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

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Date:   Mon, 21 Jan 2019 19:28:30 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Michel Dänzer <michel@...nzer.net>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will.deacon@....com>,
        David Zhou <David1.Zhou@....com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org, Junwei Zhang <Jerry.Zhang@....com>,
        Huang Rui <ray.huang@....com>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Michael Ellerman <mpe@...erman.id.au>,
        Alex Deucher <alexander.deucher@....com>,
        Sean Paul <sean@...rly.run>,
        Christian Koenig <christian.koenig@....com>,
        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 Mon, 21 Jan 2019 at 19:24, Michel Dänzer <michel@...nzer.net> wrote:
>
> On 2019-01-21 7:20 p.m., Ard Biesheuvel wrote:
> > On Mon, 21 Jan 2019 at 19:04, Michel Dänzer <michel@...nzer.net> wrote:
> >>
> >> On 2019-01-21 6:59 p.m., Ard Biesheuvel wrote:
> >>> On Mon, 21 Jan 2019 at 18:55, Michel Dänzer <michel@...nzer.net> wrote:
> >>>>
> >>>> On 2019-01-21 5:30 p.m., Ard Biesheuvel wrote:
> >>>>> On Mon, 21 Jan 2019 at 17:22, Christoph Hellwig <hch@...radead.org> wrote:
> >>>>>
> >>>>>> Until that happens we should just change the driver ifdefs to default
> >>>>>> the hacks to off and only enable them on setups where we 100%
> >>>>>> positively know that they actually work.  And document that fact
> >>>>>> in big fat comments.
> >>>>>
> >>>>> Well, as I mentioned in my commit log as well, if we default to off
> >>>>> unless CONFIG_X86, we may break working setups on MIPS and Power where
> >>>>> the device is in fact non-cache coherent, and relies on this
> >>>>> 'optimization' to get things working.
> >>>>
> >>>> FWIW, the amdgpu driver doesn't rely on non-snooped transfers for
> >>>> correct basic operation (the scenario Christian brought up is a very
> >>>> specialized use-case), so that shouldn't be an issue.
> >>>>
> >>>
> >>> The point is that this is only true for x86.
> >>>
> >>> On other architectures, the use of non-cached mappings on the CPU side
> >>> means that you /do/ rely on non-snooped transfers, since if those
> >>> transfers turn out not to snoop inadvertently, the accesses are
> >>> incoherent with the CPU's view of memory.
> >>
> >> The driver generally only uses non-cached mappings if
> >> drm_arch/device_can_wc_memory returns true.
> >>
> >
> > Indeed. And so we should take care to only return 'true' from that
> > function if it is guaranteed that non-cached CPU mappings are coherent
> > with the mappings used by the GPU, either because that is always the
> > case (like on x86), or because we know that the platform in question
> > implements NoSnoop correctly throughout the interconnect.
> >
> > What seems to be complicating matters is that in some cases, the
> > device is non-cache coherent to begin with, so regardless of whether
> > the NoSnoop attribute is used or not, those accesses will not snoop in
> > the caches and be coherent with the non-cached mappings used by the
> > CPU. So if we restrict this optimization [on non-X86] to platforms
> > that are known to implement NoSnoop correctly, we may break platforms
> > that are implicitly NoSnoop all the time.
>
> Since the driver generally doesn't rely on non-snooped accesses for
> correctness, that couldn't "break" anything that hasn't always been broken.
>

Again, that is only true on x86.

On other architectures, DMA writes from the device may allocate in the
caches, and be invisible to the CPU when it uses non-cached mappings.