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>

arch/powerpc/kernel/entry_32.S

@@ -88,7 +88,7 @@ crit_transfer_to_handler:
 	mfspr	r0,SPRN_SRR1
 	stw	r0,_SRR1(r11)
 
-	mfspr	r8,SPRN_SPRG3
+	mfspr	r8,SPRN_SPRG_THREAD
 	lwz	r0,KSP_LIMIT(r8)
 	stw	r0,SAVED_KSP_LIMIT(r11)
 	rlwimi	r0,r1,0,0,(31-THREAD_SHIFT)
@@ -108,7 +108,7 @@ crit_transfer_to_handler:
 	mfspr	r0,SPRN_SRR1
 	stw	r0,crit_srr1@l(0)
 
-	mfspr	r8,SPRN_SPRG3
+	mfspr	r8,SPRN_SPRG_THREAD
 	lwz	r0,KSP_LIMIT(r8)
 	stw	r0,saved_ksp_limit@l(0)
 	rlwimi	r0,r1,0,0,(31-THREAD_SHIFT)
@@ -138,7 +138,7 @@ transfer_to_handler:
 	mfspr	r2,SPRN_XER
 	stw	r12,_CTR(r11)
 	stw	r2,_XER(r11)
-	mfspr	r12,SPRN_SPRG3
+	mfspr	r12,SPRN_SPRG_THREAD
 	addi	r2,r12,-THREAD
 	tovirt(r2,r2)			/* set r2 to current */
 	beq	2f			/* if from user, fix up THREAD.regs */
@@ -680,7 +680,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_SPE)
 
 	tophys(r0,r4)
 	CLR_TOP32(r0)
-	mtspr	SPRN_SPRG3,r0	/* Update current THREAD phys addr */
+	mtspr	SPRN_SPRG_THREAD,r0	/* Update current THREAD phys addr */
 	lwz	r1,KSP(r4)	/* Load new stack pointer */
 
 	/* save the old current 'last' for return value */
@@ -1057,7 +1057,7 @@ exc_exit_restart_end:
 #ifdef CONFIG_40x
 	.globl	ret_from_crit_exc
 ret_from_crit_exc:
-	mfspr	r9,SPRN_SPRG3
+	mfspr	r9,SPRN_SPRG_THREAD
 	lis	r10,saved_ksp_limit@ha;
 	lwz	r10,saved_ksp_limit@l(r10);
 	tovirt(r9,r9);
@@ -1074,7 +1074,7 @@ ret_from_crit_exc:
 #ifdef CONFIG_BOOKE
 	.globl	ret_from_crit_exc
 ret_from_crit_exc:
-	mfspr	r9,SPRN_SPRG3
+	mfspr	r9,SPRN_SPRG_THREAD
 	lwz	r10,SAVED_KSP_LIMIT(r1)
 	stw	r10,KSP_LIMIT(r9)
 	RESTORE_xSRR(SRR0,SRR1);
@@ -1083,7 +1083,7 @@ ret_from_crit_exc:
 
 	.globl	ret_from_debug_exc
 ret_from_debug_exc:
-	mfspr	r9,SPRN_SPRG3
+	mfspr	r9,SPRN_SPRG_THREAD
 	lwz	r10,SAVED_KSP_LIMIT(r1)
 	stw	r10,KSP_LIMIT(r9)
 	lwz	r9,THREAD_INFO-THREAD(r9)
@@ -1097,7 +1097,7 @@ ret_from_debug_exc:
 
 	.globl	ret_from_mcheck_exc
 ret_from_mcheck_exc:
-	mfspr	r9,SPRN_SPRG3
+	mfspr	r9,SPRN_SPRG_THREAD
 	lwz	r10,SAVED_KSP_LIMIT(r1)
 	stw	r10,KSP_LIMIT(r9)
 	RESTORE_xSRR(SRR0,SRR1);
@@ -1255,7 +1255,7 @@ _GLOBAL(enter_rtas)
 	MTMSRD(r0)		/* don't get trashed */
 	li	r9,MSR_KERNEL & ~(MSR_IR|MSR_DR)
 	mtlr	r6
-	mtspr	SPRN_SPRG2,r7
+	mtspr	SPRN_SPRG_RTAS,r7
 	mtspr	SPRN_SRR0,r8
 	mtspr	SPRN_SRR1,r9
 	RFI
@@ -1265,7 +1265,7 @@ _GLOBAL(enter_rtas)
 	FIX_SRR1(r9,r0)
 	addi	r1,r1,INT_FRAME_SIZE
 	li	r0,0
-	mtspr	SPRN_SPRG2,r0
+	mtspr	SPRN_SPRG_RTAS,r0
 	mtspr	SPRN_SRR0,r8
 	mtspr	SPRN_SRR1,r9
 	RFI			/* return to caller */