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>

1 files changed, 21 insertions, 2 deletions

diff --git a/arch/sparc/boot/piggyback_32.c b/arch/sparc/boot/piggyback_32.c
index 5c86a7d03b6f..5e856f2e9e0f 100644
--- 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)