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

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Message-ID: <dca6e28e-8bde-be3e-bc3c-e97e349b3f04@quicinc.com>
Date: Wed, 17 Jan 2024 08:28:36 +0530
From: Sibi Sankar <quic_sibis@...cinc.com>
To: Lukasz Luba <lukasz.luba@....com>
CC: <linux-arm-kernel@...ts.infradead.org>, <viresh.kumar@...aro.org>,
        <rafael@...nel.org>, <cristian.marussi@....com>,
        <sudeep.holla@....com>, <linux-pm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <quic_mdtipton@...cinc.com>,
        <linux-arm-msm@...r.kernel.org>,
        "Morten
 Rasmussen" <morten.rasmussen@....com>,
        Dietmar Eggemann
	<dietmar.eggemann@....com>
Subject: Re: [PATCH 3/3] cpufreq: scmi: Register for limit change
 notifications



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@...cinc.com>
>> ---
>>   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