linux-kernel - Re: DMA-buf and uncached system memory

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Date:   Thu, 23 Jun 2022 12:27:26 +0100
From:   Daniel Stone <daniel@...ishbar.org>
To:     Christian König <christian.koenig@....com>
Cc:     Lucas Stach <l.stach@...gutronix.de>,
        Pekka Paalanen <ppaalanen@...il.com>,
        "Sharma, Shashank" <Shashank.Sharma@....com>,
        lkml <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Nicolas Dufresne <nicolas@...fresne.ca>,
        linaro-mm-sig@...ts.linaro.org,
        Sumit Semwal <sumit.semwal@...aro.org>,
        linux-media <linux-media@...r.kernel.org>
Subject: Re: DMA-buf and uncached system memory

Hi Christian,

On Thu, 23 Jun 2022 at 12:11, Christian König <christian.koenig@....com> wrote:
> > In fact DMA-buf sharing works fine on most of those SoCs because
> > everyone just assumes that all the accelerators don't snoop, so the
> > memory shared via DMA-buf is mostly CPU uncached. It only falls apart
> > for uses like the UVC cameras, where the shared buffer ends up being
> > CPU cached.
>
> Well then the existing DMA-buf framework is not what you want to use for
> this.
>
> > Non-coherent without explicit domain transfer points is just not going
> > to work. So why can't we solve the issue for DMA-buf in the same way as
> > the DMA API already solved it years ago: by adding the equivalent of
> > the dma_sync calls that do cache maintenance when necessary? On x86 (or
> > any system where things are mostly coherent) you could still no-op them
> > for the common case and only trigger cache cleaning if the importer
> > explicitly says that is going to do a non-snooping access.
>
> Because DMA-buf is a framework for buffer sharing between cache coherent
> devices which don't signal transitions.
>
> We intentionally didn't implemented any of the dma_sync_* functions
> because that would break the intended use case.
>
> You can of course use DMA-buf in an incoherent environment, but then you
> can't expect that this works all the time.
>
> This is documented behavior and so far we have bluntly rejected any of
> the complains that it doesn't work on most ARM SoCs and I don't really
> see a way to do this differently.

For some strange reason, 'let's do buffer sharing but make sure it
doesn't work on Arm' wasn't exactly the intention of the groups who
came together under the Linaro umbrella to create dmabuf.

If it's really your belief that dmabuf requires universal snooping, I
recommend you send the patch to update the documentation, as well as
to remove DRIVER_PRIME from, realistically, most non-PCIE drivers.

Cheers,
Daniel