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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)
To:     Maxime Ripard <maxime@cerno.tech>
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From:   Philipp Rossak <embed3d@gmail.com>
Message-ID: <b5a06c19-7a3e-bcb8-5ae3-76901b9c6c35@gmail.com>
Date:   Tue, 21 Apr 2020 11:57:33 +0200
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Hi,

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. 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?


>> 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?

>> 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.
> 
Good to know.

>> 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?
> 
What I know till now this is part of the PowerVR IP and not separated. 
So it should use the same memory region, clocks and interrupts.

Cheers
Philipp