170 lines
6.7 KiB
Plaintext
170 lines
6.7 KiB
Plaintext
|
====================
|
||
|
|
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 }
|
||
|
|
|
||
|
|
|
||
|
|
4. Support for legacy Energy Models (DEPRECATED)
|
||
|
|
------------------------------------------------
|
||
|
|
|
||
|
|
The Android kernel version 4.14 and before used a different type of EM for EAS,
|
||
|
|
referred to as the 'legacy' EM. The legacy EM relies on the out-of-tree
|
||
|
|
'sched-energy-costs' devicetree bindings to provide the kernel with power costs.
|
||
|
|
The usage of such bindings in Android has now been DEPRECATED in favour of the
|
||
|
|
mainline equivalents.
|
||
|
|
|
||
|
|
The currently supported alternatives to populate the EM include:
|
||
|
|
- using a firmware-based solution such as Arm SCMI (supported in
|
||
|
|
drivers/cpufreq/scmi-cpufreq.c);
|
||
|
|
- using the 'dynamic-power-coefficient' devicetree binding together with
|
||
|
|
PM_OPP. See the of_dev_pm_opp_get_cpu_power() helper in PM_OPP, and the
|
||
|
|
reference implementation in drivers/cpufreq/cpufreq-dt.c.
|
||
|
|
|
||
|
|
In order to ease the transition to the new EM format, Android 4.19 also provides
|
||
|
|
a compatibility driver able to load a legacy EM from DT into the EM framework.
|
||
|
|
*** Please note that THIS FEATURE WILL NOT BE AVAILABLE in future Android
|
||
|
|
kernels, and as such it must be considered only as a temporary workaround. ***
|
||
|
|
|
||
|
|
If you know what you're doing and still want to use this driver, you need to set
|
||
|
|
CONFIG_LEGACY_ENERGY_MODEL_DT=y in your kernel configuration to enable it.
|