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: "H. Nikolaus Schaller" <hns@goldelico.com>
To: Maxime Ripard <maxime@cerno.tech>
Cc: "Tony Lindgren" <tony@atomide.com>,
	"Philipp Rossak" <embed3d@gmail.com>,
	"Jonathan Bakker" <xc-racer2@live.ca>,
	"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>,
	"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: Wed, 22 Apr 2020 09:10:57 +0200	[thread overview]
Message-ID: <1AA57A0C-48E6-49BB-BB9A-2AAFFB371BCD@goldelico.com> (raw)
In-Reply-To: <20200422065859.quy6ane5v7vsy5tf@gilmour.lan>

Hi Maxime,

> Am 22.04.2020 um 08:58 schrieb Maxime Ripard <maxime@cerno.tech>:
> 
> On Tue, Apr 21, 2020 at 07:29:32PM +0200, H. Nikolaus Schaller wrote:
>> 
>>> Am 21.04.2020 um 16:15 schrieb Tony Lindgren <tony@atomide.com>:
>>> 
>>> * Maxime Ripard <maxime@cerno.tech> [200421 11:22]:
>>>> On Tue, Apr 21, 2020 at 11:57:33AM +0200, Philipp Rossak wrote:
>>>>> I had a look on genpd and I'm not really sure if that fits.
>>>>> 
>>>>> It is basically some bit that verify that the clocks should be enabled or
>>>>> disabled.
>>>> 
>>>> No, it can do much more than that. It's a framework to control the SoCs power
>>>> domains, so clocks might be a part of it, but most of the time it's going to be
>>>> about powering up a particular device.
>>> 
>>> Note that on omaps there are actually SoC module specific registers.
>> 
>> Ah, I see. This is of course a difference that the TI glue logic has
>> its own registers in the same address range as the sgx and this can't
>> be easily handled by a common sgx driver.
>> 
>> This indeed seems to be unique with omap.
>> 
>>> And there can be multiple devices within a single target module on
>>> omaps. So the extra dts node and device is justified there.
>>> 
>>> For other SoCs, the SGX clocks are probably best handled directly
>>> in pvr-drv.c PM runtime functions unless a custom hardware wrapper
>>> with SoC specific registers exists.
>> 
>> That is why we need to evaluate what the better strategy is.
>> 
>> So we have
>> a) omap which has a custom wrapper around the sgx
>> b) others without, i.e. an empty (or pass-through) wrapper
>> 
>> Which one do we make the "standard" and which one the "exception"?
>> What are good reasons for either one?
>> 
>> 
>> I am currently in strong favour of a) being standard because it
>> makes the pvr-drv.c simpler and really generic (independent of
>> wrapping into any SoC).
>> 
>> This will likely avoid problems if we find more SoC with yet another
>> scheme how the SGX clocks are wrapped.
>> 
>> It also allows to handle different number of clocks (A31 seems to
>> need 4, Samsung, A83 and JZ4780 one) without changing the sgx bindings
>> or making big lists of conditionals. This variance would be handled
>> outside the sgx core bindings and driver.
> 
> I disagree. Every other GPU binding and driver is handling that just fine, and
> the SGX is not special in any case here.

Can you please better explain this? With example or a description
or a proposal?

I simply do not have your experience with "every other GPU" as you have.
And I admit that I can't read from your statement what we should do
to bring this topic forward.

So please make a proposal how it should be in your view.

BR and thanks,
Nikolaus