powerpc: Use names rather than numbers for SPRGs (v2)

The kernel uses SPRG registers for various purposes, typically in
low level assembly code as scratch registers or to hold per-cpu
global infos such as the PACA or the current thread_info pointer.

We want to be able to easily shuffle the usage of those registers
as some implementations have specific constraints realted to some
of them, for example, some have userspace readable aliases, etc..
and the current choice isn't always the best.

This patch should not change any code generation, and replaces the
usage of SPRN_SPRGn everywhere in the kernel with a named replacement
and adds documentation next to the definition of the names as to
what those are used for on each processor family.

The only parts that still use the original numbers are bits of KVM
or suspend/resume code that just blindly needs to save/restore all
the SPRGs.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
index 52ff8c53b93c..6ded19d01891 100644
--- a/arch/powerpc/kernel/head_8xx.S
+++ b/arch/powerpc/kernel/head_8xx.S
@@ -110,8 +110,8 @@ turn_on_mmu:
  * task's thread_struct.
  */
 #define EXCEPTION_PROLOG        \
-        mtspr   SPRN_SPRG0,r10; \
-        mtspr   SPRN_SPRG1,r11; \
+        mtspr   SPRN_SPRG_SCRATCH0,r10; \
+        mtspr   SPRN_SPRG_SCRATCH1,r11; \
         mfcr    r10;            \
         EXCEPTION_PROLOG_1;     \
         EXCEPTION_PROLOG_2
@@ -121,7 +121,7 @@ turn_on_mmu:
         andi.   r11,r11,MSR_PR; \
         tophys(r11,r1);                 /* use tophys(r1) if kernel */ \
         beq     1f;             \
-        mfspr   r11,SPRN_SPRG3; \
+        mfspr   r11,SPRN_SPRG_THREAD;   \
         lwz     r11,THREAD_INFO-THREAD(r11);    \
         addi    r11,r11,THREAD_SIZE;    \
         tophys(r11,r11);        \
@@ -133,9 +133,9 @@ turn_on_mmu:
         stw     r10,_CCR(r11);          /* save registers */ \
         stw     r12,GPR12(r11); \
         stw     r9,GPR9(r11);   \
-        mfspr   r10,SPRN_SPRG0; \
+        mfspr   r10,SPRN_SPRG_SCRATCH0; \
         stw     r10,GPR10(r11); \
-        mfspr   r12,SPRN_SPRG1; \
+        mfspr   r12,SPRN_SPRG_SCRATCH1; \
         stw     r12,GPR11(r11); \
         mflr    r10;            \
         stw     r10,_LINK(r11); \
@@ -603,8 +603,9 @@ start_here:
         /* ptr to phys current thread */
         tophys(r4,r2)
         addi    r4,r4,THREAD    /* init task's THREAD */
-        mtspr   SPRN_SPRG3,r4
+        mtspr   SPRN_SPRG_THREAD,r4
         li      r3,0
+        /* XXX What is that for ? SPRG2 appears otherwise unused on 8xx */
         mtspr   SPRN_SPRG2,r3   /* 0 => r1 has kernel sp */
 
         /* stack */