PPC32: Better handling of program check exceptions on 4xx, patch from Kumar Gala.

With this patch we read the ESR (exception syndrome register) early on in handling
the program check exception, before reenabling the MMU and save it in the stack
frame.  This avoids having the value overwritten by a subsequent exception.

arch/ppc/kernel/head_4xx.S

@@ -447,7 +447,12 @@ label:
 	EXC_XFER_EE(0x600, AlignmentException)
 
 /* 0x0700 - Program Exception */
-	EXCEPTION(0x700, ProgramCheck, ProgramCheckException, EXC_XFER_EE)
+	START_EXCEPTION(0x0700, ProgramCheck)
+	NORMAL_EXCEPTION_PROLOG
+	mfspr	r4,SPRN_ESR		/* Grab the ESR and save it */
+	stw	r4,_ESR(r11)
+	addi	r3,r1,STACK_FRAME_OVERHEAD
+	EXC_XFER_EE(0x700, ProgramCheckException)
 
 	EXCEPTION(0x0800, Trap_08, UnknownException, EXC_XFER_EE)
 	EXCEPTION(0x0900, Trap_09, UnknownException, EXC_XFER_EE)
@@ -469,7 +474,7 @@ label:
 	lis	r0,TSR_PIS@h
 	mtspr	SPRN_TSR,r0		/* Clear the PIT exception */
 	addi	r3,r1,STACK_FRAME_OVERHEAD
-	EXC_XFER_EE(0x1000, timer_interrupt)
+	EXC_XFER_LITE(0x1000, timer_interrupt)
 
 #if 0
 /* NOTE:

arch/ppc/kernel/traps.c

@@ -302,7 +302,7 @@ ProgramCheckException(struct pt_regs *regs)
 	int errcode;
 
 #if defined(CONFIG_4xx)
-	unsigned int esr = mfspr(SPRN_ESR);
+	unsigned int esr = regs->dsisr;
 	int isbpt = esr & ESR_PTR;
 	extern int do_mathemu(struct pt_regs *regs);
 

include/asm-ppc/ptrace.h

@@ -33,7 +33,7 @@ struct pt_regs {
 	/* N.B. for critical exceptions on 4xx, the dar and dsisr
 	   fields are overloaded to hold srr0 and srr1. */
 	unsigned long dar;		/* Fault registers */
-	unsigned long dsisr;
+	unsigned long dsisr;		/* on 4xx/Book-E used for ESR */
 	unsigned long result; 		/* Result of a system call */
 };