linux-kernel - Re: [PATCH v6 04/10] PM / EM: add support for other devices than CPUs in Energy Model

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Message-ID: <56053904-0314-340e-fdaa-4a8f649d7b6f@arm.com>
Date:   Fri, 10 Apr 2020 12:34:56 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Luis Gerhorst <linux-kernel@...sgerhorst.de>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, linux-omap@...r.kernel.org,
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Subject: Re: [PATCH v6 04/10] PM / EM: add support for other devices than CPUs
 in Energy Model



On 4/10/20 12:12 PM, Luis Gerhorst wrote:
> 
> Lukasz Luba writes:
> 
>> +/**
>> + * em_dev_unregister_perf_domain() - Unregister Energy Model (EM) for 
>> a device
>> + * @dev        : Device for which the EM is registered
>> + *
>> + * Try to unregister the EM for the specified device (it checks current
>> + * reference counter). The EM for CPUs will not be freed.
>> + */
>> +void em_dev_unregister_perf_domain(struct device *dev)
>> +{
>> +    struct em_device *em_dev, *tmp;
>> +
>> +    if (IS_ERR_OR_NULL(dev))
>> +        return;
>> +
>> +    /* We don't support freeing CPU structures in hotplug */
>> +    if (_is_cpu_device(dev)) {
>> +        dev_dbg_once(dev, "EM: the structures are not going to be 
>> removed\n");
>> +        return;
>> +    }
>> +
>> +    mutex_lock(&em_pd_mutex);
>> +
>> +    if (list_empty(&em_pd_dev_list)) {
>> +        mutex_unlock(&em_pd_mutex);
>> +        return;
>> +    }
>> +
>> +    list_for_each_entry_safe(em_dev, tmp, &em_pd_dev_list, 
>> em_dev_list) {
>> +        if (em_dev->dev == dev) {
>> +            kref_put(&em_dev->kref, _em_release);
>> +            break;
>> +        }
>> +    }
>> +
>> +    mutex_unlock(&em_pd_mutex);
>> +}
>> +EXPORT_SYMBOL_GPL(em_dev_unregister_perf_domain);
> 
> Ok, so em_dev_unregister_perf_domain() does not support the CPU device
> and a subsequent em_register_perf_domain() will fail with EEXIST.

Correct. At the current mainline Energy Model we don't even have
em_unregister_perf_domain function. I had to introduce it in order
to support other devices which might have drivers loaded/unloaded
as modules.


> 
> Is there a way to unregister/change the CPU's energy model during
> runtime without restarting the whole system?

Not for the CPU for now.

It is possible for other devices. When you have i.e. a module driver for
a device and in your code there is a (*active_power)(), then it is
possible to change EM by unloading/loading the module.

For the CPU we don't have this requirement for production code. I will
add you to CC list when something like this would pop-up for a
prototyping/experimentation code. We have been analyzing some options.

Regards,
Lukasz