linux-kernel - Re: [RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM

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Message-ID: <ed2dd9042d1afede4843d7fd3728a4d701a19fdd.camel@kernel.crashing.org>
Date:   Wed, 16 Jan 2019 11:33:06 +1100
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will.deacon@....com>,
        "Koenig, Christian" <Christian.Koenig@....com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Michel Dänzer <michel@...nzer.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Carsten Haitzler <Carsten.Haitzler@....com>,
        David Airlie <airlied@...ux.ie>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "Huang, Ray" <Ray.Huang@....com>,
        "Zhang, Jerry" <Jerry.Zhang@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Bernhard Rosenkränzer 
        <Bernhard.Rosenkranzer@...aro.org>
Subject: Re: [RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM

On Tue, 2019-01-15 at 22:31 +1100, Michael Ellerman wrote:
> > > As far as I know Power doesn't really supports un-cached memory at all, 
> > > except for a very very old and odd configuration with AGP.
> > 
> > Hopefully Michael/Ben can elaborate here, but I was under the (possibly
> > mistaken) impression that mismatched attributes could cause a machine-check
> > on Power.
> 
> That's what I've always been told, but I can't actually find where it's
> documented, I'll keep searching.
> 
> But you're right that mixing cached / uncached is not really supported,
> and probably results in a machine check or worse.

 .. or worse :) It could checkstop.

It's also my understanding that on ARM v7 and above, it's technically
forbidden to map the same physical page with both cached and non-cached 
mappings, since the cached one could prefetch (or speculatively load),
thus creating collisions and inconsistencies. Am I wrong here ?

The old hack of using non-cached mapping to avoid snoop cost in AGP and
others is just that ... an ugly and horrible hacks that should have
never eventuated, when the search for performance pushes HW people into
utter insanity :)

Cheers,
Ben.