Re: Question for an accepted patch: use of DMA-BUF based videobuf2 capture buffer with no-HW-cache-coherent HW

[Hans Verkuil]

Hi Yuji,

On 10/24/22 06:02, yuji2.ishikawa@xxxxxxxxxxxxx wrote:
> Hi,
> 
> I'm porting a V4L2 capture driver from 4.19.y to 5.10.y [1].
> 
> When I test the ported driver, I sometimes find a corruption on a captured image.
> 
> Because the corruption is exactly aligned with cacheline, I started investigation from map/unmap of DMA-BUF.
> 
>  
> 
> The capture driver uses DMA-BUF for videobuf2.
> 
> The capture hardware does not have HW-mantained cache coherency with CPU, that is, explicit map/unmap is essential on QBUF/DQBUF.
> 
> After some hours of struggle, I found a patch removing cache synchronizations on QBUF/DQBUF.
> 
>  
> 
> https://patchwork.kernel.org/project/linux-media/patch/20190124095156.21898-1-paul.kocialkowski@xxxxxxxxxxx/ <https://patchwork.kernel.org/project/linux-media/patch/20190124095156.21898-1-paul.kocialkowski@xxxxxxxxxxx/>
> 
>  
> 
> When I removed this patch from my 5.10.y working-tree, the driver yielded images without any defects.v
> 
>  
> 
> ***************
> 
> Sorry for a mention to a patch released 4 years ago.
> 
> The patch removes map/unmap on QBUF/DQBUF to improve the performance of V4L2 decoder device, by reusing previously decoded frames.
> 
> However, there seems no cares nor compensations for modifying lifecycle of DMA-BUF, especially on video capture devices.

I'm not entirely sure what you mean exactly.

> 
>  
> 
> Would you tell me some idea on this patch:
> 
> * Do well-implemented capture drivers work well even if this patch is applied?

Yes, dmabuf is used extensively and I have not had any reports of issues.

> 
> * How should a video capture driver call V4L2/videobuf2 APIs, especially when the hardware does not support cache coherency?

It should all be handled correctly by the core frameworks.

I think you need to debug more inside videobuf2-core.c. Some printk's that show the
dmabuf fd when the buffer is mapped and when it is unmapped + the length it is
mapping should hopefully help a bit.

Regards,

	Hans

> 
>  
> 
> ***************
> 
> [1] FYI: the capture driver is not on mainline yet; the candidate is,
> 
> https://lore.kernel.org/all/20220810132822.32534-1-yuji2.ishikawa@xxxxxxxxxxxxx/ <https://lore.kernel.org/all/20220810132822.32534-1-yuji2.ishikawa@xxxxxxxxxxxxx/>
> 
>  
> 
>  
> 
> Regards,
> 
>               Yuji Ishikawa
>