linux-kernel - Re: [PATCH] Documentation/cpu-freq: add intel-pstate.txt

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Date:	Fri, 03 Jan 2014 13:09:18 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Ramkumar Ramachandra <artagnon@...il.com>,
	Dirk Brandewie <dirk.brandewie@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] Documentation/cpu-freq: add intel-pstate.txt

On Friday, January 03, 2014 05:01:13 PM Ramkumar Ramachandra wrote:
> The Intel P-state driver is currently undocumented. Add some
> documentation based on the cover-letter sent with the original series.
> 
> Cc: Dirk Brandewie <dirk.brandewie@...il.com>

Dirk, what do you think?

> Cc: Rafael J. Wysocki <rjw@...ysocki.net>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Ramkumar Ramachandra <artagnon@...il.com>
> ---
>  I was looking for documentation on my cpufreq driver, intel_pstate,
>  and found nothing: so, here's a small start. Although the original
>  message said SandyBridge only, I noticed that
>  /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver says intel_pstate
>  on my IvyBridge; so I figured subsequent patches introduced support
>  for SandyBridge+.
> 
>  I noticed two "policies" in the code: CPUFREQ_POLICY_PERFORMANCE and
>  CPUFREQ_POLICY_POWERSAVE, but I have no idea how to switch from one
>  to the other. The userspace tool cpupower seems to be too tightly
>  tied to the acpi-cpufreq driver, and I'm not able to find any others.
> 
>  Also, how well is the driver performing? I seem to be getting
>  terrible battery life on my 2013 MBP.
> 
>  Thanks.
> 
>  Documentation/cpu-freq/intel-pstate.txt | 40 +++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
>  create mode 100644 Documentation/cpu-freq/intel-pstate.txt
> 
> diff --git a/Documentation/cpu-freq/intel-pstate.txt b/Documentation/cpu-freq/intel-pstate.txt
> new file mode 100644
> index 0000000..fda74ab
> --- /dev/null
> +++ b/Documentation/cpu-freq/intel-pstate.txt
> @@ -0,0 +1,40 @@
> +Intel P-state driver
> +--------------------
> +
> +This driver implements a scaling driver with an internal governor for
> +Intel Core processors.  The driver follows the same model as the
> +Transmeta scaling driver (longrun.c) and implements the setpolicy()
> +instead of target().  Scaling drivers that implement setpolicy() are
> +assmuned to implement internal governors by the cpufreq core. All the
> +logic for selecting the current P state is contained within the driver
> +no external governor is used by the cpufreq core.
> +
> +Only Intel SandyBridge+ processors are supported.

That's not the case any more.

> +
> +New sysfs files for controlling P state selection have been added to
> +/sys/devices/system/cpu/intel_pstate/
> +
> +      max_perf_pct: limits the maximum P state that will be requested by
> +      the driver stated as a percentage of the avail performance.
> +
> +      min_perf_pct: limits the minimum P state that will be  requested by
> +      the driver stated as a percentage of the avail performance.
> +
> +      no_turbo: limits the driver to selecting P states below the turbo
> +      frequency range.
> +

It would be good to document them in Documentation/ABI/testing/ as well.

> +The units for these for these files are purposely abstract and stated
> +in terms of available performance and not frequency.  In idea that

I would say "For contemporary Intel processor the frequency is controlled
by the processor itself and the P-states exposed to software are related to 
performance levels" instead.

> +frequency can be set to a single frequency is a fiction for Intel Core
> +processors. Even if the scaling driver selects a single P state the
> +actual frequency the processor will run at is selected by the
> +processor itself.
> +
> +New debugfs files have also been added to /sys/kernel/debug/pstate_snb/
> +
> +      deadband
> +      d_gain_pct
> +      i_gain_pct
> +      p_gain_pct
> +      sample_rate_ms
> +      setpoint
> 

Thanks!

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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