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 11:23:44 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
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

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.

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.

Christian.