Re: [PATCH v2 3/7] arch_topology: disable frequency invariance for CONFIG_BL_SWITCHER

From: Viresh Kumar <viresh.kumar@linaro.org>
To: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Ionela Voinescu <ionela.voinescu@arm.com>,
	rjw@rjwysocki.net, catalin.marinas@arm.com, sudeep.holla@arm.com,
	will@kernel.org, linux@armlinux.org.uk, mingo@redhat.com,
	peterz@infradead.org, linux-pm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/7] arch_topology: disable frequency invariance for CONFIG_BL_SWITCHER
Date: Tue, 4 Aug 2020 12:00:46 +0530	[thread overview]
Message-ID: <20200804063046.a2hw5cxwiewhb3aw@vireshk-mac-ubuntu> (raw)
In-Reply-To: <1db4317a-0018-1590-f0ae-ed5e235b174f@arm.com>

On 30-07-20, 12:29, Dietmar Eggemann wrote:
> On 30/07/2020 06:24, Viresh Kumar wrote:
> > On 22-07-20, 10:37, Ionela Voinescu wrote:
> >> +++ b/drivers/base/arch_topology.c
> >> @@ -27,6 +27,7 @@ __weak bool arch_freq_counters_available(struct cpumask *cpus)
> >>  }
> >>  DEFINE_PER_CPU(unsigned long, freq_scale) = SCHED_CAPACITY_SCALE;
> >>  
> >> +#ifndef CONFIG_BL_SWITCHER
> >>  void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
> >>  			 unsigned long max_freq)
> >>  {
> >> @@ -46,6 +47,7 @@ void arch_set_freq_scale(struct cpumask *cpus, unsigned long cur_freq,
> >>  	for_each_cpu(i, cpus)
> >>  		per_cpu(freq_scale, i) = scale;
> >>  }
> >> +#endif
> > 
> > I don't really like this change, the ifdef hackery is disgusting and
> > then we are putting that in a completely different part of the kernel.
> > 
> > There are at least these two ways of solving this, maybe more:
> > 
> > - Fix the bl switcher driver and add the complexity in it (which you
> >   tried to do earlier).
> > 
> > - Add a cpufreq flag to skip arch-set-freq-scale call.
> 
> I agree it's not nice but IMHO the cpufreq flag is worse since we would
> introduce new infrastructure only for a deprecated feature. I'm assuming
> that BL SWITCHER is the only feature needing this CPUfreq flag extension.
> 
> #ifdef CONFIG_BL_SWITCHER is already in drivers/irqchip/irq-gic.c so
> it's ugly already.
> 
> Runtime detecting (via bL_switching_enabled) of BL SWITCHER is right now
> also only handled inside vexpress-spc-cpufreq.c via a
> bL_switcher_notifier. A mechanism which also sits behind a #ifdef
> CONFIG_BL_SWITCHER.

Vexpress one is a driver and so ugliness could be ignored here :)

So here is option number 3 (in continuation of the earlier two
options):
- Don't do anything for bL switcher, just add a TODO/NOTE in the
  driver that FIE is broken for switcher. And I don't think anyone
  will care about FIE for the switcher anyway :)

-- 
viresh