linux-kernel - Re: [PATCH] media: videobuf2: sync caches for dmabuf memory

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Date: Tue, 18 Jun 2024 16:47:45 +0900
From: Tomasz Figa <tfiga@...omium.org>
To: TaoJiang <tao.jiang_2@....com>
Cc: mchehab@...nel.org, hverkuil-cisco@...all.nl, nicolas@...fresne.ca, 
	shawnguo@...nel.org, robh+dt@...nel.org, s.hauer@...gutronix.de, 
	kernel@...gutronix.de, festevam@...il.com, linux-imx@....com, 
	xiahong.bao@....com, eagle.zhou@....com, ming.qian@....nxp.com, 
	imx@...ts.linux.dev, linux-media@...r.kernel.org, 
	linux-kernel@...r.kernel.org, m.szyprowski@...sung.com, 
	sumit.semwal@...aro.org, christian.koenig@....com, 
	dri-devel@...ts.freedesktop.org, linaro-mm-sig@...ts.linaro.org, 
	Ming Qian <ming.qian@....com>
Subject: Re: [PATCH] media: videobuf2: sync caches for dmabuf memory

Hi TaoJiang,

On Tue, Jun 18, 2024 at 4:30 PM TaoJiang <tao.jiang_2@....com> wrote:
>
> From: Ming Qian <ming.qian@....com>
>
> When the memory type is VB2_MEMORY_DMABUF, the v4l2 device can't know
> whether the dma buffer is coherent or synchronized.
>
> The videobuf2-core will skip cache syncs as it think the DMA exporter
> should take care of cache syncs
>
> But in fact it's likely that the client doesn't
> synchronize the dma buf before qbuf() or after dqbuf(). and it's
> difficult to find this type of error directly.
>
> I think it's helpful that videobuf2-core can call
> dma_buf_end_cpu_access() and dma_buf_begin_cpu_access() to handle the
> cache syncs.
>
> Signed-off-by: Ming Qian <ming.qian@....com>
> Signed-off-by: TaoJiang <tao.jiang_2@....com>
> ---
>  .../media/common/videobuf2/videobuf2-core.c   | 22 +++++++++++++++++++
>  1 file changed, 22 insertions(+)
>

Sorry, that patch is incorrect. I believe you're misunderstanding the
way DMA-buf buffers should be managed in the userspace. It's the
userspace responsibility to call the DMA_BUF_IOCTL_SYNC ioctl [1] to
signal start and end of CPU access to the kernel and imply necessary
cache synchronization.

[1] https://docs.kernel.org/driver-api/dma-buf.html#dma-buffer-ioctls

So, really sorry, but it's a NAK.

Best regards,
Tomasz

> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index 358f1fe42975..4734ff9cf3ce 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -340,6 +340,17 @@ static void __vb2_buf_mem_prepare(struct vb2_buffer *vb)
>         vb->synced = 1;
>         for (plane = 0; plane < vb->num_planes; ++plane)
>                 call_void_memop(vb, prepare, vb->planes[plane].mem_priv);
> +
> +       if (vb->memory != VB2_MEMORY_DMABUF)
> +               return;
> +       for (plane = 0; plane < vb->num_planes; ++plane) {
> +               struct dma_buf *dbuf = vb->planes[plane].dbuf;
> +
> +               if (!dbuf)
> +                       continue;
> +
> +               dma_buf_end_cpu_access(dbuf, vb->vb2_queue->dma_dir);
> +       }
>  }
>
>  /*
> @@ -356,6 +367,17 @@ static void __vb2_buf_mem_finish(struct vb2_buffer *vb)
>         vb->synced = 0;
>         for (plane = 0; plane < vb->num_planes; ++plane)
>                 call_void_memop(vb, finish, vb->planes[plane].mem_priv);
> +
> +       if (vb->memory != VB2_MEMORY_DMABUF)
> +               return;
> +       for (plane = 0; plane < vb->num_planes; ++plane) {
> +               struct dma_buf *dbuf = vb->planes[plane].dbuf;
> +
> +               if (!dbuf)
> +                       continue;
> +
> +               dma_buf_begin_cpu_access(dbuf, vb->vb2_queue->dma_dir);
> +       }
>  }
>
>  /*
> --
> 2.43.0-rc1
>