[PATCH v2] riscv,mmio: Fix readX()-to-delay() ordering

[PATCH v2] riscv,mmio: Fix readX()-to-delay() ordering

[Andrea Parri]

Section 2.1 of the Platform Specification [1] states:

  Unless otherwise specified by a given I/O device, I/O devices are on
  ordering channel 0 (i.e., they are point-to-point strongly ordered).

which is not sufficient to guarantee that a readX() by a hart completes
before a subsequent delay() on the same hart (cf. memory-barriers.txt,
"Kernel I/O barrier effects").

Set the I(nput) bit in __io_ar() to restore the ordering, align inline
comments.

[1] https://github.com/riscv/riscv-platform-specs

Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
Changes since v1:
  https://lore.kernel.org/lkml/20230711133348.151383-1-parri.andrea@gmail.com/

  - surrendered to the intricacies of NOMMU/RV32 builds: dropping #2
    (takers are welcome! ;-) )


 arch/riscv/include/asm/mmio.h | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/riscv/include/asm/mmio.h b/arch/riscv/include/asm/mmio.h
index aff6c33ab0c08..4c58ee7f95ecf 100644
--- a/arch/riscv/include/asm/mmio.h
+++ b/arch/riscv/include/asm/mmio.h
@@ -101,9 +101,9 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
  * Relaxed I/O memory access primitives. These follow the Device memory
  * ordering rules but do not guarantee any ordering relative to Normal memory
  * accesses.  These are defined to order the indicated access (either a read or
- * write) with all other I/O memory accesses. Since the platform specification
- * defines that all I/O regions are strongly ordered on channel 2, no explicit
- * fences are required to enforce this ordering.
+ * write) with all other I/O memory accesses to the same peripheral. Since the
+ * platform specification defines that all I/O regions are strongly ordered on
+ * channel 0, no explicit fences are required to enforce this ordering.
  */
 /* FIXME: These are now the same as asm-generic */
 #define __io_rbr()		do {} while (0)
@@ -125,14 +125,14 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
 #endif
 
 /*
- * I/O memory access primitives. Reads are ordered relative to any
- * following Normal memory access. Writes are ordered relative to any prior
- * Normal memory access.  The memory barriers here are necessary as RISC-V
+ * I/O memory access primitives.  Reads are ordered relative to any following
+ * Normal memory read and delay() loop.  Writes are ordered relative to any
+ * prior Normal memory write.  The memory barriers here are necessary as RISC-V
  * doesn't define any ordering between the memory space and the I/O space.
  */
 #define __io_br()	do {} while (0)
-#define __io_ar(v)	__asm__ __volatile__ ("fence i,r" : : : "memory")
-#define __io_bw()	__asm__ __volatile__ ("fence w,o" : : : "memory")
+#define __io_ar(v)	({ __asm__ __volatile__ ("fence i,ir" : : : "memory"); })
+#define __io_bw()	({ __asm__ __volatile__ ("fence w,o" : : : "memory"); })
 #define __io_aw()	mmiowb_set_pending()
 
 #define readb(c)	({ u8  __v; __io_br(); __v = readb_cpu(c); __io_ar(__v); __v; })
-- 
2.34.1


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Re: [PATCH v2] riscv,mmio: Fix readX()-to-delay() ordering

[patchwork-bot+linux-riscv]

Hello:

This patch was applied to riscv/linux.git (fixes)
by Palmer Dabbelt <palmer@rivosinc.com>:

On Thu,  3 Aug 2023 06:27:38 +0200 you wrote:
> Section 2.1 of the Platform Specification [1] states:
> 
>   Unless otherwise specified by a given I/O device, I/O devices are on
>   ordering channel 0 (i.e., they are point-to-point strongly ordered).
> 
> which is not sufficient to guarantee that a readX() by a hart completes
> before a subsequent delay() on the same hart (cf. memory-barriers.txt,
> "Kernel I/O barrier effects").
> 
> [...]

Here is the summary with links:
  - [v2] riscv,mmio: Fix readX()-to-delay() ordering
    https://git.kernel.org/riscv/c/4eb2eb1b4c0e

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html



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Re: [PATCH v2] riscv,mmio: Fix readX()-to-delay() ordering

[Palmer Dabbelt]

On Thu, 03 Aug 2023 06:27:38 +0200, Andrea Parri wrote:
> Section 2.1 of the Platform Specification [1] states:
> 
>   Unless otherwise specified by a given I/O device, I/O devices are on
>   ordering channel 0 (i.e., they are point-to-point strongly ordered).
> 
> which is not sufficient to guarantee that a readX() by a hart completes
> before a subsequent delay() on the same hart (cf. memory-barriers.txt,
> "Kernel I/O barrier effects").
> 
> [...]

Applied, thanks!

[1/1] riscv,mmio: Fix readX()-to-delay() ordering
      https://git.kernel.org/palmer/c/4eb2eb1b4c0e

Best regards,
-- 
Palmer Dabbelt <palmer@rivosinc.com>


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