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

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Message-ID: <6b34596d-34c4-bd99-c5a7-5a4742c5b886@arm.com>
Date:   Wed, 9 Dec 2020 13:05:11 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>, Tomasz Figa <tfiga@...omium.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.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

On 2020-12-09 11:12, Christoph Hellwig wrote:
> On Tue, Dec 08, 2020 at 01:54:00PM +0900, Tomasz Figa wrote:
>> >From the media perspective, it would be good to have the vmap
>> optional, similarly to the DMA_ATTR_NO_KERNEL_MAPPING attribute for
>> coherent allocations. Actually, in the media drivers, the need to have
>> a kernel mapping of the DMA buffers corresponds to a minority of the
>> drivers. Most of them only need to map them to the userspace.
>>
>> Nevertheless, that minority actually happens to be quite widely used,
>> e.g. the uvcvideo driver, so we can't go to the other extreme and just
>> drop the vmap at all.
> 
> My main problem is that the DMA_ATTR_NO_KERNEL_MAPPING makes a mess
> of an API.  I'd much rather have low-level API that returns the
> discontiguous allocations and another one that vmaps them rather
> than starting to overload arguments like in dma_alloc_attrs with
> DMA_ATTR_NO_KERNEL_MAPPING.

Agreed - if iommu-dma's dma_alloc_coherent() ends up as little more than 
a thin wrapper around those two functions I think that would be a good 
sign. It also seems like it might be a good idea for this API to use 
scatterlists rather than page arrays as it's fundamental format, to help 
reduce impedance with dma-buf - if we can end up with a wider redesign 
that also gets rid of dma_get_sgtable(), all the better!

Robin.