Kevin Hilman wrote:
"Dasgupta, Romit" <romit@xxxxxx> 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
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