sparc: additional comments to piggyback_32.c

While reverse engineering the functionality of piggyback
I missed that the code was actually commented.
So I added a few comments.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/arch/sparc/boot/piggyback_32.c b/arch/sparc/boot/piggyback_32.c
index 5c86a7d..5e856f2 100644 (file)
--- a/arch/sparc/boot/piggyback_32.c
+++ b/arch/sparc/boot/piggyback_32.c
@@ -36,6 +36,8 @@
  * as PROM looks for a.out image only.
  */
 
+#define AOUT_TEXT_OFFSET   32
+
 /* read two bytes as big endian */
 static unsigned short ld2(char *p)
 {
@@ -154,12 +156,20 @@ int main(int argc,char **argv)
                        die(argv[1]);
                j = 0;
        } else if (memcmp(buffer, aout_magic, 4) == 0) {
-               i = j = 32;
+               i = j = AOUT_TEXT_OFFSET;
        } else {
                fprintf (stderr, "Not ELF nor a.out. Don't blame me.\n");
                exit(1);
        }
        k = i;
+       /*
+        * We need to fill in values for sparc_ramdisk_image + sparc_ramdisk_size
+        * To locate these symbols search for the "HdrS" text which appear
+        * in the image a little before the gokernel symbol.
+        * See definition of these in init_32.S
+        */
+
+       /*  Find the gokernel label */
        i += (ld2(buffer + j + 2) << 2) - 512;
        if (lseek(image, i, 0) < 0)
                die("lseek");
@@ -177,6 +187,13 @@ int main(int argc,char **argv)
        if (lseek(image, offset, 0) < 0)
                die("lseek");
 
+       /*
+        * root_flags = 0
+        * root_dev = 1 (RAMDISK_MAJOR)
+        * ram_flags = 0
+        * sparc_ramdisk_image = "PAGE aligned address after _end")
+        * sparc_ramdisk_size = size of image
+        */
        st4(buffer, 0);
        st4(buffer + 4, 0x01000000);
        st4(buffer + 8, (end + 32 + 4095) & ~4095);
@@ -184,11 +201,13 @@ int main(int argc,char **argv)
 
        if (write(image, buffer + 2, 14) != 14)
                die(argv[1]);
+
+       /* seek page aligned boundary in the image file and add boot image */
        if (lseek(image, k - start + ((end + 32 + 4095) & ~4095), 0) < 0)
                die("lseek");
        if ((tail = open(argv[3],O_RDONLY)) < 0)
                die(argv[3]);
-       while ((i = read (tail, buffer, 1024)) > 0)
+       while ((i = read(tail, buffer, 1024)) > 0)
                if (write(image, buffer, i) != i)
                        die(argv[1]);
        if (close(image) < 0)