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

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Message-ID: <20201209111639.GB22806@lst.de>
Date:   Wed, 9 Dec 2020 12:16:39 +0100
From:   ". Christoph Hellwig" <hch@....de>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Tomasz Figa <tfiga@...omium.org>,
        ". Christoph Hellwig" <hch@....de>,
        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 04:13:20PM +0900, Sergey Senozhatsky wrote:
> On (20/12/08 13:54), Tomasz Figa wrote:
> > 
> > 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.
> 
> HUGE apologies for the previous screw up! I replied in the
> gmail web-interface and that did not work out as expected
> (at all, big times).

Actually the previous mail was a mime multipart one, and the plain text
version displayed just fine here.  My the gmail engineers finally learned
something after all.

> Another thing to notice is that the new API requires us to have two execution branches
> in allocators - one for the current API; and one for the new API (if it's supported and
> if user-space requested non-coherent allocation).

So I think we do want these branches for coherent vs non-coherent as they
have very different semantics and I do not think that hiding them under
the same API helps people to understand those vastly different semantics.

OTOH we should look into a fallback for DMA API instances that do not
support the discontigous allocations.

I think between your comments and those from Ricardo I have a good idea
for a somewhat updated API.  I'll try to post something in the next days.