Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API

[. Christoph Hellwig]

On Wed, Jan 27, 2021 at 12:29:08AM +0100, Ricardo Ribalda wrote:
> - 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)

dev->coherent_dma_mask is set by the driver.  So the only reason why
dma_alloc_noncontiguos will fail is because is because it can't
allocate any memory.

> - 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 ?

Yes, we could probably do that.

> 
> 
> 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.

I always found the USB dma alloc API a little weird, but we might have
to follow the scheme of the usb coherent wrappers there.