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Subject[PATCH 4/4] MIPS: Provide actually relaxed MMIO accessors
Improve performance for the relevant systems and remove the DMA ordering 
barrier from `readX_relaxed' and `writeX_relaxed' MMIO accessors, where
it is not needed according to our requirements[1]. For consistency make
the same arrangement with low-level port I/O accessors, but do not
actually provide any accessors making use of it.

References:

[1] "LINUX KERNEL MEMORY BARRIERS", Documentation/memory-barriers.txt,
Section "KERNEL I/O BARRIER EFFECTS"

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
---
arch/mips/include/asm/io.h | 48 ++++++++++++++++++++++++++-------------------
1 file changed, 28 insertions(+), 20 deletions(-)

linux-mips-readx-writex-relaxed.patch
Index: linux-20180930-3maxp/arch/mips/include/asm/io.h
===================================================================
--- linux-20180930-3maxp.orig/arch/mips/include/asm/io.h
+++ linux-20180930-3maxp/arch/mips/include/asm/io.h
@@ -51,6 +51,11 @@
# define __raw_ioswabq(a, x) (x)
# define ____raw_ioswabq(a, x) (x)

+# define __relaxed_ioswabb ioswabb
+# define __relaxed_ioswabw ioswabw
+# define __relaxed_ioswabl ioswabl
+# define __relaxed_ioswabq ioswabq
+
/* ioswab[bwlq], __mem_ioswab[bwlq] are defined in mangle-port.h */

#define IO_SPACE_LIMIT 0xffff
@@ -337,7 +342,7 @@ static inline void iounmap(const volatil
#define war_io_reorder_wmb() barrier()
#endif

-#define __BUILD_MEMORY_SINGLE(pfx, bwlq, type, barrier, irq) \
+#define __BUILD_MEMORY_SINGLE(pfx, bwlq, type, barrier, relax, irq) \
\
static inline void pfx##write##bwlq(type val, \
volatile void __iomem *mem) \
@@ -411,11 +416,12 @@ static inline type pfx##read##bwlq(const
} \
\
/* prevent prefetching of coherent DMA data prematurely */ \
- rmb(); \
+ if (!relax) \
+ rmb(); \
return pfx##ioswab##bwlq(__mem, __val); \
}

-#define __BUILD_IOPORT_SINGLE(pfx, bwlq, type, barrier, p, slow) \
+#define __BUILD_IOPORT_SINGLE(pfx, bwlq, type, barrier, relax, p, slow) \
\
static inline void pfx##out##bwlq##p(type val, unsigned long port) \
{ \
@@ -454,19 +460,21 @@ static inline type pfx##in##bwlq##p(unsi
slow; \
\
/* prevent prefetching of coherent DMA data prematurely */ \
- rmb(); \
+ if (!relax) \
+ rmb(); \
return pfx##ioswab##bwlq(__addr, __val); \
}

-#define __BUILD_MEMORY_PFX(bus, bwlq, type) \
+#define __BUILD_MEMORY_PFX(bus, bwlq, type, relax) \
\
-__BUILD_MEMORY_SINGLE(bus, bwlq, type, 1, 1)
+__BUILD_MEMORY_SINGLE(bus, bwlq, type, 1, relax, 1)

#define BUILDIO_MEM(bwlq, type) \
\
-__BUILD_MEMORY_PFX(__raw_, bwlq, type) \
-__BUILD_MEMORY_PFX(, bwlq, type) \
-__BUILD_MEMORY_PFX(__mem_, bwlq, type) \
+__BUILD_MEMORY_PFX(__raw_, bwlq, type, 0) \
+__BUILD_MEMORY_PFX(__relaxed_, bwlq, type, 1) \
+__BUILD_MEMORY_PFX(__mem_, bwlq, type, 0) \
+__BUILD_MEMORY_PFX(, bwlq, type, 0)

BUILDIO_MEM(b, u8)
BUILDIO_MEM(w, u16)
@@ -474,8 +482,8 @@ BUILDIO_MEM(l, u32)
BUILDIO_MEM(q, u64)

#define __BUILD_IOPORT_PFX(bus, bwlq, type) \
- __BUILD_IOPORT_SINGLE(bus, bwlq, type, 1, ,) \
- __BUILD_IOPORT_SINGLE(bus, bwlq, type, 1, _p, SLOW_DOWN_IO)
+ __BUILD_IOPORT_SINGLE(bus, bwlq, type, 1, 0, ,) \
+ __BUILD_IOPORT_SINGLE(bus, bwlq, type, 1, 0, _p, SLOW_DOWN_IO)

#define BUILDIO_IOPORT(bwlq, type) \
__BUILD_IOPORT_PFX(, bwlq, type) \
@@ -490,19 +498,19 @@ BUILDIO_IOPORT(q, u64)

#define __BUILDIO(bwlq, type) \
\
-__BUILD_MEMORY_SINGLE(____raw_, bwlq, type, 1, 0)
+__BUILD_MEMORY_SINGLE(____raw_, bwlq, type, 1, 0, 0)

__BUILDIO(q, u64)

-#define readb_relaxed readb
-#define readw_relaxed readw
-#define readl_relaxed readl
-#define readq_relaxed readq
+#define readb_relaxed __relaxed_readb
+#define readw_relaxed __relaxed_readw
+#define readl_relaxed __relaxed_readl
+#define readq_relaxed __relaxed_readq

-#define writeb_relaxed writeb
-#define writew_relaxed writew
-#define writel_relaxed writel
-#define writeq_relaxed writeq
+#define writeb_relaxed __relaxed_writeb
+#define writew_relaxed __relaxed_writew
+#define writel_relaxed __relaxed_writel
+#define writeq_relaxed __relaxed_writeq

#define readb_be(addr) \
__raw_readb((__force unsigned *)(addr))
\
 
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