Commit 47ab0dee authored by Russell King's avatar Russell King
Browse files

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: default avatarCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: default avatarRussell King <rmk+kernel@arm.linux.org.uk>
parent 8c0b742c

arch/arm/include/asm/pgtable.h

+1 −1
Original line number Diff line number Diff line
@@ -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

arch/arm/include/asm/system.h

+1 −1
Original line number Diff line number Diff line
@@ -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()

arch/arm/mm/Kconfig

+19 −0
Original line number Diff line number Diff line
@@ -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.

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