[PATCH] ARM: v6: prevent gcc from reordering extended CP15 reads above is_smp() test

[Tony Lindgren]

Hi,

* Paul Walmsley <paul at pwsan.com> [130728 13:23]:
> 
> Commit 621a0147d5c921f4cc33636ccd0602ad5d7cbfbc ("ARM: 7757/1: mm:
> don't flush icache in switch_mm with hardware broadcasting") breaks
> the boot on OMAP2430SDP with omap2plus_defconfig.  Tracked to an
> undefined instruction abort from the CP15 read in
> cache_ops_need_broadcast().  It turns out that gcc reorders the
> extended CP15 read above the is_smp() test.  This breaks ARM1136 r0
> cores, since they don't support several CP15 registers that later ARM
> cores do.  ARM1136JF-S TRM section 3.2.1 "Register allocation" has the
> details.
> 
> So, when the kernel is built for ARMv6 cores, mark the extended CP15
> read as clobbering memory, which seems to prevent the compiler from
> reordering it before the is_smp() test.  Russell states that the code
> generated from this approach is preferable to marking the inline asm
> as volatile.
> 
> This patch was developed in collaboration with Will Deacon and Russell
> King.
> 
> Signed-off-by: Paul Walmsley <paul at pwsan.com>
> Cc: Will Deacon <will.deacon at arm.com>
> Cc: Russell King <rmk+kernel at arm.linux.org.uk>

Sorry to be late to this party, I was offline last week. This
patch fixes the issue for me:

Acked-by: Tony Lindgren <tony at atomide.com>