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 [off-list ref] 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