Re: [PATCH] omap-pm: Fixes behaviour of some shared resource framework functions

[Mike Turquette <mturquette@ti.com>]

Kevin Hilman wrote:
> "Dasgupta, Romit" <romit@ti.com> writes:
> 
>> Hello Benoit,
>>                         One comment below:
>>> In fact, this is Mike who started that analysis. We discussed that internally and
>>> our point is that if the CPUFreq ondemand or conservative heuristic is not able
>>> to increase quickly enough the CPU need to handle correctly the UI, we have
>>> to somehow improve or modify the governor in order to provide it a extra
>>> information in term of constraint maybe in order to increase immediately the
>>> frequency.
>> The information as you mention needs to be supplied by the
>> driver. The governor would then act on behalf of the driver!  This
>> begs for a new governor API or a signature change to an existing
>> governor API.
>>
>>> This should not be done in the low level omap_pm code; this is not
>>> the right level to do that. The issue is in the ondemand and must
>>> be fixed there.
>> At the end of the day it would still be the driver making the
>> decision!
> 
> No.  The drivers can give hints about their requirements, but the
> drivers don't make decisions that are system wide.  The govenor acts
> on behalf of the entire system based on multiple inputs, not any one
> driver.
> 
> Benoit's point (and I agree with) is that this is a *system wide*
> problem that needs a *system wide* solution.  I agree that tweaked or
> new governor is the right approach to solving this for the long term,

An assumption in this thread is that ondemand/conservative can't scale 
fast enough, but that is not true.  The Android UI sluggishness 
mentioned by Benoit was solved by lowering the cpufreq 
transition_latency time from 10 million ns to 300,000ns.  I have not 
gotten the exact worst case time that it takes for voltage to scale up 
and down from the hardware guys, but through some email exchanges it was 
agreed that this value is safe.

I've pushed the patch that fixed transition_latency to the list.  Please 
see thread "decrease cpufreq transition latency".  This should hopefully 
cure a lot of performance/user experience pain and help us remove policy 
from drivers.

> In the mean time, I have a couple ideas for experimentation.
> 
> Ultimately, we're still talking about a power vs. perfomance tradeoff,
> which is a system wide choice that should be left to the system
> integrator (or maybe even end user.)  If performance is desired over
> power (like maybe when the UI is active), there are couple things that
> could be done
> 
> 1) Switch to performance governor, 
> 
> 2) or better, keep ondemand but use with CPUfreq policy changes
> 
> With CPUfreq policies, you can change which OPPs are available to the
> system.
> 
> To see the currently available OPPs and the min/max settings:
> 
> /sys/devices/system/cpu/cpu0/cpufreq # cat scaling_available_frequencies        
> 600000 550000 500000 250000 125000                                              
> /sys/devices/system/cpu/cpu0/cpufreq # cat scaling_max_freq                     
> 600000                                                                          
> /sys/devices/system/cpu/cpu0/cpufreq # cat scaling_min_freq                     
> 125000                                                                          
> 
> To make OPP3 the minimum OPP, all that's needed is:
> 
> /sys/devices/system/cpu/cpu0/cpufreq # echo 500000 > scaling_min_freq           
> 
> Changing the min freq is what you are trying to do from the MMC
> driver.  The difference here is that since this is a system wide
> policy decision, it should be done a system wide level.
> 
> Kevin
>