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SubjectRe: [PATCH v2 1/4] PM / EM: add devices to Energy Model
On Thursday 06 Feb 2020 at 13:46:37 (+0000), lukasz.luba@arm.com wrote:
> 2. Core APIs
> @@ -70,14 +72,16 @@ CONFIG_ENERGY_MODEL must be enabled to use the EM framework.
> Drivers are expected to register performance domains into the EM framework by
> calling the following API::
>
> - int em_register_perf_domain(cpumask_t *span, unsigned int nr_states,
> - struct em_data_callback *cb);
> + int em_register_perf_domain(struct device *dev, unsigned int nr_states,
> + struct em_data_callback *cb, cpumask_t *cpus);
>
> -Drivers must specify the CPUs of the performance domains using the cpumask
> +Drivers must specify the device pointer of the performance domains as first

I find this sentence a little odd no?

> argument, and provide a callback function returning <frequency, power> tuples
> -for each capacity state. The callback function provided by the driver is free
> +for each performance state. The callback function provided by the driver is free
> to fetch data from any relevant location (DT, firmware, ...), and by any mean
> -deemed necessary. See Section 3. for an example of driver implementing this
> +deemed necessary. For other devices than CPUs the last argumant must be set to

s/argumant/argument

> +NULL. Only for CPUfreq drivers it is obligatory to specify the cpumask.

Please note that as of today nothing mandates the caller to be a CPUFreq
driver -- it could be anything in theory. I'd say 'only for CPU devices'
instead.

<snip>
> @@ -24,51 +27,65 @@ struct em_cap_state {
>
> /**
> * em_perf_domain - Performance domain
> - * @table: List of capacity states, in ascending order
> - * @nr_cap_states: Number of capacity states
> - * @cpus: Cpumask covering the CPUs of the domain
> + * @table: List of performance states, in ascending order
> + * @nr_perf_states: Number of performance states
> + * @priv: In case of EM for CPU device it is a Cpumask
> + * covering the CPUs of the domain

Could you turn @priv back into 'unsigned long priv[0];' and keep the
allocation as it is today ? That is, append the cpumask to the struct.

This empty pointer for non-CPU devices is just wasted space, and pointer
chasing isn't good for your caches. Given that you pre-allocate the pd
in em_create_pd() you could just have a special case for CPUs there I
suppose. And _is_cpu_em() will have to check the bus like you did in v1.

> *
> - * A "performance domain" represents a group of CPUs whose performance is
> - * scaled together. All CPUs of a performance domain must have the same
> - * micro-architecture. Performance domains often have a 1-to-1 mapping with
> - * CPUFreq policies.
> + * In case of CPU device, a "performance domain" represents a group of CPUs
> + * whose performance is scaled together. All CPUs of a performance domain
> + * must have the same micro-architecture. Performance domains often have
> + * a 1-to-1 mapping with CPUFreq policies.
> + * In case of other devices the 'priv' field is unused.
> */
> struct em_perf_domain {
> - struct em_cap_state *table;
> - int nr_cap_states;
> - unsigned long cpus[0];
> + struct em_perf_state *table;
> + int nr_perf_states;
> + void *priv;
> };

<snip>
> struct em_data_callback {
> /**
> - * active_power() - Provide power at the next capacity state of a CPU
> - * @power : Active power at the capacity state in mW (modified)
> - * @freq : Frequency at the capacity state in kHz (modified)
> - * @cpu : CPU for which we do this operation
> + * active_power() - Provide power at the next performance state of a
> + * device
> + * @power : Active power at the performance state in mW (modified)
> + * @freq : Frequency at the performance state in kHz (modified)
> + * @dev : Device for which we do this operation (can be a CPU)
> *
> - * active_power() must find the lowest capacity state of 'cpu' above
> + * active_power() must find the lowest performance state of 'dev' above
> * 'freq' and update 'power' and 'freq' to the matching active power
> * and frequency.
> *
> - * The power is the one of a single CPU in the domain, expressed in
> - * milli-watts. It is expected to fit in the [0, EM_CPU_MAX_POWER]
> - * range.
> + * In case of CPUs, the power is the one of a single CPU in the domain,
> + * expressed in milli-watts. It is expected to fit in the
> + * [0, EM_MAX_POWER] range.
> *
> * Return 0 on success.
> */
> - int (*active_power)(unsigned long *power, unsigned long *freq, int cpu);
> + int (*active_power)(unsigned long *power, unsigned long *freq,
> + struct device *dev);

Given that you've made explicit in the doc of struct em_perf_state that
'power' can be a 'total' value (static + dynamic), this could be renamed
I suppose.

<snip>
> /**
> * em_cpu_get() - Return the performance domain for a CPU
> * @cpu : CPU to find the performance domain for
> *
> - * Return: the performance domain to which 'cpu' belongs, or NULL if it doesn't
> + * Returns the performance domain to which 'cpu' belongs, or NULL if it doesn't
> * exist.
> */
> struct em_perf_domain *em_cpu_get(int cpu)
> {
> - return READ_ONCE(per_cpu(em_data, cpu));

Since CPU perf domains are guaranteed to never go away, it'd be safe to
keep that per-CPU variable and avoid the locking and list manipulation
below. No strong opinion, though.

> + struct em_device *em_dev;
> +
> + mutex_lock(&em_pd_mutex);
> +
> + if (list_empty(&em_pd_dev_list))
> + goto unlock;
> +
> + list_for_each_entry(em_dev, &em_pd_dev_list, em_dev_list) {
> + if (!_is_cpu_em(em_dev->em_pd))
> + continue;
> +
> + if (cpumask_test_cpu(cpu, em_span_cpus(em_dev->em_pd))) {
> + mutex_unlock(&em_pd_mutex);
> + return em_dev->em_pd;
> + }
> + }
> +
> +unlock:
> + mutex_unlock(&em_pd_mutex);
> + return NULL;
> }
> EXPORT_SYMBOL_GPL(em_cpu_get);

<snip>
> /**
> - * em_register_perf_domain() - Register the Energy Model of a performance domain
> - * @span : Mask of CPUs in the performance domain
> - * @nr_states : Number of capacity states to register
> + * em_register_perf_domain() - Register the Energy Model (EM) of a performance
> + * domain for the device
> + * @dev : Device for which the EM is to register
> + * @nr_states : Number of performance states to register
> * @cb : Callback functions providing the data of the Energy Model
> + * @cpus : Pointer to cpumask_t, which in case of a CPU device is
> + * obligatory. It can be taken from i.e. 'policy->cpus'. For other

It should be policy->related_cpus actually (or 'real_cpus' even) -- PM_EM
ignores hotplug ATM. Perhaps we should document that somewhere ...

> + * type of devices this should be set to NULL.
> *
> * Create Energy Model tables for a performance domain using the callbacks
> * defined in cb.
> @@ -196,63 +361,129 @@ EXPORT_SYMBOL_GPL(em_cpu_get);

Thanks,
Quentin

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