netdev - Re: [PATCH net-next 2/4] io: add function to flush the write combine buffer to device immediately

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Date:   Fri, 30 Jul 2021 10:00:57 +0100
From:   Will Deacon <will@...nel.org>
To:     Guangbin Huang <huangguangbin2@...wei.com>
Cc:     davem@...emloft.net, kuba@...nel.org, catalin.marinas@....com,
        maz@...nel.org, mark.rutland@....com, dbrazdil@...gle.com,
        qperret@...gle.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        lipeng321@...wei.com, peterz@...radead.org
Subject: Re: [PATCH net-next 2/4] io: add function to flush the write combine
 buffer to device immediately

Hi,

On Fri, Jul 30, 2021 at 11:14:22AM +0800, Guangbin Huang wrote:
> From: Xiongfeng Wang <wangxiongfeng2@...wei.com>
> 
> Device registers can be mapped as write-combine type. In this case, data
> are not written into the device immediately. They are temporarily stored
> in the write combine buffer and written into the device when the buffer
> is full. But in some situation, we need to flush the write combine
> buffer to device immediately for better performance. So we add a general
> function called 'flush_wc_write()'. We use DGH instruction to implement
> this function for ARM64.
> 
> Signed-off-by: Xiongfeng Wang <wangxiongfeng2@...wei.com>
> Signed-off-by: Guangbin Huang <huangguangbin2@...wei.com>
> ---
>  arch/arm64/include/asm/io.h | 2 ++
>  include/linux/io.h          | 6 ++++++
>  2 files changed, 8 insertions(+)

-ENODOCUMENTATION

> diff --git a/arch/arm64/include/asm/io.h b/arch/arm64/include/asm/io.h
> index 7fd836bea7eb..5315d023b2dd 100644
> --- a/arch/arm64/include/asm/io.h
> +++ b/arch/arm64/include/asm/io.h
> @@ -112,6 +112,8 @@ static inline u64 __raw_readq(const volatile void __iomem *addr)
>  #define __iowmb()		dma_wmb()
>  #define __iomb()		dma_mb()
>  
> +#define flush_wc_write()	dgh()

I think it would be worthwhile to look at what architectures other than
arm64 offer here. For example, is there anything similar to this on riscv,
x86 or power? Doing a quick survery of what's out there might help us define
a macro that can be used across multiple architectures.

Thanks,

Will

>  /*
>   * Relaxed I/O memory access primitives. These follow the Device memory
>   * ordering rules but do not guarantee any ordering relative to Normal memory
> diff --git a/include/linux/io.h b/include/linux/io.h
> index 9595151d800d..469d53444218 100644
> --- a/include/linux/io.h
> +++ b/include/linux/io.h
> @@ -166,4 +166,10 @@ static inline void arch_io_free_memtype_wc(resource_size_t base,
>  }
>  #endif
>  
> +/* IO barriers */
> +
> +#ifndef flush_wc_write
> +#define flush_wc_write()		do { } while (0)
> +#endif
> +
>  #endif /* _LINUX_IO_H */
> -- 
> 2.8.1
>