Messages in this thread | | | From | Ricardo Ribalda <> | Date | Wed, 27 Jan 2021 00:29:08 +0100 | Subject | Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API |
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Hi Christoph
Thanks for the series!
I have a couple of questions:
- Is there any platform where dma_alloc_noncontiguos can fail? This is, !ops->alloc_noncontiguous and !dev->coherent_dma_mask If yes then we need to add a function to let the driver know in advance that it has to use the coherent allocator (usb_alloc_coherent for uvc)
- In dma_alloc_noncontiguos, on the dma_alloc_pages fallback. If we have a device where the dma happens in only one direction, could not get more performance with DMA_FROM/TO_DEVICE instead of DMA_BIDIRECTIONAL ?
Then I have tried to use the API, and I have encountered a problem: on uvcvideo the device passed to the memory allocator is different for DMA_PAGES and NON_CONTIGUOUS: https://github.com/ribalda/linux/blob/042cd497739f71c8d4a83a67ee970369e2baca4a/drivers/media/usb/uvc/uvc_video.c#L1236
I need to dig a bit tomorrow to figure out why this is, I have hardware to test both paths, so it should not be too difficult.
Thanks again
On Tue, Jan 26, 2021 at 6:07 PM . Christoph Hellwig <hch@lst.de> wrote: > > Please take a quick look at this branch: > > http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma_alloc_noncontiguous > > Warning: hot off the press, and only with the v4l conversion as that > seemed at little easier than uvcvideo.
-- Ricardo Ribalda
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