Re: [PATCH 3/3] cpufreq: scmi: Register for limit change notifications

[Sibi Sankar]

On 1/10/24 13:56, Lukasz Luba wrote:
> Hi Sibi,

Hey Lukasz,
Thanks for taking time to review the series!

> + Morten and Dietmar on CC
>
> On 1/8/24 14:01, Sibi Sankar wrote:
> > Register for limit change notifications if supported with the help of
> > perf_notify_support interface and determine the throttled frequency
> > using the perf_opp_xlate to apply HW pressure.
> >
> > Signed-off-by: Sibi Sankar <quic_sibis@xxxxxxxxxxx>
> > ---
> >   drivers/cpufreq/scmi-cpufreq.c | 42 +++++++++++++++++++++++++++++++++-
> >   1 file changed, 41 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpufreq/scmi-cpufreq.c 
> > b/drivers/cpufreq/scmi-cpufreq.c
> > index 4ee23f4ebf4a..53bc8868455d 100644
> > --- a/drivers/cpufreq/scmi-cpufreq.c
> > +++ b/drivers/cpufreq/scmi-cpufreq.c
> > @@ -25,9 +25,13 @@ struct scmi_data {
> >       int domain_id;
> >       int nr_opp;
> >       struct device *cpu_dev;
> > +    struct cpufreq_policy *policy;
> >       cpumask_var_t opp_shared_cpus;
> > +    struct notifier_block limit_notify_nb;
> >   };
> > +const struct scmi_handle *handle;
> > +static struct scmi_device *scmi_dev;
> >   static struct scmi_protocol_handle *ph;
> >   static const struct scmi_perf_proto_ops *perf_ops;
> > @@ -144,6 +148,22 @@ scmi_get_cpu_power(struct device *cpu_dev, 
> > unsigned long *power,
> >       return 0;
> >   }
> > +static int scmi_limit_notify_cb(struct notifier_block *nb, unsigned 
> > long event, void *data)
> > +{
> > +    unsigned long freq_hz;
> > +    struct scmi_perf_limits_report *limit_notify = data;
> > +    struct scmi_data *priv = container_of(nb, struct scmi_data, 
> > limit_notify_nb);
> > +    struct cpufreq_policy *policy = priv->policy;
> > +
> > +    if (perf_ops->perf_opp_xlate(ph, priv->domain_id, 
> > limit_notify->range_max, &freq_hz))
> > +        return NOTIFY_OK;
> > +
> > +    /* Update HW pressure (the boost frequencies are accepted) */
> > +    arch_update_hw_pressure(policy->related_cpus, (freq_hz / 
> > HZ_PER_KHZ));
>
> This is wrong. The whole idea of the new HW pressure was that I wanted
> to get rid of the 'signal smoothing' mechanism in order to get
> instantaneous value from FW to task scheduler. Vincent created
> 2 interfaces in that new HW pressure:
> 1. cpufreq_update_pressure(policy) - raw variable
> 2. arch_update_hw_pressure(policy->related_cpus, (freq_hz / HZ_PER_KHZ))
>     - smoothing PELT mechanism, good for raw IRQ in drivers
>
> In our SCMI cpufreq driver we need the 1st one:
> cpufreq_update_pressure(policy)
>
> The FW will do the 'signal smoothing or filtering' and won't
> flood the kernel with hundreds of notifications.

Ack, even though I see no mention of filtering being mandated in the 
SCMI specification, the scmi notification by itself will serve as a
rate limiter I guess.

> So, please change that bit and add me, Morten and Dietmar on CC.
> I would like to review it.

ack

-Sibi

> Regards,
> Lukasz