Re: [PATCH v6 00/12] ARM/MIPS: DTS: add child nodes describing the PVRSGX GPU present in some OMAP SoC and JZ4780 (and many more)

From: Maxime Ripard <maxime@cerno.tech>
To: Philipp Rossak <embed3d@gmail.com>
Cc: "H. Nikolaus Schaller" <hns@goldelico.com>,
	"David Airlie" <airlied@linux.ie>,
	"Daniel Vetter" <daniel@ffwll.ch>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Benoît Cousson" <bcousson@baylibre.com>,
	"Tony Lindgren" <tony@atomide.com>,
	"Paul Cercueil" <paul@crapouillou.net>,
	"Ralf Baechle" <ralf@linux-mips.org>,
	"Paul Burton" <paulburton@kernel.org>,
	"James Hogan" <jhogan@kernel.org>,
	"Kukjin Kim" <kgene@kernel.org>,
	"Krzysztof Kozlowski" <krzk@kernel.org>,
	"Chen-Yu Tsai" <wens@csie.org>,
	"Thomas Bogendoerfer" <tsbogend@alpha.franken.de>,
	"open list:DRM PANEL DRIVERS" <dri-devel@lists.freedesktop.org>,
	"open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS"
	<devicetree@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	linux-omap <linux-omap@vger.kernel.org>,
	"OpenPVRSGX Linux Driver Group" <openpvrsgx-devgroup@letux.org>,
	"Discussions about the Letux Kernel"
	<letux-kernel@openphoenux.org>,
	kernel@pyra-handheld.com, linux-mips@vger.kernel.org,
	arm-soc <linux-arm-kernel@lists.infradead.org>,
	linux-samsung-soc@vger.kernel.org
Subject: Re: [PATCH v6 00/12] ARM/MIPS: DTS: add child nodes describing the PVRSGX GPU present in some OMAP SoC and JZ4780 (and many more)
Date: Mon, 20 Apr 2020 09:38:42 +0200	[thread overview]
Message-ID: <20200420073842.nx4xb3zqvu23arkc@gilmour.lan> (raw)
In-Reply-To: <10969e64-fe1f-d692-4984-4ba916bd2161@gmail.com>

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Hi,

On Fri, Apr 17, 2020 at 02:09:06PM +0200, Philipp Rossak wrote:
> > > I'm a bit skeptical on that one since it doesn't even list the
> > > interrupts connected to the GPU that the binding mandates.
> > 
> > I think he left it out for a future update.
> > But best he comments himself.
> 
> I'm currently working on those bindings. They are now 90% done, but they are
> not finished till now. Currently there is some mainline support missing to
> add the full binding. The A83T and also the A31/A31s have a GPU Power Off
> Gating Register in the R_PRCM module, that is not supported right now in
> Mainline. The Register need to be written when the GPU is powered on and
> off.
> 
> @Maxime: I totally agree on your point that a demo needs to be provided
> before the related DTS patches should be provided. That's the reason why I
> added the gpu placeholder patches.
> Do you have an idea how a driver for the R_PRCM stuff can look like? I'm not
> that experienced with the clock driver framework.

It looks like a power-domain to me, so you'd rather plug that into the genpd
framework.

> The big question is right now how to proceed with the A83T and A31s patches.
> I see there three options, which one do you prefer?:
> 
> 1. Provide now placeholder patches and send new patches, if everything is
> clear and other things are mainlined
> 2. Provide now patches as complete as possible and provide later patches to
> complete them when the R_PRCM things are mainlined
> 3. Leave them out, till the related work is mainlined and the bindings are
> final.

Like I said, the DT *has* to be backward-compatible, so for any DT patch that
you are asking to be merged, you should be prepared to support it indefinitely
and be able to run from it, and you won't be able to change the bindings later
on.

> Since this GPU IP core is very flexible and the SOC manufactures can
> configure it on their needs, I think the binding will extend in the future.
> For example the SGX544 GPU is available in different configurations: there
> is a SGX544 core and SGX544MPx core. The x stands for the count of the USSE
> (Universal Scalable Shader Engine) cores. For example the GPU in the A83T is
> a MP1 and the A31/A31s a MP2.

Mali is in the same situation and it didn't cause much trouble.

> In addition to that some of the GPU's have also a 2D engine.

In the same memory region, running from the same interrupts, or is it a
completely separate IP that happens to be sold by the same vendor?

> There might be even more differences in the GPU's that we don't know right
> now and should be described in the Devicetree, but that's a different topic
> that we should keep in mind.

Like I said, it's not a completely different topic.

Maxime

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