linux-kernel - Re: [RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86

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Date:   Thu, 24 Jan 2019 10:28:45 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     "Koenig, Christian" <Christian.Koenig@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Alex Deucher <alexdeucher@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Will Deacon <will.deacon@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx list <amd-gfx@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        "Huang, Ray" <Ray.Huang@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Zhang, Jerry" <Jerry.Zhang@....com>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        Sean Paul <sean@...rly.run>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH] drm: disable WC optimization for cache coherent
 devices on non-x86

On Thu, 24 Jan 2019 at 10:25, Koenig, Christian
<Christian.Koenig@....com> wrote:
>
> Am 24.01.19 um 10:13 schrieb Christoph Hellwig:
> > On Wed, Jan 23, 2019 at 05:52:50PM +0100, Ard Biesheuvel wrote:
> >> But my concern is that it seems likely that non-cache coherent
> >> implementations are relying on this hack as well. There must be a
> >> reason that this hack is only disabled for PowerPC platforms if they
> >> are cache coherent, for instance, and I suspect that that reason is
> >> that the hack is the only thing ensuring that the CPU mapping
> >> attributes match the device ones used for these buffers (the vmap()ed
> >> ones), whereas the rings and other consistent data structures are
> >> using the DMA API as intended, and thus getting uncached attributes in
> >> the correct way.
> > Dave, who added that commit is on Cc together with just about everyone
> > involved in the review chain.  Based on the previous explanation
> > that idea what we might want an uncached mapping for some non-coherent
> > architectures for this to work at all makes sense, but then again
> > the way to create those mappings is entirely architecture specific,
> > and also need a cache flushing before creating the mapping to work
> > properly.  So my working theory is that this code never properly
> > worked on architectures without DMA coherent for PCIe at all, but
> > I'd love to be corrected by concrete examples including an explanation
> > of how it actually ends up working.
>
> Cache coherency is mandatory for modern GPU operation.
>
> Otherwise you can't implement a bunch of the requirements of the
> userspace APIs.
>
> In other words the applications doesn't inform the driver that the GPU
> or the CPU is accessing data, it just does it and assumes that it works.
>

Wonderful!

In that case, do you have any objections to the patch proposed by
Christoph above?