linux-kernel - Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync

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Message-ID: <CAF6AEGsT9RO2RmgBTa0HQRjRUiRKy9N3vQxvD7RVWp4HpnWn+w@mail.gmail.com>
Date:   Thu, 29 Nov 2018 13:57:38 -0500
From:   Rob Clark <robdclark@...il.com>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, hch@....de,
        David Airlie <airlied@...ux.ie>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        Robin Murphy <robin.murphy@....com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH v3 1/1] drm: msm: Replace dma_map_sg with dma_sync_sg*

On Thu, Nov 29, 2018 at 12:24 PM Tomasz Figa <tfiga@...omium.org> wrote:
>
> [CC Marek]
>
> On Thu, Nov 29, 2018 at 9:09 AM Daniel Vetter <daniel@...ll.ch> wrote:
> >
> > On Thu, Nov 29, 2018 at 5:57 PM Christoph Hellwig <hch@....de> wrote:
> > >
> > > Note that one thing I'd like to avoid is exposing these funtions directly
> > > to drivers, as that will get us into all kinds of abuses.
> >
> > What kind of abuse do you expect? It could very well be that gpu folks
> > call that "standard use case" ... At least on x86 with the i915 driver
> > we pretty much rely on architectural guarantees for how cache flushes
> > work very much. Down to userspace doing the cache flushing for
> > mappings the kernel has set up.
>
> i915 is a very specific case of a fully contained,
> architecture-specific hardware subsystem, where you can just hardcode
> all integration details inside the driver, because nobody else would
> care.
>
> In ARM world, you can have the same IP blocks licensed by multiple SoC
> vendors with different integration details and that often includes the
> option of coherency.

fwiw, I believe all the GPU IP blocks that are used across multiple
SoCs have their own GPU MMU (potentially in addition to an iommu?).
So the dma-api is a much better fit for them..  drm/msm is a lot
closer to drm/i915 scenario, so I don't so much care if the solution
to our unique problem isn't something that would work for other
drivers ;-)

BR,
-R