA mappings
-
-There are two types of DMA mappings:
-
-- Consistent DMA mappings which are usually mapped at driver
- initialization, unmapped at the end and for which the hardware should
- guarantee that the device and the CPU can access the data
- in parallel and will see updates made by each other without any
- explicit software flushing.
-
- Think of "consistent" as "synchronous" or "coherent".
-
- The current default is to return consistent memory in the low 32
- bits of the bus space. However, for future compatibility you should
- set the consistent mask even if this default is fine for your
- driver.
-
- Good examples of what to use consistent mappings for are:
-
- - Network card DMA ring descriptors.
- - SCSI adapter mailbox command data structures.
- - Device firmware microcode executed out of
- main memory.
-
- The invariant these examples all require is that any CPU store
- to memory is immediately visible to the device, and vice
- versa. Consistent mappings guarantee this.
-
- IMPORTANT: Consistent DMA memory does not preclude the usage of
- proper memory barriers. The CPU may reorder stores to
- consistent memory just as it may normal memory. Example:
- if it is important for the device to see the first word
- of a descriptor updated before the second, you must do
- something like:
-
- desc->word0 = address;
- wmb();
- desc->word1 = DESC_VALID;
-
- in order to get correct behavior on all platforms.
-
- Also, on some platforms your driver may need to flush CPU write
- buffers in much the same way as it needs to flush write buffers
- found in PCI bridges (such as by reading a register's value
- after writing it).
-
-- Streaming DMA mappings which are usually mapped for one DMA
- transfer, unmapped right after it (unless you use dma_sync_* below)
- and for which hardware can optimize for sequential accesses.
-
- This of "streaming" as "asynchronous" or "outside the coherency
- domain".
-
- Good examples of what to use streaming mappings for are:
-
- - Networking buffers transmitted/received by a device.
- - Filesystem buffers written/read by a SCSI device.
-
- The interfaces for using this type of mapping were designed in
- such a way that an implementation can make whatever performance
- optimizations the hardware allows. To this end, when using
- such mappings you must be explicit about what you want to happen.
-
-Neither type of DMA mapping has alignment restrictions that come from
-the underlying bus, although some devices may have such restrictions.
-Also, systems with caches that aren't DMA-coherent will work better
-when the underlying buffers don't share cache lines with other data.
-
-
- Using Consistent DMA mappings.
-
-To allocate and map large (PAGE_SIZE or so) consistent DMA regions,
-you should do:
-
- dma_addr_t dma_handle;
-
- cpu_addr = dma_alloc_coherent(dev, size, &dma_handle, gfp);
-
-where device is a struct device *. This may be called in interrupt
-context with the GFP_ATOMIC flag.
-
-Size is the length of the region you want to allocate, in bytes.
-
-This routine will allocate RAM for that region, so it acts similarly to
-__get_free_pages (but takes size instead of a page order). If your
-driver needs regions sized smaller than a page, you may prefer using
-the dma_pool interface, described below.
-
-The consistent DMA mapping interfaces, for non-NULL dev, will by
-default return a DMA address which is 32-bit addressable. Even if the
-device indicates (via DMA mask) that it may address the upper 32-bits,
-consistent allocation will only return > 32-bit addresses for DMA if
-the consistent DMA mask has been explicitly changed via
-dma_set_coherent_mask(). This is true of the dma_pool interface as
-well.
-
-dma_alloc_coherent returns two values: the virtual address which you
-can use to access it from the CPU and dma_handle which you pass to the
-card.
-
-The cpu return address and the DMA bus master address are both
-guaranteed to be aligned to the smallest PAGE_SIZE order which
-is greater than or equal to the requested size. This invariant
-exists (for example) to guarantee that if you allocate a chunk
-which is smaller than or equal to 64 kilobytes, the extent of the
-buffer you receive will not cross a 64K boundary.
-
-To unmap and free such a DMA region, you call:
-
- dma_free_coherent(dev, size, cpu_addr, dma_handle);
-
-where dev, size are the same as in the above call and cpu_addr and
-dma_handle are the values dma_alloc_coherent returned to you.
-This function may not be called in interrupt context.
-
-If your driver needs lots of smaller memory regions, you can write
-custom code to subdivide pages returned by dma_alloc_coherent,
-or you can use the dma_pool API to do that. A dma_pool is like
-a kmem_cache, but it uses dma_alloc_coherent not __get_free_pages.
-Also, it understands common hardware constraints for alignment,
-like queue heads needing to be aligned on N byte boundaries.
-
-Create a dma_pool like this:
-
- struct dma_pool *pool;
-
- pool = dma_pool_create(name, dev, size, align, alloc);
-
-The "name" is for diagnostics (like a kmem_cache name); dev and size
-are as above. The device's hardware alignment requirement for this
-type of data is "align" (which is expressed in bytes, and must be a
-power of two). If your device has no boundary crossing restrictions,
-pass 0 for alloc; passing 4096 says memory allocated from this pool
-must not cross 4KByte boundaries (but at that time it may be better to
-go for dma_alloc_coherent directly instead).
-
-Allocate memory from a dma pool like this:
-
- cpu_addr = dma_pool_alloc(pool, flags, &dma_handle);
-
-flags are SLAB_KERNEL if blocking is permitted (not in_interrupt nor
-holding SMP locks), SLAB_ATOMIC otherwise. Like dma_alloc_coherent,
-this returns two values, cpu_addr and dma_handle.
-
-Free memory that was allocated from a dma_pool like this:
-
- dma_pool_free(pool, cpu_addr, dma_handle);
-
-where pool is what you passed to dma_pool_alloc, and cpu_addr and
-dma_handle are the values dma_pool_alloc returned. This function
-may be called in interrupt context.
-
-Destroy a dma_pool by calling:
-
- dma_pool_destroy(pool);
-
-Make sure you've called dma_pool_free for all memory allocated
-from a pool before you destroy the pool. This function may not
-be called in interrupt context.
-
- DMA Direction
-
-The interfaces described in subsequent portions of this document
-take a DMA direction argument, which is an integer and takes on
-one of the following values:
-
- DMA_BIDIRECTIONAL
- DMA_TO_DEVICE
- DMA_FROM_DEVICE
- DMA_NONE
-
-One should provide the exact DMA direction if you know it.
-
-DMA_TO_DEVICE means "from main memory to the device"
-DMA_FROM_DEVICE means "from the device to main memory"
-It is the direction in which the data moves during the DMA
-transfer.
-
-You are _strongly_ encouraged to specify this as precisely
-as you possibly can.
-
-If you absolutely cannot know the direction of the DMA transfer,
-specify DMA_BIDIRECTIONAL. It means that the DMA can go in
-either direction. The platform guarantees that you may legally
-specify this, and that it will work, but this may be at the
-cost of performance for example.
-
-The value DMA_NONE is to be used for debugging. One can
-hold this in a data structure before you come to know the
-precise direction, and this will help catch cases where your
-direction tracking logic has failed to set things up properly.
-
-Another advantage of specifying this value precisely (outside of
-potential platform-specific optimizations of such) is for debugging.
-Some platforms actually have a write permission boolean which DMA
-mappings can be marked with, much like page protections in the user
-program address space. Such platforms can and do report errors in the
-kernel logs when the DMA controller hardware detects violation of the
-permission setting.
-
-Only streaming mappings specify a direction, consistent mappings
-implicitly have a direction attribute setting of
-DMA_BIDIRECTIONAL.
-
-The SCSI subsystem tells you the direction to use in the
-'sc_data_direction' member of the SCSI command your driver is
-working on.
-
-For Networking drivers, it's a rather simple affair. For transmit
-packets, map/unmap them with the DMA_TO_DEVICE direction
-specifier. For receive packets, just the opposite, map/unmap them
-with the DMA_FROM_DEVICE direction specifier.
-
- Using Streaming DMA mappings
-
-The streaming DMA mapping routines can be called from interrupt
-context. There are two versions of each map/unmap, one which will
-map/unmap a single memory region, and one which will map/unmap a
-scatterlist.
-
-To map a single region, you do:
-
- struct device *dev = &my_dev->dev;
- dma_addr_t dma_handle;
- void *addr = buffer->ptr;
- size_t size = buffer->len;
-
- dma_handle = dma_map_single(dev, addr, size, direction);
-
-and to unmap it:
-
- dma_unmap_single(dev, dma_handle, size, direction);
-
-You should call dma_unmap_single when the DMA activity is finished, e.g.
-from the interrupt which told you that the DMA transfer is done.
-
-Using cpu pointers like this for single mappings has a disadvantage,
-you cannot reference HIGHMEM memory in this way. Thus, there is a
-map/unmap interface pair akin to dma_{map,unmap}_single. These
-interfaces deal with page/offset pairs instead of cpu pointers.
-Specifically:
-
- struct device *dev = &my_dev->dev;
- dma_addr_t dma_handle;
- struct page *page = buffer->page;
- unsigned long offset = buffer->offset;
- size_t size = buffer->len;
-
- dma_handle = dma_map_page(dev, page, offset, size, direction);
-
- ...
-
- dma_unmap_page(dev, dma_handle, size, direction);
-
-Here, "offset" means byte offset within the given page.
-
-With scatterlists, you map a region gathered from several regions by:
-
- int i, count = dma_map_sg(dev, sglist, nents, direction);
- struct scatterlist *sg;
-
- for_each_sg(sglist, sg, count, i) {
- hw_address[i] = sg_dma_address(sg);
- hw_len[i] = sg_dma_len(sg);
- }
-
-where nents is the number of entries in the sglist.
-
-The implementation is free to merge several consecutive sglist entries
-into one (e.g. if DMA mapping is done with PAGE_SIZE granularity, any
-consecutive sglist entries can be merged into one provided the first one
-ends and the second one starts on a page boundary - in fact this is a huge
-advantage for cards which either cannot do scatter-gather or have very
-limited number of scatter-gather entries) and returns the actual number
-of sg entries it mapped them to. On failure 0 is returned.
-
-Then you should loop count times (note: this can be less than nents times)
-and use sg_dma_address() and sg_dma_len() macros where you previously
-accessed sg->address and sg->length as shown above.
-
-To unmap a scatterlist, just call:
-
- dma_unmap_sg(dev, sglist, nents, direction);
-
-Again, make sure DMA activity has already finished.
-
-PLEASE NOTE: The 'nents' argument to the dma_unmap_sg call must be
- the _same_ one you passed into the dma_map_sg call,
- it should _NOT_ be the 'count' value _returned_ from the
- dma_map_sg call.
-
-Every dma_map_{single,sg} call should have its dma_unmap_{single,sg}
-counterpart, because the bus address space is a shared resource (although
-in some ports the mapping is per each BUS so less devices contend for the
-same bus address space) and you could render the machine unusable by eating
-all bus addresses.
-
-If you need to use the same streaming DMA region multiple times and touch
-the data in between the DMA transfers, the buffer needs to be synced
-properly in order for the cpu and device to see the most uptodate and
-correct copy of the DMA buffer.
-
-So, firstly, just map it with dma_map_{single,sg}, and after each DMA
-transfer call either:
-
- dma_sync_single_for_cpu(dev, dma_handle, size, direction);
-
-or:
-
- dma_sync_sg_for_cpu(dev, sglist, nents, direction);
-
-as appropriate.
-
-Then, if you wish to let the device get at the DMA area again,
-finish accessing the data with the cpu, and then before actually
-giving the buffer to the hardware call either:
-
- dma_sync_single_for_device(dev, dma_handle, size, direction);
-
-or:
-
- dma_sync_sg_for_device(dev, sglist, nents, direction);
-
-as appropriate.
-
-After the last DMA transfer call one of the DMA unmap routines
-dma_unmap_{single,sg}. If you don't touch the data from the first dma_map_*
-call till dma_unmap_*, then you don't have to call the dma_sync_*
-routines at all.
-
-Here is pseudo code which shows a situation in which you would need
-to use the dma_sync_*() interfaces.
-
- my_card_setup_receive_buffer(struct my_card *cp, char *buffer, int len)
- {
- dma_addr_t mapping;
-
- mapping = dma_map_single(cp->dev, buffer, len, DMA_FROM_DEVICE);
-
- cp->rx_buf = buffer;
- cp->rx_len = len;
- cp->rx_dma = mapping;
-
- give_rx_buf_to_card(cp);
- }
-
- ...
-
- my_card_interrupt_handler(int irq, void *devid, struct pt_regs *regs)
- {
- struct my_card *cp = devid;
-
- ...
- if (read_card_status(cp) == RX_BUF_TRANSFERRED) {
- struct my_card_header *hp;
-
- /* Examine the header to see if we wish
- * to accept the data. But synchronize
- * the DMA transfer with the CPU first
- * so that we see updated contents.
- */
- dma_sync_single_for_cpu(&cp->dev, cp->rx_dma,
- cp->rx_len,
- DMA_FROM_DEVICE);
-
- /* Now it is safe to examine the buffer. */
- hp = (struct my_card_header *) cp->rx_buf;
- if (header_is_ok(hp)) {
- dma_unmap_single(&cp->dev, cp->rx_dma, cp->rx_len,
- DMA_FROM_DEVICE);
- pass_to_upper_layers(cp->rx_buf);
- make_and_setup_new_rx_buf(cp);
- } else {
- /* Just sync the buffer and give it back
- * to the card.
- */
- dma_sync_single_for_device(&cp->dev,
- cp->rx_dma,
- cp->rx_len,
- DMA_FROM_DEVICE);
- give_rx_buf_to_card(cp);
- }
- }
- }
-
-Drivers converted fully to this interface should not use virt_to_bus any
-longer, nor should they use bus_to_virt. Some drivers have to be changed a
-little bit, because there is no longer an equivalent to bus_to_virt in the
-dynamic DMA mapping scheme - you have to always store the DMA addresses
-returned by the dma_alloc_coherent, dma_pool_alloc, and dma_map_single
-calls (dma_map_sg stores them in the scatterlist itself if the platform
-supports dynamic DMA mapping in hardware) in your driver structures and/or
-in the card registers.
-
-All drivers should be using these interfaces with no exceptions. It
-is planned to completely remove virt_to_bus() and bus_to_virt() as
-they are entirely deprecated. Some ports already do not provide these
-as it is impossible to correctly support them.
-
- Optimizing Unmap State Space Consumption
-
-On many platforms, dma_unmap_{single,page}() is simply a nop.
-Therefore, keeping track of the mapping address and length is a waste
-of space. Instead of filling your drivers up with ifdefs and the like
-to "work around" this (which would defeat the whole purpose of a
-portable API) the following facilities are provided.
-
-Actually, instead of describing the macros one by one, we'll
-transform some example code.
-
-1) Use DEFINE_DMA_UNMAP_{ADDR,LEN} in state saving structures.
- Example, before:
-
- struct ring_state {
- struct sk_buff *skb;
- dma_addr_t mapping;
- __u32 len;
- };
-
- after:
-
- struct ring_state {
- struct sk_buff *skb;
- DEFINE_DMA_UNMAP_ADDR(mapping);
- DEFINE_DMA_UNMAP_LEN(len);
- };
-
-2) Use dma_unmap_{addr,len}_set to set these values.
- Example, before:
-
- ringp->mapping = FOO;
- ringp->len = BAR;
-
- after:
-
- dma_unmap_addr_set(ringp, mapping, FOO);
- dma_unmap_len_set(ringp, len, BAR);
-
-3) Use dma_unmap_{addr,len} to access these values.
- Example, before:
-
- dma_unmap_single(dev, ringp->mapping, ringp->len,
- DMA_FROM_DEVICE);
-
- after:
-
- dma_unmap_single(dev,
- dma_unmap_addr(ringp, mapping),
- dma_unmap_len(ringp, len),
- DMA_FROM_DEVICE);
-
-It really should be self-explanatory. We treat the ADDR and LEN
-separately, because it is possible for an implementation to only
-need the address in order to perform the unmap operation.
-
- Platform Issues
-
-If you are just writing drivers for Linux and do not maintain
-an architecture port for the kernel, you can safely skip down
-to "Closing".
-
-1) Struct scatterlist requirements.
-
- Struct scatterlist must contain, at a minimum, the following
- members:
-
- struct page *page;
- unsigned int offset;
- unsigned int length;
-
- The base address is specified by a "page+offset" pair.
-
- Previous versions of struct scatterlist contained a "void *address"
- field that was sometimes used instead of page+offset. As of Linux
- 2.5., page+offset is always used, and the "address" field has been
- deleted.
-
-2) More to come...
-
- Handling Errors
-
-DMA address space is limited on some architectures and an allocation
-failure can be determined by:
-
-- checking if dma_alloc_coherent returns NULL or dma_map_sg returns 0
-
-- checking the returned dma_addr_t of dma_map_single and dma_map_page
- by using dma_mapping_error():
-
- dma_addr_t dma_handle;
-
- dma_handle = dma_map_single(dev, addr, size, direction);
- if (dma_mapping_error(dev, dma_handle)) {
- /*
- * reduce current DMA mapping usage,
- * delay and try again later or
- * reset driver.
- */
- }
-
- Closing
-
-This document, and the API itself, would not be in it's current
-form without the feedback and suggestions from numerous individuals.
-We would like to specifically mention, in no particular order, the
-following people:
-
- Russell King <rmk@arm.linux.org.uk>
- Leo Dagum <dagum@barrel.engr.sgi.com>
- Ralf Baechle <ralf@oss.sgi.com>
- Grant Grundler <grundler@cup.hp.com>
- Jay Estabrook <Jay.Estabrook@compaq.com>
- Thomas Sailer <sailer@ife.ee.ethz.ch>
- Andrea Arcangeli <andrea@suse.de>
- Jens Axboe <jens.axboe@oracle.com>
- David Mosberger-Tang <davidm@hpl.hp.com>
projects / linux-flexiantxendom0-3.2.10.git / commitdiff