ARM: 8353/1: mm: Fix Cortex-A8 erratum 430973 segfaults for bootloaders and multiarch

Looks like apps can be made to segfault easily on armhf distros
just by running cpuburn-a8 in the background, then starting apt
get update unless erratum 430973 workaround is enabled. This happens
on r3p2 also, which has 430973 fixed in hardware.

Turns out the reason for this is some bootloaders incorrectly
setting the auxilary register IBE bit, which probably causes us
to hit erratum 687067 on Cortex-A8 later than r1p2.

If the bootloader incorrectly sets the IBE bit in the auxilary control
register for Cortex-A8 revisions with 430973 fixed in hardware, we
need to call flush BTAC/BTB to avoid segfaults probably caused by
erratum 687067. So let's flush BTAC/BTB unconditionally for Cortex-A8.
It won't do anything unless the IBE bit is set.

Note that we keep the erratum 430973 Kconfig option still around and
disabled for multiarch as it may be unsafe to enable for some secure
SoC. It is known safe to be enabled for n900, but won't do anything
on n900 as the IBE bit needs to be set with SMC.

Also note that SoCs probably should also add checks and print warnings
for the misconfigured IBE bit depending on the Cortex-A8 revision
so the bootloaders can be fixed Cortex-A8 revisions later than
r1p2 to not set the IBE bit.

Tested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/proc-v7-2level.S

@@ -36,14 +36,16 @@
  *
  *	It is assumed that:
  *	- we are not using split page tables
+ *
+ *	Note that we always need to flush BTAC/BTB if IBE is set
+ *	even on Cortex-A8 revisions not affected by 430973.
+ *	If IBE is not set, the flush BTAC/BTB won't do anything.
  */
 ENTRY(cpu_ca8_switch_mm)
 #ifdef CONFIG_MMU
 	mov	r2, #0
-#ifdef CONFIG_ARM_ERRATA_430973
 	mcr	p15, 0, r2, c7, c5, 6		@ flush BTAC/BTB
 #endif
-#endif
 ENTRY(cpu_v7_switch_mm)
 #ifdef CONFIG_MMU
 	mmid	r1, r1				@ get mm->context.id