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Subject[PATCH 1/2] PM / EM: Document the Energy Model framework
Date
Introduce a documentation file summarizing the key design points and
APIs of the newly introduced Energy Model framework.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>
---
Documentation/power/energy-model.txt | 144 +++++++++++++++++++++++++++
1 file changed, 144 insertions(+)
create mode 100644 Documentation/power/energy-model.txt

diff --git a/Documentation/power/energy-model.txt b/Documentation/power/energy-model.txt
new file mode 100644
index 000000000000..a2b0ae4c76bd
--- /dev/null
+++ b/Documentation/power/energy-model.txt
@@ -0,0 +1,144 @@
+ ====================
+ Energy Model of CPUs
+ ====================
+
+1. Overview
+-----------
+
+The Energy Model (EM) framework serves as an interface between drivers knowing
+the power consumed by CPUs at various performance levels, and the kernel
+subsystems willing to use that information to make energy-aware decisions.
+
+The source of the information about the power consumed by CPUs can vary greatly
+from one platform to another. These power costs can be estimated using
+devicetree data in some cases. In others, the firmware will know better.
+Alternatively, userspace might be best positioned. And so on. In order to avoid
+each and every client subsystem to re-implement support for each and every
+possible source of information on its own, the EM framework intervenes as an
+abstraction layer which standardizes the format of power cost tables in the
+kernel, hence enabling to avoid redundant work.
+
+The figure below depicts an example of drivers (Arm-specific here, but the
+approach is applicable to any architecture) providing power costs to the EM
+framework, and interested clients reading the data from it.
+
+ +---------------+ +-----------------+ +---------------+
+ | Thermal (IPA) | | Scheduler (EAS) | | Other |
+ +---------------+ +-----------------+ +---------------+
+ | | em_pd_energy() |
+ | | em_cpu_get() |
+ +---------+ | +---------+
+ | | |
+ v v v
+ +---------------------+
+ | Energy Model |
+ | Framework |
+ +---------------------+
+ ^ ^ ^
+ | | | em_register_perf_domain()
+ +----------+ | +---------+
+ | | |
+ +---------------+ +---------------+ +--------------+
+ | cpufreq-dt | | arm_scmi | | Other |
+ +---------------+ +---------------+ +--------------+
+ ^ ^ ^
+ | | |
+ +--------------+ +---------------+ +--------------+
+ | Device Tree | | Firmware | | ? |
+ +--------------+ +---------------+ +--------------+
+
+The EM framework manages power cost tables per 'performance domain' in the
+system. A performance domain is a group of CPUs whose performance is scaled
+together. Performance domains generally have a 1-to-1 mapping with CPUFreq
+policies. All CPUs in a performance domain are required to have the same
+micro-architecture. CPUs in different performance domains can have different
+micro-architectures.
+
+
+2. Core APIs
+------------
+
+ 2.1 Config options
+
+CONFIG_ENERGY_MODEL must be enabled to use the EM framework.
+
+
+ 2.2 Registration of performance domains
+
+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);
+
+Drivers must specify the CPUs of the performance domains using the cpumask
+argument, and provide a callback function returning <frequency, power> tuples
+for each capacity 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
+callback, and kernel/power/energy_model.c for further documentation on this
+API.
+
+
+ 2.3 Accessing performance domains
+
+Subsystems interested in the energy model of a CPU can retrieve it using the
+em_cpu_get() API. The energy model tables are allocated once upon creation of
+the performance domains, and kept in memory untouched.
+
+The energy consumed by a performance domain can be estimated using the
+em_pd_energy() API. The estimation is performed assuming that the schedutil
+CPUfreq governor is in use.
+
+More details about the above APIs can be found in include/linux/energy_model.h.
+
+
+3. Example driver
+-----------------
+
+This section provides a simple example of a CPUFreq driver registering a
+performance domain in the Energy Model framework using the (fake) 'foo'
+protocol. The driver implements an est_power() function to be provided to the
+EM framework.
+
+ -> drivers/cpufreq/foo_cpufreq.c
+
+01 static int est_power(unsigned long *mW, unsigned long *KHz, int cpu)
+02 {
+03 long freq, power;
+04
+05 /* Use the 'foo' protocol to ceil the frequency */
+06 freq = foo_get_freq_ceil(cpu, *KHz);
+07 if (freq < 0);
+08 return freq;
+09
+10 /* Estimate the power cost for the CPU at the relevant freq. */
+11 power = foo_estimate_power(cpu, freq);
+12 if (power < 0);
+13 return power;
+14
+15 /* Return the values to the EM framework */
+16 *mW = power;
+17 *KHz = freq;
+18
+19 return 0;
+20 }
+21
+22 static int foo_cpufreq_init(struct cpufreq_policy *policy)
+23 {
+24 struct em_data_callback em_cb = EM_DATA_CB(est_power);
+25 int nr_opp, ret;
+26
+27 /* Do the actual CPUFreq init work ... */
+28 ret = do_foo_cpufreq_init(policy);
+29 if (ret)
+30 return ret;
+31
+32 /* Find the number of OPPs for this policy */
+33 nr_opp = foo_get_nr_opp(policy);
+34
+35 /* And register the new performance domain */
+36 em_register_perf_domain(policy->cpus, nr_opp, &em_cb);
+37
+38 return 0;
+39 }
--
2.20.1
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