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

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Date: Tue, 18 Jun 2024 16:27:56 +0800
From: ming qian <ming.qian@....nxp.com>
To: Tomasz Figa <tfiga@...omium.org>, 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, 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

Hi Tomasz,
     Thanks for your explanation, sorry for bothering due to our 
misunderstanding

Best regards,
Ming

> 
>> 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
>>