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

[Ard Biesheuvel]

On Mon, 21 Jan 2019 at 20:04, Michel Dänzer <michel at 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 at 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 at 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 at 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 at 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?