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

From: Ricardo Ribalda <ribalda@chromium.org>
To: ". Christoph Hellwig" <hch@lst.de>
Cc: Sergey Senozhatsky <senozhatsky@google.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	IOMMU DRIVERS <iommu@lists.linux-foundation.org>,
	Linux Media Mailing List <linux-media@vger.kernel.org>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API
Date: Wed, 27 Jan 2021 22:35:02 +0100	[thread overview]
Message-ID: <CANiDSCsHquQWgeGurmh83UDm3xPzkzaRTV0EOKQs=Q_a349f=Q@mail.gmail.com> (raw)
In-Reply-To: <20210127155608.GA20272@lst.de>

Hi Christoph

On Wed, Jan 27, 2021 at 4:56 PM . Christoph Hellwig <hch@lst.de> wrote:
>
> 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.

I have used the current API here:

https://git.kernel.org/pub/scm/linux/kernel/git/ribalda/linux.git/log/?h=uvc-noncontiguous

And I think the result is very clean. Great work!

I have tested it in X86 and in arm64, with similar performance as the
previous patchset.

Maybe you want to cherry pick that commit into your series I can also
send the patch to the list for review if you prefer so.

At least in 5.11 rc5 I the same dmadev worked in arm64 and x86.

Best regards!

-- 
Ricardo Ribalda
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