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Date: Wed, 18 Nov 2015 09:18:11 +0100
From: Sascha Hauer <s.hauer@pengutronix.de>
To: Javi Merino <javi.merino@arm.com>
Cc: Eduardo Valentin <edubezval@gmail.com>, mark.rutland@arm.com,
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        Matthias Brugger <matthias.bgg@gmail.com>,
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Subject: Re: [PATCH 2/3] thermal: Add Mediatek thermal controller support
Message-ID: <20151118081811.GW8526@pengutronix.de>
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On Fri, Nov 13, 2015 at 11:26:37AM +0000, Javi Merino wrote:
> On Fri, Nov 13, 2015 at 11:09:12AM +0100, Sascha Hauer wrote:
> > 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?
> 
> Fair enough.  I agree that it's easier to add thermal zones in the
> future than to remove it.  Thanks for the explanation.

I added this comment to make this a bit clearer for the next version:

/*
 * The thermal core only gets the maximum temperature of all banks, so
 * the bank concept wouldn't be necessary here. However, the SVS (Smart
 * Voltage Scaling) unit makes its decisions based on the same bank
 * data, and this indeed needs the temperatures of the individual
 * banks
 * for making better decisions.
 */

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