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 12:45:25 +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 12:37, Koenig, Christian
<Christian.Koenig@....com> wrote:
>
> Am 24.01.19 um 12:26 schrieb Ard Biesheuvel:
> > On Thu, 24 Jan 2019 at 12:23, Koenig, Christian
> > <Christian.Koenig@....com> 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