[POWERPC] zImage: Cleanup and improve zImage entry point

This patch re-organises the way the zImage wrapper code is entered, to
allow more flexibility on platforms with unusual entry conditions.
After this patch, a platform .o file has two options:

1) It can define a _zimage_start, in which case the platform code gets
   control from the very beginning of execution.  In this case the
   platform code is responsible for relocating the zImage if necessary,
   clearing the BSS, performing any platform specific initialization, and
   finally calling start() to load and enter the kernel.

2) It can define platform_init().  In this case the generic crt0.S
   handles initial entry, and calls platform_init() before calling
   start().  The signature of platform_init() is changed, however, to
   take up to 5 parameters (in r3..r7) as they come from the platform's
   initial loader, instead of a fixed set of parameters based on OF's
   usage.

   When using the generic crt0.S, the platform .o can optionally
   supply a custom stack to use, using the BSS_STACK() macro.  If this
   is not supplied, the crt0.S will assume that the loader has
   supplied a usable stack.

In either case, the platform code communicates information to the
generic code (specifically, a PROM pointer for OF systems, and/or an
initrd image address supplied by the bootloader) via a global
structure "loader_info".

In addition the wrapper script is rearranged to ensure that the
platform .o is always linked first.  This means that platforms where
the zImage entry point is at a fixed address or offset, rather than
being encoded in the binary header can be supported using option (1).

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

1 files changed, 29 insertions, 3 deletions

diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
index 70e65b13e033..3dc8d8f78499 100644
--- a/arch/powerpc/boot/crt0.S
+++ b/arch/powerpc/boot/crt0.S
@@ -16,6 +16,7 @@
 _zimage_start_opd:
         .long   _zimage_start, 0, 0, 0
 
+        .weak   _zimage_start
         .globl  _zimage_start
 _zimage_start:
         /* Work out the offset between the address we were linked at
@@ -44,7 +45,7 @@ _zimage_start:
         addi    r9,r9,4
         bdnz    2b
 
-        /* Do a cache flush for our text, in case OF didn't */
+        /* Do a cache flush for our text, in case the loader didn't */
 3:      lis     r9,_start@ha
         addi    r9,r9,_start@l
         add     r9,r0,r9
@@ -59,6 +60,31 @@ _zimage_start:
         sync
         isync
 
-        mr      r6,r1
-        b       start
+        /* Clear the BSS */
+        lis     r9,__bss_start@ha
+        addi    r9,r9,__bss_start@l
+        lis     r8,_end@ha
+        addi    r8,r8,_end@l
+        li      r0,0
+5:      stw     r0,0(r9)
+        addi    r9,r9,4
+        cmplw   cr0,r9,r8
+        blt     5b
 
+        /* Possibly set up a custom stack */
+.weak   _platform_stack_top
+        lis     r8,_platform_stack_top@ha
+        addi    r8,r8,_platform_stack_top@l
+        cmpwi   r8,0
+        beq     6f
+        lwz     r1,0(r8)
+        li      r0,0
+        stwu    r0,-16(r1)      /* establish a stack frame */
+6:
+
+        /* Call platform_init() */
+        bl      platform_init
+
+        /* Call start */
+        mr      r3,r1
+        b       start