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/head_fsl_booke.S

@@ -361,7 +361,7 @@ skpinv:	addi	r6,r6,1				/* Increment */
 
 	/* ptr to current thread */
 	addi	r4,r2,THREAD	/* init task's THREAD */
-	mtspr	SPRN_SPRG3,r4
+	mtspr	SPRN_SPRG_THREAD,r4
 
 	/* stack */
 	lis	r1,init_thread_union@h
@@ -532,12 +532,12 @@ interrupt_base:
 
 	/* Data TLB Error Interrupt */
 	START_EXCEPTION(DataTLBError)
-	mtspr	SPRN_SPRG0, r10		/* Save some working registers */
-	mtspr	SPRN_SPRG1, r11
-	mtspr	SPRN_SPRG4W, r12
-	mtspr	SPRN_SPRG5W, r13
+	mtspr	SPRN_SPRG_WSCRATCH0, r10 /* Save some working registers */
+	mtspr	SPRN_SPRG_WSCRATCH1, r11
+	mtspr	SPRN_SPRG_WSCRATCH2, r12
+	mtspr	SPRN_SPRG_WSCRATCH3, r13
 	mfcr	r11
-	mtspr	SPRN_SPRG7W, r11
+	mtspr	SPRN_SPRG_WSCRATCH4, r11
 	mfspr	r10, SPRN_DEAR		/* Get faulting address */
 
 	/* If we are faulting a kernel address, we have to use the
@@ -557,7 +557,7 @@ interrupt_base:
 
 	/* Get the PGD for the current thread */
 3:
-	mfspr	r11,SPRN_SPRG3
+	mfspr	r11,SPRN_SPRG_THREAD
 	lwz	r11,PGDIR(r11)
 
 4:
@@ -598,12 +598,12 @@ interrupt_base:
 	/* The bailout.  Restore registers to pre-exception conditions
 	 * and call the heavyweights to help us out.
 	 */
-	mfspr	r11, SPRN_SPRG7R
+	mfspr	r11, SPRN_SPRG_RSCRATCH4
 	mtcr	r11
-	mfspr	r13, SPRN_SPRG5R
-	mfspr	r12, SPRN_SPRG4R
-	mfspr	r11, SPRN_SPRG1
-	mfspr	r10, SPRN_SPRG0
+	mfspr	r13, SPRN_SPRG_RSCRATCH3
+	mfspr	r12, SPRN_SPRG_RSCRATCH2
+	mfspr	r11, SPRN_SPRG_RSCRATCH1
+	mfspr	r10, SPRN_SPRG_RSCRATCH0
 	b	DataStorage
 
 	/* Instruction TLB Error Interrupt */
@@ -613,12 +613,12 @@ interrupt_base:
 	 * to a different point.
 	 */
 	START_EXCEPTION(InstructionTLBError)
-	mtspr	SPRN_SPRG0, r10		/* Save some working registers */
-	mtspr	SPRN_SPRG1, r11
-	mtspr	SPRN_SPRG4W, r12
-	mtspr	SPRN_SPRG5W, r13
+	mtspr	SPRN_SPRG_WSCRATCH0, r10 /* Save some working registers */
+	mtspr	SPRN_SPRG_WSCRATCH1, r11
+	mtspr	SPRN_SPRG_WSCRATCH2, r12
+	mtspr	SPRN_SPRG_WSCRATCH3, r13
 	mfcr	r11
-	mtspr	SPRN_SPRG7W, r11
+	mtspr	SPRN_SPRG_WSCRATCH4, r11
 	mfspr	r10, SPRN_SRR0		/* Get faulting address */
 
 	/* If we are faulting a kernel address, we have to use the
@@ -638,7 +638,7 @@ interrupt_base:
 
 	/* Get the PGD for the current thread */
 3:
-	mfspr	r11,SPRN_SPRG3
+	mfspr	r11,SPRN_SPRG_THREAD
 	lwz	r11,PGDIR(r11)
 
 4:
@@ -666,12 +666,12 @@ interrupt_base:
 	/* The bailout.  Restore registers to pre-exception conditions
 	 * and call the heavyweights to help us out.
 	 */
-	mfspr	r11, SPRN_SPRG7R
+	mfspr	r11, SPRN_SPRG_RSCRATCH4
 	mtcr	r11
-	mfspr	r13, SPRN_SPRG5R
-	mfspr	r12, SPRN_SPRG4R
-	mfspr	r11, SPRN_SPRG1
-	mfspr	r10, SPRN_SPRG0
+	mfspr	r13, SPRN_SPRG_RSCRATCH3
+	mfspr	r12, SPRN_SPRG_RSCRATCH2
+	mfspr	r11, SPRN_SPRG_RSCRATCH1
+	mfspr	r10, SPRN_SPRG_RSCRATCH0
 	b	InstructionStorage
 
 #ifdef CONFIG_SPE
@@ -790,12 +790,12 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_BIG_PHYS)
 	tlbwe
 
 	/* Done...restore registers and get out of here.  */
-	mfspr	r11, SPRN_SPRG7R
+	mfspr	r11, SPRN_SPRG_RSCRATCH4
 	mtcr	r11
-	mfspr	r13, SPRN_SPRG5R
-	mfspr	r12, SPRN_SPRG4R
-	mfspr	r11, SPRN_SPRG1
-	mfspr	r10, SPRN_SPRG0
+	mfspr	r13, SPRN_SPRG_RSCRATCH3
+	mfspr	r12, SPRN_SPRG_RSCRATCH2
+	mfspr	r11, SPRN_SPRG_RSCRATCH1
+	mfspr	r10, SPRN_SPRG_RSCRATCH0
 	rfi					/* Force context change */
 
 #ifdef CONFIG_SPE
@@ -839,7 +839,7 @@ load_up_spe:
 #endif /* !CONFIG_SMP */
 	/* enable use of SPE after return */
 	oris	r9,r9,MSR_SPE@h
-	mfspr	r5,SPRN_SPRG3		/* current task's THREAD (phys) */
+	mfspr	r5,SPRN_SPRG_THREAD	/* current task's THREAD (phys) */
 	li	r4,1
 	li	r10,THREAD_ACC
 	stw	r4,THREAD_USED_SPE(r5)
@@ -1118,7 +1118,7 @@ __secondary_start:
 
 	/* ptr to current thread */
 	addi	r4,r2,THREAD	/* address of our thread_struct */
-	mtspr	SPRN_SPRG3,r4
+	mtspr	SPRN_SPRG_THREAD,r4
 
 	/* Setup the defaults for TLB entries */
 	li	r4,(MAS4_TSIZED(BOOK3E_PAGESZ_4K))@l