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

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Date:   Mon, 21 Jan 2019 08:35:39 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        amd-gfx@...ts.freedesktop.org,
        Christian Koenig <christian.koenig@....com>,
        Alex Deucher <alexander.deucher@....com>,
        David Zhou <David1.Zhou@....com>,
        Huang Rui <ray.huang@....com>,
        Junwei Zhang <Jerry.Zhang@....com>,
        Michel Daenzer <michel.daenzer@....com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [RFC PATCH] drm: disable WC optimization for cache coherent
 devices on non-x86

On Mon, Jan 21, 2019 at 05:30:00PM +0100, Ard Biesheuvel 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. The same could be true for
> non-coherent ARM systems, hence my approach to disable this hack for
> cache coherent devices on non-X86 only.

Do we break existing setups or just reduce performance due to the lack
of WC mappings?  I thought it was the latter.  The point is that
even your check won't do what you actually said.  At lot of non-loongson
mips platforms are not cache coherent.  As are a lot of arm setups
and all sparc64 ones for example.  And chances that someone will
hacks this file out in a random subsystem when adding news ports also
is rather slim, so we'll remaing broken by default.

That is why I want at very least:  a whitelist instead of a blacklist
and some very big comments explaining what is going on here.  And in
the mid to long term even drm really needs to learn to use the
proper APIs :(