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

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Date:   Mon, 24 Oct 2022 04:27:26 +0000
From:   <yuji2.ishikawa@...hiba.co.jp>
To:     <posciak@...omium.org>, <paul.kocialkowski@...tlin.com>,
        <hverkuil-cisco@...all.nl>, <mchehab+samsung@...nel.org>,
        <linux-media@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Question for an accepted patch: use of DMA-BUF based videobuf2
 capture buffer with no-HW-cache-coherent HW

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/

When I removed this patch from my 5.10.y working-tree, the driver yielded images without any defects.

***************
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.

Would you tell me some idea on this patch:
* Do well-implemented capture drivers work well even if this patch is applied?
* How should a video capture driver call V4L2/videobuf2 APIs, especially when the hardware does not support cache coherency?

***************
[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/

***************
Sorry for sending the same email message again. I wrongly posted previous one with HTML format.

Regards,
              Yuji Ishikawa