read_cpuid_id() in arch/arm/kernel/setup.c

[Mason]

On 13/03/2015 17:45, Russell King - ARM Linux wrote:

> Yes, this one I like - and it probably fixes a potential latent bug
> where the compiler was free to re-order that mrc outside of the if()
> statement.
> 
> Please wrap it up as a normal submission, thanks.

Proposed patch at the end of this message.

I'm now puzzling over why it's required to have "memory"
in read_cpuid_ext's clobber list, and not in read_cpuid's?

Regards.


-- >8 --
Date: Sun, 15 Mar 2015 17:59:53 +0100
Subject: [PATCH] Use read_cpuid_ext() macro instead of inline asm

In commit 067e710b9a982a92cc8294d7fa0f1e924c65bba1, Paul Walmsley fixed
read_cpuid_ext() and added the following comment.

    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.
---
 arch/arm/kernel/setup.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
index e55408e..1d60beb 100644
--- a/arch/arm/kernel/setup.c
+++ b/arch/arm/kernel/setup.c
@@ -246,12 +246,9 @@ static int __get_cpu_architecture(void)
 		if (cpu_arch)
 			cpu_arch += CPU_ARCH_ARMv3;
 	} else if ((read_cpuid_id() & 0x000f0000) == 0x000f0000) {
-		unsigned int mmfr0;
-
 		/* Revised CPUID format. Read the Memory Model Feature
 		 * Register 0 and check for VMSAv7 or PMSAv7 */
-		asm("mrc	p15, 0, %0, c0, c1, 4"
-		    : "=r" (mmfr0));
+		unsigned int mmfr0 = read_cpuid_ext(CPUID_EXT_MMFR0);
 		if ((mmfr0 & 0x0000000f) >= 0x00000003 ||
 		    (mmfr0 & 0x000000f0) >= 0x00000030)
 			cpu_arch = CPU_ARCH_ARMv7;
-- 
2.3.2