[PATCH] io: prevent compiler reordering on the default writeX() implementation

[PATCH] io: prevent compiler reordering on the default writeX() implementation

[Sinan Kaya]

The default implementation of mapping writeX() to __raw_writeX() is wrong.
writeX() has stronger ordering semantics. Compiler is allowed to reorder
__raw_writeX().

In the abscence of a write barrier or when using a strongly ordered
architecture, writeX() should at least have a compiler barrier in
it to prevent commpiler from clobbering the execution order.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
---
 include/asm-generic/io.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/asm-generic/io.h b/include/asm-generic/io.h
index b4531e3..fbbf2bb 100644
--- a/include/asm-generic/io.h
+++ b/include/asm-generic/io.h
@@ -153,6 +153,7 @@ static inline void writeb(u8 value, volatile void __iomem *addr)
 static inline void writew(u16 value, volatile void __iomem *addr)
 {
 	__raw_writew(cpu_to_le16(value), addr);
+	barrier();
 }
 #endif
 
@@ -161,6 +162,7 @@ static inline void writew(u16 value, volatile void __iomem *addr)
 static inline void writel(u32 value, volatile void __iomem *addr)
 {
 	__raw_writel(__cpu_to_le32(value), addr);
+	barrier();
 }
 #endif
 
@@ -170,6 +172,7 @@ static inline void writel(u32 value, volatile void __iomem *addr)
 static inline void writeq(u64 value, volatile void __iomem *addr)
 {
 	__raw_writeq(__cpu_to_le64(value), addr);
+	barrier();
 }
 #endif
 #endif /* CONFIG_64BIT */
-- 
2.7.4

[PATCH] io: prevent compiler reordering on the default writeX() implementation

[Sinan Kaya]

On 3/30/2018 10:29 AM, Sinan Kaya wrote:
> In the abscence of a write barrier or when using a strongly ordered
> architecture, writeX() should at least have a compiler barrier in
> it to prevent commpiler from clobbering the execution order.

Same is true for readX(). I'll wait for review feedback on this before
posting another change for readX().

-- 
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.

[PATCH] io: prevent compiler reordering on the default writeX() implementation

[Russell King - ARM Linux]

On Fri, Mar 30, 2018 at 10:29:58AM -0400, Sinan Kaya wrote:
> The default implementation of mapping writeX() to __raw_writeX() is wrong.
> writeX() has stronger ordering semantics. Compiler is allowed to reorder
> __raw_writeX().
> 
> In the abscence of a write barrier or when using a strongly ordered
> architecture, writeX() should at least have a compiler barrier in
> it to prevent commpiler from clobbering the execution order.

You want the barrier _before_ the call to __raw_writel() - you need to
ensure that writes to memory are emitted by the compiler _before_ the
write to the hardware - the write to the hardware may start DMA, and it
may be reading data that the program thinks it previously wrote.

Similarly, for readl(), you need the barrier after __raw_readl() to
ensure that other reads in the program aren't scheduled before a
potential DMA status register read.

-- 
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