From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: "Michel Dänzer" <michel@daenzer.net>
Cc: Christoph Hellwig <hch@infradead.org>,
Will Deacon <will.deacon@arm.com>,
David Zhou <David1.Zhou@amd.com>,
Maxime Ripard <maxime.ripard@bootlin.com>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
David Airlie <airlied@linux.ie>,
Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
amd-gfx@lists.freedesktop.org, Junwei Zhang <Jerry.Zhang@amd.com>,
Huang Rui <ray.huang@amd.com>,
dri-devel <dri-devel@lists.freedesktop.org>,
Daniel Vetter <daniel@ffwll.ch>,
Michael Ellerman <mpe@ellerman.id.au>,
Alex Deucher <alexander.deucher@amd.com>,
Sean Paul <sean@poorly.run>,
Christian Koenig <christian.koenig@amd.com>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86
Date: Tue, 22 Jan 2019 09:38:10 +0100 [thread overview]
Message-ID: <CAKv+Gu_VBd8agHASc5RS0eCgyXP64Lt_i4yPuBjKwGQ+5Lpd5Q@mail.gmail.com> (raw)
In-Reply-To: <047667fd-17be-1c37-5d2a-26768cfd6ab8@daenzer.net>
On Mon, 21 Jan 2019 at 20:04, Michel Dänzer <michel@daenzer.net> wrote:
>
> On 2019-01-21 7:28 p.m., Ard Biesheuvel wrote:
> > On Mon, 21 Jan 2019 at 19:24, Michel Dänzer <michel@daenzer.net> wrote:
> >> On 2019-01-21 7:20 p.m., Ard Biesheuvel wrote:
> >>> On Mon, 21 Jan 2019 at 19:04, Michel Dänzer <michel@daenzer.net> wrote:
> >>>> On 2019-01-21 6:59 p.m., Ard Biesheuvel wrote:
> >>>>> On Mon, 21 Jan 2019 at 18:55, Michel Dänzer <michel@daenzer.net> wrote:
> >>>>>> On 2019-01-21 5:30 p.m., Ard Biesheuvel wrote:
> >>>>>>> On Mon, 21 Jan 2019 at 17:22, Christoph Hellwig <hch@infradead.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.
>
> Let me try one last time:
>
> If drm_arch_can_wc_memory returns false, the driver falls back to the
> normal mode of operation, using a cacheable CPU mapping and snooped GPU
> transfers, even if userspace asks (as a performance optimization) for a
> write-combined CPU mapping and non-snooped GPU transfers via
> AMDGPU_GEM_CREATE_CPU_GTT_USWC.
Another question: when userspace requests for such a mapping to be
created, does this involve pages that are mapped cacheable into the
userland process?
next prev parent reply other threads:[~2019-01-22 8:38 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-21 10:06 [RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86 Ard Biesheuvel
2019-01-21 10:11 ` Koenig, Christian
2019-01-21 15:07 ` Christoph Hellwig
2019-01-21 15:33 ` Ard Biesheuvel
2019-01-21 15:59 ` Christoph Hellwig
2019-01-21 16:14 ` Ard Biesheuvel
2019-01-21 16:22 ` Christoph Hellwig
2019-01-21 16:30 ` Ard Biesheuvel
2019-01-21 16:35 ` Christoph Hellwig
2019-01-21 16:50 ` Ard Biesheuvel
2019-01-21 17:55 ` Michel Dänzer
2019-01-21 17:59 ` Ard Biesheuvel
2019-01-21 18:04 ` Michel Dänzer
2019-01-21 18:20 ` Ard Biesheuvel
2019-01-21 18:23 ` Michel Dänzer
2019-01-21 18:28 ` Ard Biesheuvel
2019-01-21 19:04 ` Michel Dänzer
2019-01-21 19:18 ` Ard Biesheuvel
2019-01-22 8:38 ` Ard Biesheuvel [this message]
2019-01-22 20:56 ` Alex Deucher
2019-01-22 21:07 ` Ard Biesheuvel
2019-01-23 7:15 ` Christoph Hellwig
2019-01-23 9:08 ` Ard Biesheuvel
2019-01-23 16:44 ` Christoph Hellwig
2019-01-23 16:52 ` Ard Biesheuvel
2019-01-24 9:13 ` Christoph Hellwig
2019-01-24 9:25 ` Koenig, Christian
2019-01-24 9:28 ` Ard Biesheuvel
2019-01-24 9:45 ` Koenig, Christian
2019-01-24 9:59 ` Ard Biesheuvel
2019-01-24 11:23 ` Koenig, Christian
2019-01-24 11:26 ` Ard Biesheuvel
2019-01-24 11:37 ` Koenig, Christian
2019-01-24 11:45 ` Ard Biesheuvel
2019-01-24 13:54 ` Alex Deucher
2019-01-24 13:57 ` Ard Biesheuvel
2019-01-24 14:00 ` Alex Deucher
2019-01-24 16:04 ` Michel Dänzer
2019-01-24 9:31 ` Michel Dänzer
2019-01-24 9:37 ` Ard Biesheuvel
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