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/ 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@xxxxxxxxxxxxx/ *************** Sorry for sending the same email message again. I wrongly posted previous one with HTML format. Regards, Yuji Ishikawa
