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 12:26:14 +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 12:23, 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.

Otherwise, the driver will vmap() DMA pages with cacheable attributes,
while the non-cache coherent device uses uncached attributes, breaking
coherency.