linux-kernel - 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)

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Date:   Tue, 21 Apr 2020 18:42:17 +0200
From:   Philipp Rossak <embed3d@...il.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     "H. Nikolaus Schaller" <hns@...delico.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        Paul Cercueil <paul@...pouillou.net>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paulburton@...nel.org>,
        James Hogan <jhogan@...nel.org>, Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-omap <linux-omap@...r.kernel.org>,
        OpenPVRSGX Linux Driver Group <openpvrsgx-devgroup@...ux.org>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>, kernel@...a-handheld.com,
        linux-mips@...r.kernel.org,
        arm-soc <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc@...r.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)

Hi,

On 21.04.20 13:21, Maxime Ripard wrote:
> Hi,
> 
> On Tue, Apr 21, 2020 at 11:57:33AM +0200, Philipp Rossak wrote:
>> On 20.04.20 09:38, Maxime Ripard wrote:
>>> 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.
>>
>> 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.
> 
So I think I've found now the right piece of documentation and a driver 
that implements something similar [1].

So I will write a similar driver like linked above that only sets the 
right bits for A83T and A31/A31s.
Do you think this is the right approach?

>> I think this is better placed somewhere in the clocking framework.
>> I see there more similarities to the gating stuff.
>> Do you think it is suitable to implement it like the clock gating?
> 
> I'm really not sure what makes you think that this should be modelled as a
> clock?
> 

Looks like I looked in the wrong place and got some information that are 
not suitable for this.

>>>> 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.
>>
>> I agree on your points. But is this also suitable to drivers that are
>> currently off tree and might be merged in one or two years?
> 
> This is what we done for the Mali. The devicetree binding was first done for the
> out-of-tree driver, and then lima/panfrost reused it.
> 
> The key thing here is to have enough confidence about how the hardware works so
> that you can accurately describe it.

Ok thanks! So I will resend my patches when the work got a more mature 
state and we know enough about the Hardware.

Cheers,
Philipp


[1]: 
https://elixir.bootlin.com/linux/latest/source/drivers/soc/amlogic/meson-gx-pwrc-vpu.c