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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Benjamin Gaignard <benjamin.gaignard@st.com>
Cc:     hugues.fruchet@st.com, mchehab@kernel.org,
        mcoquelin.stm32@gmail.com, alexandre.torgue@st.com,
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        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        vincent.guittot@linaro.org, rjw@rjwysocki.net
Subject: Re: [PATCH v2 2/3] media: stm32-dcmi: Set minimum cpufreq requirement
In-reply-to: <20200603124559.22652-3-benjamin.gaignard@st.com>
Date:   Wed, 03 Jun 2020 16:25:01 +0100
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On 03/06/20 13:45, Benjamin Gaignard wrote:
> +static void dcmi_set_min_frequency(struct stm32_dcmi *dcmi, u64 freq)
> +{
> +	struct cpufreq_policy *p;
> +	int cpu;
> +
> +	for_each_cpu(cpu, irq_get_affinity_mask(dcmi->irq)) {
> +		p = per_cpu(policy, cpu);
> +		if (!p)
> +			continue;
> +
> +		freq_qos_update_request(&per_cpu(qos_req, cpu), freq);
> +	}
> +}
> +

You may want to use a "visited" cpumask as I suggested in the previous
thread, since a policy can cover more than one CPU (IOW, a frequency domain
can span more than one CPU). It's not required per-se, AFAICT, but it makes
things a bit neater.

I also think you'll have to use the affinity notifier
(irq_set_affinity_notifier()), since AFAICT userspace can change the
affinity of that IRQ. I suppose you'll want something like:
- Check if we currently are in streaming mode
- Clear the QoS request for CPUs that were previously boosted but that
  aren't in the new mask
- Add the request for the new CPUs.

You'll probably need serialize the reading of the mask in the regular
dcmi_set_min_frequency() as well. I concur all of that is somewhat
annoying, but AFAICT that's required for a sturdy implementation.

>  static int dcmi_start_streaming(struct vb2_queue *vq, unsigned int count)
>  {
>       struct stm32_dcmi *dcmi = vb2_get_drv_priv(vq);