Revert "ARM: 6943/1: mm: use TTBR1 instead of reserved context ID"

This reverts commit 52af9c6c.

Will Deacon reports that:

 In 52af9c6c ("ARM: 6943/1: mm: use TTBR1 instead of reserved context ID")
 I updated the ASID rollover code to use only the kernel page tables
 whilst updating the ASID.

 Unfortunately, the code to restore the user page tables was part of a
 later patch which isn't yet in mainline, so this leaves the code
 quite broken.

We're also in the process of eliminating __ARCH_WANT_INTERRUPTS_ON_CTXSW
from ARM, so lets revert these until we can properly sort out what we're
doing with the ARM context switching.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/context.c

@@ -24,7 +24,9 @@ DEFINE_PER_CPU(struct mm_struct *, current_mm);
 
 /*
  * We fork()ed a process, and we need a new context for the child
- * to run in.
+ * to run in.  We reserve version 0 for initial tasks so we will
+ * always allocate an ASID. The ASID 0 is reserved for the TTBR
+ * register changing sequence.
  */
 void __init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 {
@@ -34,11 +36,8 @@ void __init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 
 static void flush_context(void)
 {
-	u32 ttb;
-	/* Copy TTBR1 into TTBR0 */
-	asm volatile("mrc	p15, 0, %0, c2, c0, 1\n"
-		     "mcr	p15, 0, %0, c2, c0, 0"
-		     : "=r" (ttb));
+	/* set the reserved ASID before flushing the TLB */
+	asm("mcr	p15, 0, %0, c13, c0, 1\n" : : "r" (0));
 	isb();
 	local_flush_tlb_all();
 	if (icache_is_vivt_asid_tagged()) {

arch/arm/mm/proc-v7.S

@@ -108,16 +108,18 @@ ENTRY(cpu_v7_switch_mm)
 #ifdef CONFIG_ARM_ERRATA_430973
 	mcr	p15, 0, r2, c7, c5, 6		@ flush BTAC/BTB
 #endif
-	mrc	p15, 0, r2, c2, c0, 1		@ load TTB 1
-	mcr	p15, 0, r2, c2, c0, 0		@ into TTB 0
+#ifdef CONFIG_ARM_ERRATA_754322
+	dsb
+#endif
+	mcr	p15, 0, r2, c13, c0, 1		@ set reserved context ID
+	isb
+1:	mcr	p15, 0, r0, c2, c0, 0		@ set TTB 0
 	isb
 #ifdef CONFIG_ARM_ERRATA_754322
 	dsb
 #endif
 	mcr	p15, 0, r1, c13, c0, 1		@ set context ID
 	isb
-	mcr	p15, 0, r0, c2, c0, 0		@ set TTB 0
-	isb
 #endif
 	mov	pc, lr
 ENDPROC(cpu_v7_switch_mm)