linux-kernel - Re: [PATCH 1/11] ARM: tegra: add function to control the GPU rail clamp

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Message-ID: <20150107101900.GP10073@tbergstrom-lnx.Nvidia.com>
Date:	Wed, 7 Jan 2015 12:19:00 +0200
From:	Peter De Schrijver <pdeschrijver@...dia.com>
To:	Thierry Reding <thierry.reding@...il.com>
CC:	Vince Hsu <vinceh@...dia.com>, Lucas Stach <dev@...xeye.de>,
	<swarren@...dotorg.org>, <gnurou@...il.com>, <bskeggs@...hat.com>,
	<martin.peres@...e.fr>, <seven@...rod-online.com>,
	<samuel.pitoiset@...il.com>, <nouveau@...ts.freedesktop.org>,
	<linux-tegra@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/11] ARM: tegra: add function to control the GPU rail
 clamp

On Mon, Jan 05, 2015 at 04:09:33PM +0100, Thierry Reding wrote:
> * PGP Signed by an unknown key
> 
> On Thu, Dec 25, 2014 at 10:28:08AM +0800, Vince Hsu wrote:
> > On 12/24/2014 09:16 PM, Lucas Stach wrote:
> > >Am Dienstag, den 23.12.2014, 18:39 +0800 schrieb Vince Hsu:
> > >>The Tegra124 and later Tegra SoCs have a sepatate rail gating register
> > >>to enable/disable the clamp. The original function
> > >>tegra_powergate_remove_clamping() is not sufficient for the enable
> > >>function. So add a new function which is dedicated to the GPU rail
> > >>gating. Also don't refer to the powergate ID since the GPU ID makes no
> > >>sense here.
> > >>
> > >>Signed-off-by: Vince Hsu <vinceh@...dia.com>
> > >To be honest I don't see the point of this patch.
> > >You are bloating the PMC interface by introducing another exported
> > >function that does nothing different than what the current function
> > >already does.
> > >
> > >If you need a way to assert the clamp I would have expected you to
> > >introduce a common function to do this for all power partitions.
> > I thought about adding an tegra_powergate_assert_clamping(), but that
> > doesn't make sense to all the power partitions except GPU. Note the
> > difference in TRM. Any suggestion for the common function?
> 
> I don't think extending the powergate API is useful at this point. We've
> long had an open TODO item to replace this with a generic API. I did
> some prototyping a while ago to use generic power domains for this, that
> way all the details and dependencies between the partitions could be
> properly modeled.
> 
> Can you take a look at my staging/powergate branch here:
> 
> 	https://github.com/thierryreding/linux/commits/staging/powergate
> 
> and see if you can use that instead? The idea is to completely hide the
> details of power partitions from drivers and use runtime PM instead.
> 
> Also adding Peter whom I had discussed this with earlier. Can we finally
> get this converted? I'd rather not keep complicating this custom API to
> avoid making the conversion even more difficult.

Conceptually I fully agree that we should use runtime PM and powerdomains.
However I don't think the implementation you mentioned is correct. The resets 
of all modules in a domain need to be asserted and the memory clients need to
be flushed. All this needs to be done with module clocks enabled (resets are
synchronous).  Then all module clocks need to be disabled and then the
partition can be powergated. After ungating, the module resets need to be
deasserted and the FLUSH bit cleared with clocks enabled.

Cheers,

Peter.
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