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

[Ard Biesheuvel]

On Thu, 24 Jan 2019 at 14:54, Alex Deucher <alexdeucher@xxxxxxxxx> wrote:
>
> On Thu, Jan 24, 2019 at 6:45 AM Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
> >
> > On Thu, 24 Jan 2019 at 12:37, Koenig, Christian
> > <Christian.Koenig@xxxxxxx> wrote:
> > >
> > > Am 24.01.19 um 12:26 schrieb Ard Biesheuvel:
> > > > On Thu, 24 Jan 2019 at 12:23, Koenig, Christian
> > > > <Christian.Koenig@xxxxxxx> wrote:
> > > >> Am 24.01.19 um 10:59 schrieb Ard Biesheuvel:
> > > >>> [SNIP]
> > > >>> This is *exactly* my point the whole time.
> > > >>>
> > > >>> The current code has
> > > >>>
> > > >>> static inline bool drm_arch_can_wc_memory(void)
> > > >>> {
> > > >>> #if defined(CONFIG_PPC) && !defined(CONFIG_NOT_COHERENT_CACHE)
> > > >>>      return false;
> > > >>>
> > > >>> which means the optimization is disabled *unless the system is
> > > >>> non-cache coherent*
> > > >>>
> > > >>> So if you have reports that the optimization works on some PowerPC, it
> > > >>> must be non-cache coherent PowerPC, because that is the only place
> > > >>> where it is enabled in the first place.
> > > >>>
> > > >>>> The only problematic here actually seems to be ARM, so you should
> > > >>>> probably just add an "#ifdef .._ARM return false;".
> > > >>>>
> > > >>> ARM/arm64 does not have a Kconfig symbol like
> > > >>> CONFIG_NOT_COHERENT_CACHE, so we can only disable it everywhere. If
> > > >>> there are non-coherent ARM systems that are currently working in the
> > > >>> same way as those non-coherent PowerPC systems, we will break them by
> > > >>> doing this.
> > > >> Summing the things I've read so far for ARM up I actually think it
> > > >> depends on a runtime configuration and not on compile time one.
> > > >>
> > > >> So the whole idea of providing the device to the drm_*_can_wc_memory()
> > > >> function isn't so far fetched.
> > > >>
> > > > Thank you.
> > > >
> > > >> But for now I do prefer working and slightly slower system over broken
> > > >> one, so I think we should just disable this on ARM for now.
> > > >>
> > > > Again, this is not about non-cache coherent being slower without the
> > > > optimization, it is about non-cache coherent likely not working *at
> > > > all* unless the optimization is enabled.
> > >
> > > As Michel tried to explain this CAN'T happen. The optimization is a
> > > purely optional request from userspace.
> > >
> >
> > Right.
> >
> > So in that case, we can assume that the following test
> >
> > static inline bool drm_arch_can_wc_memory(void)
> > {
> > #if defined(CONFIG_PPC) && !defined(CONFIG_NOT_COHERENT_CACHE)
> >     return false;
> >
> > is bogus, and it was just unnecessary caution on the part of the
> > author to disregard non-cache coherent devices.
> > Unfortunately, those commits have no log messages whatsoever, so it is
> > difficult to infer the intent retroactively.
> >
> > > > Otherwise, the driver will vmap() DMA pages with cacheable attributes,
> > > > while the non-cache coherent device uses uncached attributes, breaking
> > > > coherency.
> > >
> > > Again this is mandated by the userspace APIs anyway. E.g. we can't
> > > vmap() pages in any other way or our userspace APIs would break.
> > >
> >
> > OK,
> >
> > So let's just disable this for all ARM and arm64 then, given that
> > non-cache coherent is not supported in any case
>
> So I think we are back to this patch:
> https://patchwork.kernel.org/patch/10739023/
>

Apart from the fact that the issue has nothing to do with write-combining, yes.