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)

[Philipp Rossak]

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