[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>

8 files changed, 61 insertions, 23 deletions

diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
index 3628d8681844..b1fc029e5ea7 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -40,11 +40,11 @@ zliblinuxheader := zlib.h zconf.h zutil.h
 $(addprefix $(obj)/,$(zlib) main.o): $(addprefix $(obj)/,$(zliblinuxheader)) \
                 $(addprefix $(obj)/,$(zlibheader))
 
-src-wlib := string.S stdio.c main.c flatdevtree.c flatdevtree_misc.c \
+src-wlib := string.S crt0.S stdio.c main.c flatdevtree.c flatdevtree_misc.c \
                 ns16550.c serial.c simple_alloc.c div64.S util.S \
                 gunzip_util.c $(zlib)
 src-plat := of.c
-src-boot := crt0.S $(src-wlib) $(src-plat) empty.c
+src-boot := $(src-wlib) $(src-plat) empty.c
 
 src-boot := $(addprefix $(obj)/, $(src-boot))
 obj-boot := $(addsuffix .o, $(basename $(src-boot)))
@@ -97,7 +97,7 @@ $(obj)/wrapper.a: $(obj-wlib)
 
 hostprogs-y     := addnote addRamDisk hack-coff mktree
 
-extra-y         := $(obj)/crt0.o $(obj)/wrapper.a $(obj-plat) $(obj)/empty.o \
+extra-y         := $(obj)/wrapper.a $(obj-plat) $(obj)/empty.o \
                    $(obj)/zImage.lds $(obj)/zImage.coff.lds
 
 wrapper         :=$(srctree)/$(src)/wrapper
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
diff --git a/arch/powerpc/boot/main.c b/arch/powerpc/boot/main.c
index 05de6cfafeeb..8a60e13777d7 100644
--- a/arch/powerpc/boot/main.c
+++ b/arch/powerpc/boot/main.c
@@ -254,21 +254,15 @@ static void set_cmdline(char *buf)
 struct platform_ops platform_ops;
 struct dt_ops dt_ops;
 struct console_ops console_ops;
+struct loader_info loader_info;
 
-void start(unsigned long a1, unsigned long a2, void *promptr, void *sp)
+void start(void *sp)
 {
         struct addr_range vmlinux, initrd;
         kernel_entry_t kentry;
         char cmdline[COMMAND_LINE_SIZE];
         unsigned long ft_addr = 0;
 
-        memset(__bss_start, 0, _end - __bss_start);
-        memset(&platform_ops, 0, sizeof(platform_ops));
-        memset(&dt_ops, 0, sizeof(dt_ops));
-        memset(&console_ops, 0, sizeof(console_ops));
-
-        if (platform_init(promptr, _dtb_start, _dtb_end))
-                exit();
         if (console_ops.open && (console_ops.open() < 0))
                 exit();
         if (platform_ops.fixups)
@@ -278,7 +272,8 @@ void start(unsigned long a1, unsigned long a2, void *promptr, void *sp)
                _start, sp);
 
         vmlinux = prep_kernel();
-        initrd = prep_initrd(vmlinux, a1, a2);
+        initrd = prep_initrd(vmlinux, loader_info.initrd_addr,
+                             loader_info.initrd_size);
 
         /* If cmdline came from zimage wrapper or if we can edit the one
          * in the dt, print it out and edit it, if possible.
@@ -298,7 +293,7 @@ void start(unsigned long a1, unsigned long a2, void *promptr, void *sp)
         if (ft_addr)
                 printf(" flat tree at 0x%lx\n\r", ft_addr);
         else
-                printf(" using OF tree (promptr=%p)\n\r", promptr);
+                printf(" using OF tree (promptr=%p)\n\r", loader_info.promptr);
 
         if (console_ops.close)
                 console_ops.close();
@@ -307,7 +302,8 @@ void start(unsigned long a1, unsigned long a2, void *promptr, void *sp)
         if (ft_addr)
                 kentry(ft_addr, 0, NULL);
         else
-                kentry((unsigned long)initrd.addr, initrd.size, promptr);
+                kentry((unsigned long)initrd.addr, initrd.size,
+                       loader_info.promptr);
 
         /* console closed so printf below may not work */
         printf("Error: Linux kernel returned to zImage boot wrapper!\n\r");
diff --git a/arch/powerpc/boot/of.c b/arch/powerpc/boot/of.c
index 044f34770b96..c6f0d9701485 100644
--- a/arch/powerpc/boot/of.c
+++ b/arch/powerpc/boot/of.c
@@ -267,7 +267,7 @@ static void of_console_write(char *buf, int len)
         call_prom("write", 3, 1, of_stdout_handle, buf, len);
 }
 
-int platform_init(void *promptr, char *dt_blob_start, char *dt_blob_end)
+void platform_init(unsigned long a1, unsigned long a2, void *promptr)
 {
         platform_ops.image_hdr = of_image_hdr;
         platform_ops.malloc = of_try_claim;
@@ -282,5 +282,7 @@ int platform_init(void *promptr, char *dt_blob_start, char *dt_blob_end)
         console_ops.write = of_console_write;
 
         prom = (int (*)(void *))promptr;
-        return 0;
+        loader_info.promptr = promptr;
+        loader_info.initrd_addr = a1;
+        loader_info.initrd_size = a2;
 }
diff --git a/arch/powerpc/boot/ops.h b/arch/powerpc/boot/ops.h
index fa62ff223e70..cad4eee599fb 100644
--- a/arch/powerpc/boot/ops.h
+++ b/arch/powerpc/boot/ops.h
@@ -59,7 +59,13 @@ struct serial_console_data {
         void            (*close)(void);
 };
 
-int platform_init(void *promptr, char *dt_blob_start, char *dt_blob_end);
+struct loader_info {
+        void *promptr;
+        unsigned long initrd_addr, initrd_size;
+};
+extern struct loader_info loader_info;
+
+void start(void *sp);
 int ft_init(void *dt_blob, unsigned int max_size, unsigned int max_find_device);
 int serial_console_init(void);
 int ns16550_console_init(void *devp, struct serial_console_data *scdp);
@@ -100,4 +106,8 @@ static inline void exit(void)
         for(;;);
 }
 
+#define BSS_STACK(size) \
+        static char _bss_stack[size]; \
+        void *_platform_stack_top = _bss_stack + sizeof(_bss_stack);
+
 #endif /* _PPC_BOOT_OPS_H_ */
diff --git a/arch/powerpc/boot/wrapper b/arch/powerpc/boot/wrapper
index 024e4d425c59..157d8c89e138 100755
--- a/arch/powerpc/boot/wrapper
+++ b/arch/powerpc/boot/wrapper
@@ -191,7 +191,7 @@ fi
 
 if [ "$platform" != "miboot" ]; then
     ${CROSS}ld -m elf32ppc -T $lds -o "$ofile" \
-        $object/crt0.o $platformo $tmp $object/wrapper.a
+        $platformo $tmp $object/wrapper.a
     rm $tmp
 fi
 
@@ -201,7 +201,9 @@ pseries|chrp)
     $object/addnote "$ofile"
     ;;
 pmaccoff)
-    ${CROSS}objcopy -O aixcoff-rs6000 --set-start 0x500000 "$ofile"
+    entry=`objdump -f "$ofile" | grep '^start address ' | \
+        cut -d' ' -f3`
+    ${CROSS}objcopy -O aixcoff-rs6000 --set-start "$entry" "$ofile"
     $object/hack-coff "$ofile"
     ;;
 esac
diff --git a/arch/powerpc/boot/zImage.coff.lds.S b/arch/powerpc/boot/zImage.coff.lds.S
index a360905e5428..fe87a90ce7f1 100644
--- a/arch/powerpc/boot/zImage.coff.lds.S
+++ b/arch/powerpc/boot/zImage.coff.lds.S
@@ -1,5 +1,6 @@
 OUTPUT_ARCH(powerpc:common)
-ENTRY(_start)
+ENTRY(_zimage_start_opd)
+EXTERN(_zimage_start_opd)
 SECTIONS
 {
   . = (5*1024*1024);
diff --git a/arch/powerpc/boot/zImage.lds.S b/arch/powerpc/boot/zImage.lds.S
index 4be3c6414b04..f6e380fdb388 100644
--- a/arch/powerpc/boot/zImage.lds.S
+++ b/arch/powerpc/boot/zImage.lds.S
@@ -1,5 +1,6 @@
 OUTPUT_ARCH(powerpc:common)
 ENTRY(_zimage_start)
+EXTERN(_zimage_start)
 SECTIONS
 {
   . = (4*1024*1024);