[PATCH v4 1/2] MIPS: io: prevent compiler reordering on the default writeX() implementation

[PATCH v4 1/2] MIPS: io: prevent compiler reordering on the default writeX() implementation

[Sinan Kaya]

writeX() has a strong ordering semantics with respect to memory updates.
In the abscence of a write barrier or a compiler barrier, commpiler can
reorder register and memory update instructions. This breaks the writeX()
API.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
---
 arch/mips/include/asm/io.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/mips/include/asm/io.h b/arch/mips/include/asm/io.h
index 0cbf3af..fd00ddaf 100644
--- a/arch/mips/include/asm/io.h
+++ b/arch/mips/include/asm/io.h
@@ -307,7 +307,7 @@ static inline void iounmap(const volatile void __iomem *addr)
 #if defined(CONFIG_CPU_CAVIUM_OCTEON) || defined(CONFIG_LOONGSON3_ENHANCEMENT)
 #define war_io_reorder_wmb()		wmb()
 #else
-#define war_io_reorder_wmb()		do { } while (0)
+#define war_io_reorder_wmb()		barrier()
 #endif
 
 #define __BUILD_MEMORY_SINGLE(pfx, bwlq, type, irq)			\
-- 
2.7.4

[PATCH v4 2/2] MIPS: io: add a barrier after register read in readX()

[Sinan Kaya]

While a barrier is present in writeX() function before the register write,
a similar barrier is missing in the readX() function after the register
read. This could allow memory accesses following readX() to observe
stale data.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
---
 arch/mips/include/asm/io.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/mips/include/asm/io.h b/arch/mips/include/asm/io.h
index fd00ddaf..d96af41 100644
--- a/arch/mips/include/asm/io.h
+++ b/arch/mips/include/asm/io.h
@@ -377,6 +377,8 @@ static inline type pfx##read##bwlq(const volatile void __iomem *mem)	\
 		BUG();							\
 	}								\
 									\
+	/* prevent prefetching of coherent DMA dma prematurely */	\
+	rmb();								\
 	return pfx##ioswab##bwlq(__mem, __val);				\
 }
 
-- 
2.7.4

Re: [PATCH v4 2/2] MIPS: io: add a barrier after register read in readX()

[Sinan Kaya]

On 4/12/2018 10:30 PM, Sinan Kaya wrote:
> +	/* prevent prefetching of coherent DMA dma prematurely */	\

I tried to write DMA data but my keyboard is not cooperating. I'll hold onto
posting another version until I hear back from you for wmb().

-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

Re: [PATCH v4 2/2] MIPS: io: add a barrier after register read in readX()

[James Hogan]

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On Thu, Apr 12, 2018 at 10:33:42PM -0400, Sinan Kaya wrote:
> On 4/12/2018 10:30 PM, Sinan Kaya wrote:
> > +	/* prevent prefetching of coherent DMA dma prematurely */	\
> 
> I tried to write DMA data but my keyboard is not cooperating. I'll hold onto
> posting another version until I hear back from you for wmb().

No problem, I've applied both for 4.17 and tweaked the comment.

Thanks
James

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