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sparc: additional comments to piggyback_32.c

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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>
This commit is contained in:
Sam Ravnborg
2011-01-04 11:39:13 +00:00
committed by David S. Miller
parent b2a39b0d8e
commit 9c23905304
1 changed files with 21 additions and 2 deletions
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23
arch/sparc/boot/piggyback_32.c
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@@ -36,6 +36,8 @@
* as PROM looks for a.out image only. * as PROM looks for a.out image only.
*/ */
#define AOUT_TEXT_OFFSET 32
/* read two bytes as big endian */ /* read two bytes as big endian */
static unsigned short ld2(char *p) static unsigned short ld2(char *p)
{ {
@@ -154,12 +156,20 @@ int main(int argc,char **argv)
die(argv[1]); die(argv[1]);
j = 0; j = 0;
} else if (memcmp(buffer, aout_magic, 4) == 0) { } else if (memcmp(buffer, aout_magic, 4) == 0) {
i = j = 32; i = j = AOUT_TEXT_OFFSET;
} else { } else {
fprintf (stderr, "Not ELF nor a.out. Don't blame me.\n"); fprintf (stderr, "Not ELF nor a.out. Don't blame me.\n");
exit(1); exit(1);
} }
k = i; 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; i += (ld2(buffer + j + 2) << 2) - 512;
if (lseek(image, i, 0) < 0) if (lseek(image, i, 0) < 0)
die("lseek"); die("lseek");
@@ -177,6 +187,13 @@ int main(int argc,char **argv)
if (lseek(image, offset, 0) < 0) if (lseek(image, offset, 0) < 0)
die("lseek"); 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, 0);
st4(buffer + 4, 0x01000000); st4(buffer + 4, 0x01000000);
st4(buffer + 8, (end + 32 + 4095) & ~4095); st4(buffer + 8, (end + 32 + 4095) & ~4095);
@@ -184,11 +201,13 @@ int main(int argc,char **argv)
if (write(image, buffer + 2, 14) != 14) if (write(image, buffer + 2, 14) != 14)
die(argv[1]); 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) if (lseek(image, k - start + ((end + 32 + 4095) & ~4095), 0) < 0)
die("lseek"); die("lseek");
if ((tail = open(argv[3],O_RDONLY)) < 0) if ((tail = open(argv[3],O_RDONLY)) < 0)
die(argv[3]); die(argv[3]);
while ((i = read (tail, buffer, 1024)) > 0) while ((i = read(tail, buffer, 1024)) > 0)
if (write(image, buffer, i) != i) if (write(image, buffer, i) != i)
die(argv[1]); die(argv[1]);
if (close(image) < 0) if (close(image) < 0)