linux-kernel - Re: [PATCH 2/3] thermal: Add Mediatek thermal controller support

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Message-ID: <20151113100912.GE8526@pengutronix.de>
Date:	Fri, 13 Nov 2015 11:09:12 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Eduardo Valentin <edubezval@...il.com>
Cc:	mark.rutland@....com, devicetree@...r.kernel.org,
	Javi Merino <javi.merino@....com>, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, robh+dt@...nel.org,
	linux-mediatek@...ts.infradead.org, kernel@...gutronix.de,
	Matthias Brugger <matthias.bgg@...il.com>,
	Zhang Rui <rui.zhang@...el.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/3] thermal: Add Mediatek thermal controller support

On Wed, Nov 11, 2015 at 08:27:47AM +0100, Sascha Hauer wrote:
> On Tue, Nov 10, 2015 at 10:26:30AM -0800, Eduardo Valentin wrote:
> > On Tue, Nov 10, 2015 at 12:05:54PM +0000, Javi Merino wrote:
> > > On Mon, Nov 09, 2015 at 11:13:32AM +0100, Sascha Hauer wrote:
> > 
> > <cut>
> > 
> > > > +
> > > > +/*
> > > > + * The MT8173 thermal controller has four banks. Each bank can read up to
> > > > + * four temperature sensors simultaneously. The MT8173 has a total of 5
> > > > + * temperature sensors. We use each bank to measure a certain area of the
> > > > + * SoC. Since TS2 is located centrally in the SoC it is influenced by multiple
> > > > + * areas, hence is used in different banks.
> > > > + */
> > > > +static const struct mtk_thermal_bank_cfg bank_data[] = {
> > > > +	{
> > > > +		.num_sensors = 2,
> > > > +		.sensors = { MT8173_TS2, MT8173_TS3 },
> > > > +	}, {
> > > > +		.num_sensors = 2,
> > > > +		.sensors = { MT8173_TS2, MT8173_TS4 },
> > > > +	}, {
> > > > +		.num_sensors = 3,
> > > > +		.sensors = { MT8173_TS1, MT8173_TS2, MT8173_TSABB },
> > > > +	}, {
> > > > +		.num_sensors = 1,
> > > > +		.sensors = { MT8173_TS2 },
> > > > +	},
> > > > +};
> > 
> > Would it make sense to simply expose all sensors and let the
> > configuration of their aggregation be done by DT?
> 
> This particular layout has been chosen because there's also the Smart
> Voltage Scaler (SVS) in the SoC. The SVS uses the same banks for
> measuring temperatures. I don't know the details yet, I just asked the
> Mediatek guys.

Ok, the job of the SVS is to always pick the best voltage for a given
CPU frequency based on the temperature of the CPU cluster. How I
understand it the SVS engine automatically reads temperatures from bank0
for the first CPU cluster and from bank1 for the second CPU cluster. For
this to work we are not free to assign the sensors to the banks
arbitrarily.

I was told that controlling the CPU frequency the performance is better
if we use the maximum temperature of the whole die rather than the
temperature of individual clusters.

I would prefer to keep the sensor/bank association like it currently is
as it allows for easy SVS engine integration. Also I would prefer to
expose a single thermal zone for now, it will be easier to add
additional zones later than it is to remove them later once we have
exposed them to the device tree.

Is that ok with you?

Sascha


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