From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: "Koenig, Christian" <Christian.Koenig@amd.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Will Deacon <will.deacon@arm.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: Thu, 17 Jan 2019 16:59:53 +1100 [thread overview]
Message-ID: <db8cd68b82cda0ee0d352fdb709a76151798f9f5.camel@kernel.crashing.org> (raw)
In-Reply-To: <9dc27cbf-e109-0c32-fc92-6fce1b224cda@amd.com>
On Wed, 2019-01-16 at 07:35 +0000, Koenig, Christian wrote:
> No, but you answer the wrong question.
>
> See we don't want to have different mappings of cached and non-cached on
> the CPU, but rather want to know if a snooped DMA from the PCIe counts
> as cached access as well.
>
> As far as I know on x86 it doesn't, so when you have an un-cached page
> you can still access it with a snooping DMA read/write operation and
> don't cause trouble.
Hrm... well, if you map it uncached on the CPU on powerpc, a snoop DMA
will work fine too, it won't hit any cache. The only problem I'm aware
of is a core (or CAPI device) emiting non-cached load/stores colliding
with a cache snooper.
> > 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 :)
>
> Well I agree that un-cached system memory makes things much more
> complicated for a questionable gain.
>
> But fact is we now have to deal with the mess, so no point in
> complaining about it to much :)
I wish we could just sent the HW designers home and tell them we won't
support that crap... oh well.
Ben.
> Cheers,
> Christian.
>
> > Cheers,
> > Ben.
> >
> >
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prev parent reply other threads:[~2019-01-17 6:00 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
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 [this message]
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