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ARM: Optionally allow ARMv6 to use 'normal, bufferable' memory for DMA 

Provide a configuration option to allow the ARMv6 to use normal
bufferable memory for coherent DMA.  This option is forced to 'y'
for ARMv7, and offered as a configuration option on ARMv6.

Enabling this option requires drivers to have the necessary barriers
to ensure that data in DMA coherent memory is visible prior to the
DMA operation commencing.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Russell King 2010-05-15 11:02:43 +01:00
parent 8c0b742ca7
commit 47ab0dee66
3 changed files with 21 additions and 2 deletions
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2
arch/arm/include/asm/pgtable.h
View File

@ -314,7 +314,7 @@ static inline pte_t pte_mkspecial(pte_t pte) { return pte; }
__pgprot_modify(prot, L_PTE_MT_MASK, L_PTE_MT_UNCACHED)
#define pgprot_writecombine(prot) \
__pgprot_modify(prot, L_PTE_MT_MASK, L_PTE_MT_BUFFERABLE)
#if __LINUX_ARM_ARCH__ >= 7
#ifdef CONFIG_ARM_DMA_MEM_BUFFERABLE
#define pgprot_dmacoherent(prot) \
__pgprot_modify(prot, L_PTE_MT_MASK|L_PTE_EXEC, L_PTE_MT_BUFFERABLE)
#else

2
arch/arm/include/asm/system.h
View File

@ -137,7 +137,7 @@ extern unsigned int user_debug;
#define dmb() __asm__ __volatile__ ("" : : : "memory")
#endif
#if __LINUX_ARM_ARCH__ >= 7 || defined(CONFIG_SMP)
#if defined(CONFIG_ARM_DMA_MEM_BUFFERABLE) || defined(CONFIG_SMP)
#define mb() dmb()
#define rmb() dmb()
#define wmb() dmb()

19
arch/arm/mm/Kconfig
View File

@ -781,3 +781,22 @@ config ARM_L1_CACHE_SHIFT
int
default 6 if ARM_L1_CACHE_SHIFT_6
default 5
config ARM_DMA_MEM_BUFFERABLE
bool "Use non-cacheable memory for DMA" if CPU_V6 && !CPU_V7
default y if CPU_V6 || CPU_V7
help
Historically, the kernel has used strongly ordered mappings to
provide DMA coherent memory. With the advent of ARMv7, mapping
memory with differing types results in unpredictable behaviour,
so on these CPUs, this option is forced on.
Multiple mappings with differing attributes is also unpredictable
on ARMv6 CPUs, but since they do not have aggressive speculative
prefetch, no harm appears to occur.
However, drivers may be missing the necessary barriers for ARMv6,
and therefore turning this on may result in unpredictable driver
behaviour. Therefore, we offer this as an option.
You are recommended say 'Y' here and debug any affected drivers.