linux-kernel - Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc

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Date:   Wed, 27 Jan 2021 22:35:02 +0100
From:   Ricardo Ribalda <ribalda@...omium.org>
To:     ". Christoph Hellwig" <hch@....de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        IOMMU DRIVERS <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Sergey Senozhatsky <senozhatsky@...gle.com>
Subject: Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API

Hi Christoph

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