Re: [PATCH 1/3] iommu/io-pgtable-arm: Support coherency for Mali LPAE

From: Will Deacon <will@kernel.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: robh@kernel.org, tomeu.vizoso@collabora.com,
	narmstrong@baylibre.com, khilman@baylibre.com,
	dri-devel@lists.freedesktop.org, steven.price@arm.com,
	iommu@lists.linux-foundation.org,
	alyssa.rosenzweig@collabora.com,
	linux-amlogic@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org, jbrunet@baylibre.com
Subject: Re: [PATCH 1/3] iommu/io-pgtable-arm: Support coherency for Mali LPAE
Date: Mon, 21 Sep 2020 23:24:34 +0100	[thread overview]
Message-ID: <20200921222434.GB4409@willie-the-truck> (raw)
In-Reply-To: <71cc6c53-7bd1-da1a-05fa-8172510b33d8@arm.com>

On Mon, Sep 21, 2020 at 10:53:23PM +0100, Robin Murphy wrote:
> On 2020-09-21 18:57, Will Deacon wrote:
> > On Wed, Sep 16, 2020 at 12:51:05AM +0100, Robin Murphy wrote:
> > > Midgard GPUs have ACE-Lite master interfaces which allows systems to
> > > integrate them in an I/O-coherent manner. It seems that from the GPU's
> > > viewpoint, the rest of the system is its outer shareable domain, and so
> > > even when snoop signals are wired up, they are only emitted for outer
> > > shareable accesses. As such, setting the TTBR_SHARE_OUTER bit does
> > > indeed get coherent pagetable walks working nicely for the coherent
> > > T620 in the Arm Juno SoC.
> > 
> > I can't help but think some of this commentary deserves to be in the code
> > as well.
> 
> Sure, if you want.

Yes, please.

> > Do you know if this sort of thing is done for other SoCs too, or is this
> > just a Juno quirk?
> 
> Yup, this is a "Midgard working as designed" thing. Juno is the coherent
> example I have to hand, but off the top of my head I believe some of the
> Exynos SoCs can also use their GPUs coherently if a switch is flipped in the
> interconnect to change routing between the CCI and a direct-to-RAM path; I
> expect there are probably further Midgard examples that I'm not aware of.
> Then there are definitely coherent Bifrost GPUs like the Amlogic S922/A311
> that prompted me to revive this patch, which we currently drive in "Legacy"
> mode and thus behave the same way as Midgard (Bifrost's "AArch64" mode
> realigns Ish and Osh with the rest of the system, and instead invents a new
> "Internal Shareable" value in between Nsh and Ish to represent the
> shareability between cores within the GPU for which Midgard hijacked Ish).

That is more than I wanted to know :) "Internal Shareable", jeez...

Thanks,

Will
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