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

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Date:   Wed, 27 Jan 2021 00:29:08 +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

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 <hch@....de> 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