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Message-ID: <20210219094440.GA29843@arm.com> Date: Fri, 19 Feb 2021 09:44:40 +0000 From: Ionela Voinescu <ionela.voinescu@....com> To: Viresh Kumar <viresh.kumar@...aro.org> Cc: Rafael Wysocki <rjw@...ysocki.net>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, Vincent Guittot <vincent.guittot@...aro.org>, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org, Sudeep Holla <sudeep.holla@....com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org> Subject: Re: [PATCH V3 1/2] topology: Allow multiple entities to provide sched_freq_tick() callback On Friday 19 Feb 2021 at 10:28:23 (+0530), Viresh Kumar wrote: > On 18-02-21, 16:36, Ionela Voinescu wrote: > > Yes, we don't care if there is no cpufreq driver, as the use of AMUs won't > > get initialised either. But we do care if there is a cpufreq driver that > > does not support frequency invariance, which is the example above. > > > > The intention with the patches that made cpufreq based invariance generic > > a while back was for it to be present, seamlessly, for as many drivers as > > possible, as a less than accurate invariance default method is still > > better than nothing. > > Right. > > > So only a few drivers today don't support cpufreq based FI > > Only two AFAICT, both x86, and the AMU stuff doesn't conflict with > them. > > drivers/cpufreq/intel_pstate.c > drivers/cpufreq/longrun.c > > > but it's not a guarantee that it will stay this way. > > What do you mean by "no guarantee" here ? > > The very core routines (cpufreq_freq_transition_end() and > cpufreq_driver_fast_switch()) of the cpufreq core call > arch_set_freq_scale() today and this isn't going to change anytime > soon. If something gets changed there someone will need to see other > parts of the kernel which may get broken with that. > Yes, but it won't really be straightforward to notice this breakage if that happens, so in my opinion it was worth to keep that condition. > I don't see any need of complicating other parts of the kernel like, > amu or cppc code for that. They should be kept simple and they should > assume cpufreq invariance will be supported as it is today. > Fair enough! It is a corner case after all. Thanks, Ionela. > -- > viresh
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