Kernel PPS processing

[Frank Nicholas]

> On Jul 3, 2016, at 3:40 PM, Gary E. Miller <gem at rellim.com> wrote:
> 
> Yo Achim!
> 
> On Sun, 03 Jul 2016 12:10:44 +0200
> Achim Gratz <Stromeko at nexgo.de> wrote:
> 
>> Eric S. Raymond writes:
>>> Gary E. Miller <gem at rellim.com>:  
>>>> Anyone got a guess what the equivalent RasPi setting to turn off
>>>> power saving would be?  
>>> 
>>> turbo=1 in /boot/config.txt, I think. See also  
>> 
>> No.  That used to be "force_turbo=1", but is not needed anymore.
> 
> As of what kernel?  I notice RasPi's have all sorts of weird kernel
> versions and patchsets.  My Gentoo RasPi is at 4.4.14-v7,
> but wheezy is at 4.1.19-v7.  Worse my Odroid is at 3.10!
> 

I’m running Gentoo with Linux dev3-rpi 4.4.13-v7+ #894 SMP Mon Jun 13 13:13:27 BST 2016 armv7l ARMv7 Processor rev 4 (v7l) BCM2709 GNU/Linux

A recent pull from https://github.com/raspberrypi/userland.git

I believe for quite a while (1-2 years)“turbo=1” is only needed if yo want the clock speed to be above the “standard” max speed (varies by Pi version).  For the Pi 3, the max speed is 1.2Ghz.  For speeds above or below the standards, you must pass them in the config.txt.  If you only want to work with the standard speeds, nothing is needed in config.txt.  

For working with the speeds, it uses normal linux conventions, by manipulating things here:
/sys/devices/system/cpu/cpufreq/policy0

These have values that are self described:
cpuinfo_cur_freq
cpuinfo_max_freq
cpuinfo_min_freq

You can see what “governors” are supported in the current kernel by looking at:
scaling_available_governors

A full list of what frequencies are available (only a min & max with the Pi’s):
scaling_available_frequencies

For the different governors, I’ve only played with “ondemand powersave performance”.  You set the active governor by echoing it to “scaling_governor”.  “powersave” forces the clock to the “cpuinfo_min_freq”.  “performance” forces the clock to “cpuinfo_max_freq”.  “ondemand” allows the clock to vary between any frequencies in “scaling_available_frequencies” as needed.

Please be aware (as with most modern CPU’s) if the temperature exceeds the design limit (85C, I believe), the CPU will be throttled.  When using “performance” or “ondemand” and building under Gentoo, you can easily reach 85C.  Using the GPU will also increase the temperature.  

Temperatures can be measured with '/opt/vc/bin/vcgencmd measure_temp’.


>> You
>> simply define the maximum (and perhaps minimum) frequencies in
>> /boot/config.txt and chose the proper CPUfreq governor.
> 
> Do I need to do this, or is 'performance' enough?  What would you put in 
> config.txt to set the performance governor?
> 
>> Default is
>> ondemand, you will either want powersave (to keep at the low
>> frequency) or performance (force maximum frequency).  So far I
>> haven't seen a difference between those when it comes to NTP
>> performance, but I'm running two isolated DCF77 clocks only at the
>> moment.

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