Re: [RFC v2 01/11] OPP: Don't overwrite rounded clk rate

[Viresh Kumar]

On 13-06-19, 15:24, Viresh Kumar wrote:
> I am confused as hell on what we should be doing and what we are doing
> right now. And if we should do better.
> 
> Let me explain with an example.
> 
> - The clock provider supports following frequencies: 500, 600, 700,
>   800, 900, 1000 MHz.
> 
> - The OPP table contains/supports only a subset: 500, 700, 1000 MHz.
> 
> Now, the request to change the frequency starts from cpufreq
> governors, like schedutil when they calls:
> 
> __cpufreq_driver_target(policy, 599 MHz, CPUFREQ_RELATION_L);
> 
> CPUFREQ_RELATION_L means: lowest frequency at or above target. And so
> I would expect the frequency to get set to 600MHz (if we look at clock
> driver) or 700MHz (if we look at OPP table). I think we should decide
> this thing from the OPP table only as that's what the platform guys
> want us to use. So, we should end up with 700 MHz.
> 
> Then we land into dev_pm_opp_set_rate(), which does this (which is
> code copied from earlier version of cpufreq-dt driver):
> 
> - clk_round_rate(clk, 599 MHz).
> 
>   clk_round_rate() returns the highest frequency lower than target. So
>   it must return 500 MHz (I haven't tested this yet, all theoretical).
> 
> - _find_freq_ceil(opp_table, 500 MHz).
> 
>   This works like CPUFREQ_RELATION_L, so we find lowest frequency >=
>   target freq. And so we should get: 500 MHz itself as OPP table has
>   it.
> 
> - clk_set_rate(clk, 500 MHz).
> 
>   This must be doing round-rate again, but I think we will settle with
>   500 MHz eventually.
> 
> 
> Now the questionnaire:
> 
> - Is this whole exercise correct ?

No, I missed the call to cpufreq_frequency_table_target() in
__cpufreq_driver_target() which finds the exact frequency from cpufreq table
(which was created using opp table) and so we never screw up here. Sorry for
confusing everyone on this :(

> Now lets move to this patch, which makes it more confusing.
> 
> The OPP tables for CPUs and GPUs should already be somewhat like fmax
> tables for particular voltage values and that's why both cpufreq and
> OPP core try to find a frequency higher than target so we choose the
> most optimum one power-efficiency wise.
> 
> For cases where the OPP table is only a subset of the clk-providers
> table (almost always), if we let the clock provider to find the
> nearest frequency (which is lower) we will run the CPU/GPU at a
> not-so-optimal frequency. i.e. if 500, 600, 700 MHz all need voltage
> to be 1.2 V, we should be running at 700 always, while we may end up
> running at 500 MHz.

This won't happen for CPUs because of the reason I explained earlier. cpufreq
core does the rounding before calling dev_pm_opp_set_rate(). And no other
devices use dev_pm_opp_set_rate() right now apart from CPUs, so we are not going
to break anything.

> This kind of behavior (introduced by this patch) is important for
> other devices which want to run at the nearest frequency to target
> one, but not for CPUs/GPUs. So, we need to tag these IO devices
> separately, maybe from DT ? So we select the closest match instead of
> most optimal one.

Hmm, so this patch won't break anything and I am inclined to apply it again :)

Does anyone see any other issues with it, which I might be missing ?

-- 
viresh