ARM: 7801/1: v6: prevent gcc 4.5 from reordering extended CP15 reads above is_smp() test

Commit 621a0147d5 ("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 4.5 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 mark the extended CP15 read as clobbering memory, which prevents
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.  Remove the existing condition
code clobber as it's obsolete, per Nico's post:

    http://www.spinics.net/lists/arm-kernel/msg261208.html

This patch is a collaboration with Will Deacon and Russell King.

Comments from Paul Walmsley:

 Russell, if you accept this one, might you also add Will's ack from the lists:

Comments from Paul Walmsley:

 I'd also be obliged if you could add a Cc: line for Jonathan Austin, since he helped test:

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Tony Lindgren <tony@atomide.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Jonathan Austin <jonathan.austin@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/cputype.h

@@ -89,13 +89,18 @@ extern unsigned int processor_id;
 		__val;							\
 	})
 
+/*
+ * The memory clobber prevents gcc 4.5 from reordering the mrc before
+ * any is_smp() tests, which can cause undefined instruction aborts on
+ * ARM1136 r0 due to the missing extended CP15 registers.
+ */
 #define read_cpuid_ext(ext_reg)						\
 	({								\
 		unsigned int __val;					\
 		asm("mrc	p15, 0, %0, c0, " ext_reg		\
 		    : "=r" (__val)					\
 		    :							\
-		    : "cc");						\
+		    : "memory");					\
 		__val;							\
 	})