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

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Message-ID: <20160104141940.GI13058@pengutronix.de>
Date:	Mon, 4 Jan 2016 15:19:40 +0100
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Eduardo Valentin <edubezval@...il.com>
Cc:	linux-pm@...r.kernel.org, Zhang Rui <rui.zhang@...el.com>,
	linux-kernel@...r.kernel.org, kernel@...gutronix.de,
	linux-mediatek@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	Matthias Brugger <matthias.bgg@...il.com>
Subject: Re: [PATCH 2/3] thermal: Add Mediatek thermal controller support

Hi Eduardo,

On Thu, Dec 17, 2015 at 11:33:33AM -0800, Eduardo Valentin wrote:
> Sascha,
> 
> Yeah, sorry for the long delay. I was planing on applying this patch for
> the next merge window, but it just came across one point, see below.
> 
> On Mon, Nov 30, 2015 at 12:42:32PM +0100, Sascha Hauer wrote:
> > This adds support for the Mediatek thermal controller found on MT8173
> > +static const int sensor_mux_values[MT8173_NUM_SENSORS] = { 0, 1, 2, 3, 16 };
> > +
> 
> <big 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.
> > + *
> > + * 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.
> > + */
> > +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 },
> > +	},
> > +};
> 
> Why can't we expose all these as thermal zones?
> 
> That should remove the policy of computing the maximum from this driver.
> Please have a look on the work being done [1] to add grouping and
> aggregation of thermal zones. With that in place, you should be a matter
> of configuring the grouping and selecting max as the aggregation function,
> from the thermal core, instead in the driver. Which should give the
> system engineer, more flexibility to compose whatever policy based on
> the exposed sensors.

I think the aggregation of thermal zones is quite useful when it comes
to putting different chips together to a system. I am not so sure how
useful it is to expose different thermal zones of a single SoC to the
device tree.
Currently the only control knob we have is the CPU frequency. When any
of the sensors on the SoC gets too hot then the only thing we can do is
to decrease the CPU frequency. This does not leave much space for
configuration in the device tree.
What I need to be able is to attach multiple sensors to one thermal
zone. The aggregation patch series only partly solves that and I think
is inconsistent, but I commented on the series directly.

Sascha

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