Re: [PATCH V3 1/2] topology: Allow multiple entities to provide sched_freq_tick() callback

Hey,

On Thursday 18 Feb 2021 at 15:03:04 (+0530), Viresh Kumar wrote:

On 17-02-21, 00:24, Ionela Voinescu wrote:

quoted

quoted

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index 1e47dfd465f8..47fca7376c93 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c

@@ -240,7 +240,6 @@ static struct scale_freq_data amu_sfd = {
 
 static void amu_fie_setup(const struct cpumask *cpus)
 {
-	bool invariant;
 	int cpu;
 
 	/* We are already set since the last insmod of cpufreq driver */

@@ -257,25 +256,10 @@ static void amu_fie_setup(const struct cpumask *cpus)
 
 	cpumask_or(amu_fie_cpus, amu_fie_cpus, cpus);
 
-	invariant = topology_scale_freq_invariant();
-
-	/* We aren't fully invariant yet */
-	if (!invariant && !cpumask_equal(amu_fie_cpus, cpu_present_mask))
-		return;
-

You still need these checks, otherwise you could end up with only part
of the CPUs setting a scale factor, when only part of the CPUs support
AMUs and there is no cpufreq support for FIE.

Another look at it and here goes another reason (hope I don't have
another in-code comment somewhere else to kill this one) :)

We don't need to care for the reason you gave (which is a valid reason
otherwise), as we are talking specifically about amu_fie_setup() here
and it gets called from cpufreq policy-notifier. i.e. we won't support
AMUs without cpufreq being there in the first place and the same goes
for cppc-driver.

Does that sound reasonable ?

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. So only a few drivers today don't support cpufreq
based FI, but it's not a guarantee that it will stay this way.

Hope it makes sense,
Ionela.

-- 
viresh

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