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

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Date:   Wed, 9 Dec 2020 12:12:35 +0100
From:   Christoph Hellwig <hch@....de>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Christoph Hellwig <hch@....de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Ricardo Ribalda <ribalda@...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

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.

> 
> In any case, Sergey is going to share a preliminary patch on how the
> current API would be used in the V4L2 videobuf2 framework. That should
> give us more input on how such a helper could look.

Awesome!