linux-kernel - Re: [PATCH 4/4] csky: Add dma_inv_range for DMA_FROM

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Message-ID: <CAK8P3a0v3oVS5cCkORxA7na+VE7ofTQRxiv5o5xNf5v=esnN9A@mail.gmail.com>
Date:   Tue, 30 Jul 2019 15:43:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-csky@...r.kernel.org, feng_shizhu@...uatech.com,
        zhang_jian5@...uatech.com, zheng_xingjian@...uatech.com,
        zhu_peng@...uatech.com, Guo Ren <ren_guo@...ky.com>
Subject: Re: [PATCH 4/4] csky: Add dma_inv_range for DMA_FROM_DEVICE

On Tue, Jul 30, 2019 at 2:16 PM <guoren@...nel.org> wrote:
> From: Guo Ren <ren_guo@...ky.com>

> diff --git a/arch/csky/mm/dma-mapping.c b/arch/csky/mm/dma-mapping.c
> index 3f1ff9d..d8f0f81 100644
> --- a/arch/csky/mm/dma-mapping.c
> +++ b/arch/csky/mm/dma-mapping.c
> @@ -72,6 +72,8 @@ void arch_sync_dma_for_device(struct device *dev, phys_addr_t paddr,
>                 cache_op(paddr, size, dma_wb_range);
>                 break;
>         case DMA_FROM_DEVICE:
> +               cache_op(paddr, size, dma_inv_range);
> +               break;
>         case DMA_BIDIRECTIONAL:
>                 cache_op(paddr, size, dma_wbinv_range);
>                 break;
> @@ -88,6 +90,8 @@ void arch_sync_dma_for_cpu(struct device *dev, phys_addr_t paddr,
>                 cache_op(paddr, size, dma_wb_range);
>                 break;
>         case DMA_FROM_DEVICE:
> +               cache_op(paddr, size, dma_inv_range);
> +               break;
>         case DMA_BIDIRECTIONAL:
>                 cache_op(paddr, size, dma_wbinv_range);
>                 break;

When syncing 'for_cpu', you should not need to write back, because
there won't be any dirty cache lines.

If you have a CPU core that does not do speculative loads, you also don't
need to invalidate here, because you have already done that in the
_for_device() case, the only reason to invalidate the CPU cache
again is if a speculative load created a stale cache line that now
shadows the data received from the device.

        Arnd