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Date:   Wed, 27 Jan 2021 16:56:08 +0100
From:   ". Christoph Hellwig" <hch@lst.de>
To:     Ricardo Ribalda <ribalda@chromium.org>
Cc:     ". Christoph Hellwig" <hch@lst.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Tomasz Figa <tfiga@chromium.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        IOMMU DRIVERS <iommu@lists.linux-foundation.org>,
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        Sergey Senozhatsky <senozhatsky@google.com>
Subject: Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API
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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.