From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>,
Will Deacon <will.deacon@arm.com>,
"Koenig, Christian" <Christian.Koenig@amd.com>
Cc: "Carsten Haitzler" <Carsten.Haitzler@arm.com>,
"Ard Biesheuvel" <ard.biesheuvel@linaro.org>,
"David Airlie" <airlied@linux.ie>,
"Michel Dänzer" <michel@daenzer.net>,
"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
dri-devel <dri-devel@lists.freedesktop.org>,
"Huang, Ray" <Ray.Huang@amd.com>,
"Zhang, Jerry" <Jerry.Zhang@amd.com>,
linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
"Bernhard Rosenkränzer" <Bernhard.Rosenkranzer@linaro.org>
Subject: Re: [RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM
Date: Wed, 16 Jan 2019 11:33:06 +1100 [thread overview]
Message-ID: <ed2dd9042d1afede4843d7fd3728a4d701a19fdd.camel@kernel.crashing.org> (raw)
In-Reply-To: <87a7k2yx66.fsf@concordia.ellerman.id.au>
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.
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next prev parent reply other threads:[~2019-01-16 0:33 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-10 7:28 [RFC PATCH] drm/ttm: force cached mappings for system RAM on ARM Ard Biesheuvel
2019-01-10 8:36 ` Zhang, Jerry(Junwei)
2019-01-10 8:36 ` Koenig, Christian
2019-01-10 9:34 ` Michel Dänzer
2019-01-14 10:53 ` Ard Biesheuvel
2019-01-14 11:38 ` Koenig, Christian
2019-01-14 17:32 ` Ard Biesheuvel
[not found] ` <9f956898-7973-98ee-6bf1-e1d445e9d365@amd.com>
2019-01-14 19:13 ` Will Deacon
2019-01-14 19:21 ` Koenig, Christian
2019-01-14 19:35 ` Will Deacon
2019-01-15 11:31 ` Michael Ellerman
2019-01-16 0:33 ` Benjamin Herrenschmidt [this message]
2019-01-16 7:35 ` Koenig, Christian
2019-01-16 7:47 ` Ard Biesheuvel
2019-01-17 6:07 ` Benjamin Herrenschmidt
2019-01-17 8:02 ` Ard Biesheuvel
2019-01-17 5:59 ` Benjamin Herrenschmidt
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