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Message-ID: <b645f983-447b-7b4b-6dd6-d5f10da08e96@xs4all.nl>
Date: Mon, 24 Oct 2022 09:49:11 +0200
From: Hans Verkuil <hverkuil-cisco@...all.nl>
To: yuji2.ishikawa@...hiba.co.jp, posciak@...omium.org,
paul.kocialkowski@...tlin.com, mchehab+samsung@...nel.org,
linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Question for an accepted patch: use of DMA-BUF based videobuf2
capture buffer with no-HW-cache-coherent HW
Hi Yuji,
On 10/24/22 06:02, yuji2.ishikawa@...hiba.co.jp 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@bootlin.com/ <https://patchwork.kernel.org/project/linux-media/patch/20190124095156.21898-1-paul.kocialkowski@bootlin.com/>
>
>
>
> 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@toshiba.co.jp/ <https://lore.kernel.org/all/20220810132822.32534-1-yuji2.ishikawa@toshiba.co.jp/>
>
>
>
>
>
> Regards,
>
> Yuji Ishikawa
>
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