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    SubjectRe: [PATCH] media: stm32-dcmi: Set minimum cpufreq requirement
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    On 03/06/20 08:50, Vincent Guittot wrote:
    > On Wed, 3 Jun 2020 at 09:34, Benjamin GAIGNARD <benjamin.gaignard@st.com> wrote:
    >> On 6/2/20 3:35 PM, Valentin Schneider wrote:
    >> > AFAIA streaming_start() is not necessarily executing on the same CPU as the
    >> > one that will handle the interrupt. I was thinking you could use the IRQ's
    >> > effective affinity as a hint of which CPU(s) to boost, i.e. something like:
    >> >
    >> > ---
    >> > struct cpumask_var_t visited;
    >> > struct irq_data *d = irq_get_irq_data(irq);
    >> >
    >> > err = alloc_cpumask_var(visited, GFP_KERNEL);
    >> > /* ... */
    >> > for_each_cpu(cpu, irq_data_get_effective_affinity_mask(d)) {
    >> > /* check if not already spanned */
    >> > if (cpumask_test_cpu(cpu, visited))
    >> > continue;
    >> >
    >> > policy = cpufreq_cpu_get(cpu);
    >> > cpumask_or(visited, visited, policy->cpus);
    >> > /* do the boost for that policy here */
    >> > /* ... */
    >> > cpufreq_cpu_put(policy);
    >> > }
    >> > ---
    >> >
    >> > That of course falls apart when hotplug gets involved, and the effective
    >> > affinity changes... There's irq_set_affinity_notifier() out there, but it
    >> > seems it's only about the affinity, not the effective_affinity, I'm not
    >> > sure how valid it would be to query the effective_affinity in that
    >> > notifier.

    >> If I wait to be in the irq it will be too late so I think I will do a
    >> loop over all possible CPUs
    >> before start the streaming to change the policies.
    >

    Yes, that's what I was thinking as well.

    > Can't you use irq_get_affinity_mask and loop over it ?
    >

    In the end that's the only usable option, I think.

    I was looking at alternatives because on arm64 (and AFAICT that applies
    to arm too; see irq-gic.c::gic_set_affinity()) the affinity mask spans
    all CPUs by default, while the effective affinity mask spans only the
    CPU that will actually handle the IRQ (+ where its thread should run).

    That said, using the effective mask that way does feel like an
    implementation leak. Sadly I couldn't find any better way to minimize
    the number of boosted frequency domains.

    > Also You should better use freq_qos_add/remove_request during probe
    > and remove of the driver and use freq_qos_update_request in
    > dcmi_start/stop_streaming to set/unset your constraint.
    >

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