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Date:   Thu, 25 Jan 2018 10:57:15 +0000
From:   Daniel Thompson <daniel.thompson@linaro.org>
To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     edubezval@gmail.com, kevin.wangtao@linaro.org, leo.yan@linaro.org,
        vincent.guittot@linaro.org, amit.kachhap@gmail.com,
        viresh.kumar@linaro.org, linux-kernel@vger.kernel.org,
        Zhang Rui <rui.zhang@intel.com>,
        "open list:THERMAL" <linux-pm@vger.kernel.org>
Subject: Re: [PATCH 4/8] thermal/drivers/Kconfig: Convert the CPU cooling
 device to a choice
Message-ID: <20180125105715.sae3nwvz7u32v4va@oak.lan>
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On Wed, Jan 24, 2018 at 05:59:09PM +0100, Daniel Lezcano wrote:
> On 24/01/2018 17:34, Daniel Thompson wrote:
> > On Tue, Jan 23, 2018 at 04:34:27PM +0100, Daniel Lezcano wrote:
> >> The next changes will add new way to cool down a CPU. In order to
> >> sanitize and make the overall cpu cooling code consistent and robust
> >> we must prevent the cpu cooling devices to co-exists with the same
> >> purpose at the same time in the kernel.
> >>
> >> Make the CPU cooling device a choice in the Kconfig, so only one CPU
> >> cooling strategy can be chosen.
> > 
> > I puzzled by the role of Kconfig here.
> > 
> > IIUC a distro kernel will always be forced to select the combo strategy 
> > otherwise it will never be able to cool systems that don't have cpufreq
> > (I hope the combo strategy treats such system as a special case with
> > only one OPP).
> 
> Actually it does not make sense to select the combo if there is no
> cpufreq support. The cpuidle cooling device must be used instead.

Well, I said before what I hoped. This is what I feared! ;-)


> > This raises the question what the other options (cpufreq-only
> > idle-injection-only) are for? Are they just for petrol heads who want to
> > save a few bytes of code or is idle-injection undesirable for some
> > users.
> 
> The combo cooling device must be used on a system with a proper support
> for cpuidle and cpufreq and with the power numbers specified in the DT.
> 
> By proper support of cpuidle, I mean a cluster power down idle state,
> fast enough. This idle state allows to drop the dynamic power *and* the
> static leakage (the latter to prevent a thermal runaway).
> 
> If the system does not have power numbers, no (or bad) cpuidle, the
> combo cooling device must not be used. If there is no cpufreq support,
> the cpuidle cooling must be used and if there is no proper support for
> both, the CPU cooling can't be used. In this case, you have to put a fan
> on your board or reduce the frequency where the system stays in its
> thermal envelope.

How can we know (in the general case) what is going to be in the DT at
compile time?


> > If the later, how can a distro kernel mitigate the undesired effects
> > whilst still selecting the combo strategy.
> 
> I'm not sure to understand the question. Distros always use the make
> allmodconfig, so that chooses the cpufreq CPU cooling device which was
> the case before without this change.

So there's no regression. That's nice but doesn't that mean distros
cannot exploit the new features.


> However, we are talking about distros here but the CPU cooling mechanism
> is for mobile and in this case the kernel (usually Android based) comes
> with a specific configuration file and this is where the SoC vendor has
> to choose the right strategy.

I agree its hard to conceive of a modern Android device that doesn't implement
both the needed features but the performance figures in the covering 
letter come from Hikey (and they look pretty good) and Hikey is 
supported by a good number of regular Linux distros now.

Using Kconfig to force what should be runtime decisions to happen at 
compile time means that Hikey becomes an example of a platform that
is unable to run at max performance on the distros that have added 
support for it.


Daniel.


> >> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
> >> ---
> >>  drivers/thermal/Kconfig | 20 +++++++++++++++++---
> >>  1 file changed, 17 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/thermal/Kconfig b/drivers/thermal/Kconfig
> >> index 315ae29..925e73b 100644
> >> --- a/drivers/thermal/Kconfig
> >> +++ b/drivers/thermal/Kconfig
> >> @@ -142,17 +142,31 @@ config THERMAL_GOV_POWER_ALLOCATOR
> >>  	  allocating and limiting power to devices.
> >>  
> >>  config CPU_THERMAL
> >> -	bool "generic cpu cooling support"
> >> -	depends on CPU_FREQ
> >> +	bool "Generic cpu cooling support"
> >>  	depends on THERMAL_OF
> >>  	help
> >> +	  Enable the CPU cooling features. If the system has no active
> >> +	  cooling device available, this option allows to use the CPU
> >> +	  as a cooling device.
> >> +
> >> +choice
> >> +	prompt "CPU cooling strategies"
> >> +	depends on CPU_THERMAL
> >> +	default CPU_FREQ_THERMAL
> >> +	help
> >> +	  Select the CPU cooling strategy.
> >> +
> >> +config CPU_FREQ_THERMAL
> >> +        bool "CPU frequency cooling strategy"
> >> +	depends on CPU_FREQ
> >> +	help
> >>  	  This implements the generic cpu cooling mechanism through frequency
> >>  	  reduction. An ACPI version of this already exists
> >>  	  (drivers/acpi/processor_thermal.c).
> >>  	  This will be useful for platforms using the generic thermal interface
> >>  	  and not the ACPI interface.
> >>  
> >> -	  If you want this support, you should say Y here.
> > 
> > ... this may not be great advice... if you think you want this support
> > then you *probably* actually want the comboappears to be terrible advice.
> > 
> > This cooling strategies should only be selected by petrolheads making a 
> > device specific *and* are obsessive about code size.
> > 
> > The 
> >> +endchoice
> >>  
> >>  config CLOCK_THERMAL
> >>  	bool "Generic clock cooling support"
> >> -- 
> >> 2.7.4
> >>
> 
> 
> -- 
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