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1.. SPDX-License-Identifier: GPL-2.0
2
3=======================
4Energy Model of devices
5=======================
6
71. Overview
8-----------
9
10The Energy Model (EM) framework serves as an interface between drivers knowing
11the power consumed by devices at various performance levels, and the kernel
12subsystems willing to use that information to make energy-aware decisions.
13
14The source of the information about the power consumed by devices can vary greatly
15from one platform to another. These power costs can be estimated using
16devicetree data in some cases. In others, the firmware will know better.
17Alternatively, userspace might be best positioned. And so on. In order to avoid
18each and every client subsystem to re-implement support for each and every
19possible source of information on its own, the EM framework intervenes as an
20abstraction layer which standardizes the format of power cost tables in the
21kernel, hence enabling to avoid redundant work.
22
23The power values might be expressed in micro-Watts or in an 'abstract scale'.
24Multiple subsystems might use the EM and it is up to the system integrator to
25check that the requirements for the power value scale types are met. An example
26can be found in the Energy-Aware Scheduler documentation
27Documentation/scheduler/sched-energy.rst. For some subsystems like thermal or
28powercap power values expressed in an 'abstract scale' might cause issues.
29These subsystems are more interested in estimation of power used in the past,
30thus the real micro-Watts might be needed. An example of these requirements can
31be found in the Intelligent Power Allocation in
32Documentation/driver-api/thermal/power_allocator.rst.
33Kernel subsystems might implement automatic detection to check whether EM
34registered devices have inconsistent scale (based on EM internal flag).
35Important thing to keep in mind is that when the power values are expressed in
36an 'abstract scale' deriving real energy in micro-Joules would not be possible.
37
38The figure below depicts an example of drivers (Arm-specific here, but the
39approach is applicable to any architecture) providing power costs to the EM
40framework, and interested clients reading the data from it::
41
42 +---------------+ +-----------------+ +---------------+
43 | Thermal (IPA) | | Scheduler (EAS) | | Other |
44 +---------------+ +-----------------+ +---------------+
45 | | em_cpu_energy() |
46 | | em_cpu_get() |
47 +---------+ | +---------+
48 | | |
49 v v v
50 +---------------------+
51 | Energy Model |
52 | Framework |
53 +---------------------+
54 ^ ^ ^
55 | | | em_dev_register_perf_domain()
56 +----------+ | +---------+
57 | | |
58 +---------------+ +---------------+ +--------------+
59 | cpufreq-dt | | arm_scmi | | Other |
60 +---------------+ +---------------+ +--------------+
61 ^ ^ ^
62 | | |
63 +--------------+ +---------------+ +--------------+
64 | Device Tree | | Firmware | | ? |
65 +--------------+ +---------------+ +--------------+
66
67In case of CPU devices the EM framework manages power cost tables per
68'performance domain' in the system. A performance domain is a group of CPUs
69whose performance is scaled together. Performance domains generally have a
701-to-1 mapping with CPUFreq policies. All CPUs in a performance domain are
71required to have the same micro-architecture. CPUs in different performance
72domains can have different micro-architectures.
73
74To better reflect power variation due to static power (leakage) the EM
75supports runtime modifications of the power values. The mechanism relies on
76RCU to free the modifiable EM perf_state table memory. Its user, the task
77scheduler, also uses RCU to access this memory. The EM framework provides
78API for allocating/freeing the new memory for the modifiable EM table.
79The old memory is freed automatically using RCU callback mechanism when there
80are no owners anymore for the given EM runtime table instance. This is tracked
81using kref mechanism. The device driver which provided the new EM at runtime,
82should call EM API to free it safely when it's no longer needed. The EM
83framework will handle the clean-up when it's possible.
84
85The kernel code which want to modify the EM values is protected from concurrent
86access using a mutex. Therefore, the device driver code must run in sleeping
87context when it tries to modify the EM.
88
89With the runtime modifiable EM we switch from a 'single and during the entire
90runtime static EM' (system property) design to a 'single EM which can be
91changed during runtime according e.g. to the workload' (system and workload
92property) design.
93
94It is possible also to modify the CPU performance values for each EM's
95performance state. Thus, the full power and performance profile (which
96is an exponential curve) can be changed according e.g. to the workload
97or system property.
98
99
1002. Core APIs
101------------
102
1032.1 Config options
104^^^^^^^^^^^^^^^^^^
105
106CONFIG_ENERGY_MODEL must be enabled to use the EM framework.
107
108
1092.2 Registration of performance domains
110^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
111
112Registration of 'advanced' EM
113~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
114
115The 'advanced' EM gets its name due to the fact that the driver is allowed
116to provide more precised power model. It's not limited to some implemented math
117formula in the framework (like it is in 'simple' EM case). It can better reflect
118the real power measurements performed for each performance state. Thus, this
119registration method should be preferred in case considering EM static power
120(leakage) is important.
121
122Drivers are expected to register performance domains into the EM framework by
123calling the following API::
124
125 int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
126 struct em_data_callback *cb, cpumask_t *cpus, bool microwatts);
127
128Drivers must provide a callback function returning <frequency, power> tuples
129for each performance state. The callback function provided by the driver is free
130to fetch data from any relevant location (DT, firmware, ...), and by any mean
131deemed necessary. Only for CPU devices, drivers must specify the CPUs of the
132performance domains using cpumask. For other devices than CPUs the last
133argument must be set to NULL.
134The last argument 'microwatts' is important to set with correct value. Kernel
135subsystems which use EM might rely on this flag to check if all EM devices use
136the same scale. If there are different scales, these subsystems might decide
137to return warning/error, stop working or panic.
138See Section 3. for an example of driver implementing this
139callback, or Section 2.4 for further documentation on this API
140
141Registration of EM using DT
142~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
143
144The EM can also be registered using OPP framework and information in DT
145"operating-points-v2". Each OPP entry in DT can be extended with a property
146"opp-microwatt" containing micro-Watts power value. This OPP DT property
147allows a platform to register EM power values which are reflecting total power
148(static + dynamic). These power values might be coming directly from
149experiments and measurements.
150
151Registration of 'artificial' EM
152~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
153
154There is an option to provide a custom callback for drivers missing detailed
155knowledge about power value for each performance state. The callback
156.get_cost() is optional and provides the 'cost' values used by the EAS.
157This is useful for platforms that only provide information on relative
158efficiency between CPU types, where one could use the information to
159create an abstract power model. But even an abstract power model can
160sometimes be hard to fit in, given the input power value size restrictions.
161The .get_cost() allows to provide the 'cost' values which reflect the
162efficiency of the CPUs. This would allow to provide EAS information which
163has different relation than what would be forced by the EM internal
164formulas calculating 'cost' values. To register an EM for such platform, the
165driver must set the flag 'microwatts' to 0, provide .get_power() callback
166and provide .get_cost() callback. The EM framework would handle such platform
167properly during registration. A flag EM_PERF_DOMAIN_ARTIFICIAL is set for such
168platform. Special care should be taken by other frameworks which are using EM
169to test and treat this flag properly.
170
171Registration of 'simple' EM
172~~~~~~~~~~~~~~~~~~~~~~~~~~~
173
174The 'simple' EM is registered using the framework helper function
175cpufreq_register_em_with_opp(). It implements a power model which is tight to
176math formula::
177
178 Power = C * V^2 * f
179
180The EM which is registered using this method might not reflect correctly the
181physics of a real device, e.g. when static power (leakage) is important.
182
183
1842.3 Accessing performance domains
185^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
186
187There are two API functions which provide the access to the energy model:
188em_cpu_get() which takes CPU id as an argument and em_pd_get() with device
189pointer as an argument. It depends on the subsystem which interface it is
190going to use, but in case of CPU devices both functions return the same
191performance domain.
192
193Subsystems interested in the energy model of a CPU can retrieve it using the
194em_cpu_get() API. The energy model tables are allocated once upon creation of
195the performance domains, and kept in memory untouched.
196
197The energy consumed by a performance domain can be estimated using the
198em_cpu_energy() API. The estimation is performed assuming that the schedutil
199CPUfreq governor is in use in case of CPU device. Currently this calculation is
200not provided for other type of devices.
201
202More details about the above APIs can be found in ``<linux/energy_model.h>``
203or in Section 2.5
204
205
2062.4 Runtime modifications
207^^^^^^^^^^^^^^^^^^^^^^^^^
208
209Drivers willing to update the EM at runtime should use the following dedicated
210function to allocate a new instance of the modified EM. The API is listed
211below::
212
213 struct em_perf_table __rcu *em_table_alloc(struct em_perf_domain *pd);
214
215This allows to allocate a structure which contains the new EM table with
216also RCU and kref needed by the EM framework. The 'struct em_perf_table'
217contains array 'struct em_perf_state state[]' which is a list of performance
218states in ascending order. That list must be populated by the device driver
219which wants to update the EM. The list of frequencies can be taken from
220existing EM (created during boot). The content in the 'struct em_perf_state'
221must be populated by the driver as well.
222
223This is the API which does the EM update, using RCU pointers swap::
224
225 int em_dev_update_perf_domain(struct device *dev,
226 struct em_perf_table __rcu *new_table);
227
228Drivers must provide a pointer to the allocated and initialized new EM
229'struct em_perf_table'. That new EM will be safely used inside the EM framework
230and will be visible to other sub-systems in the kernel (thermal, powercap).
231The main design goal for this API is to be fast and avoid extra calculations
232or memory allocations at runtime. When pre-computed EMs are available in the
233device driver, than it should be possible to simply re-use them with low
234performance overhead.
235
236In order to free the EM, provided earlier by the driver (e.g. when the module
237is unloaded), there is a need to call the API::
238
239 void em_table_free(struct em_perf_table __rcu *table);
240
241It will allow the EM framework to safely remove the memory, when there is
242no other sub-system using it, e.g. EAS.
243
244To use the power values in other sub-systems (like thermal, powercap) there is
245a need to call API which protects the reader and provide consistency of the EM
246table data::
247
248 struct em_perf_state *em_perf_state_from_pd(struct em_perf_domain *pd);
249
250It returns the 'struct em_perf_state' pointer which is an array of performance
251states in ascending order.
252This function must be called in the RCU read lock section (after the
253rcu_read_lock()). When the EM table is not needed anymore there is a need to
254call rcu_real_unlock(). In this way the EM safely uses the RCU read section
255and protects the users. It also allows the EM framework to manage the memory
256and free it. More details how to use it can be found in Section 3.2 in the
257example driver.
258
259There is dedicated API for device drivers to calculate em_perf_state::cost
260values::
261
262 int em_dev_compute_costs(struct device *dev, struct em_perf_state *table,
263 int nr_states);
264
265These 'cost' values from EM are used in EAS. The new EM table should be passed
266together with the number of entries and device pointer. When the computation
267of the cost values is done properly the return value from the function is 0.
268The function takes care for right setting of inefficiency for each performance
269state as well. It updates em_perf_state::flags accordingly.
270Then such prepared new EM can be passed to the em_dev_update_perf_domain()
271function, which will allow to use it.
272
273More details about the above APIs can be found in ``<linux/energy_model.h>``
274or in Section 3.2 with an example code showing simple implementation of the
275updating mechanism in a device driver.
276
277
2782.5 Description details of this API
279^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
280.. kernel-doc:: include/linux/energy_model.h
281 :internal:
282
283.. kernel-doc:: kernel/power/energy_model.c
284 :export:
285
286
2873. Examples
288-----------
289
2903.1 Example driver with EM registration
291^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
292
293The CPUFreq framework supports dedicated callback for registering
294the EM for a given CPU(s) 'policy' object: cpufreq_driver::register_em().
295That callback has to be implemented properly for a given driver,
296because the framework would call it at the right time during setup.
297This section provides a simple example of a CPUFreq driver registering a
298performance domain in the Energy Model framework using the (fake) 'foo'
299protocol. The driver implements an est_power() function to be provided to the
300EM framework::
301
302 -> drivers/cpufreq/foo_cpufreq.c
303
304 01 static int est_power(struct device *dev, unsigned long *mW,
305 02 unsigned long *KHz)
306 03 {
307 04 long freq, power;
308 05
309 06 /* Use the 'foo' protocol to ceil the frequency */
310 07 freq = foo_get_freq_ceil(dev, *KHz);
311 08 if (freq < 0);
312 09 return freq;
313 10
314 11 /* Estimate the power cost for the dev at the relevant freq. */
315 12 power = foo_estimate_power(dev, freq);
316 13 if (power < 0);
317 14 return power;
318 15
319 16 /* Return the values to the EM framework */
320 17 *mW = power;
321 18 *KHz = freq;
322 19
323 20 return 0;
324 21 }
325 22
326 23 static void foo_cpufreq_register_em(struct cpufreq_policy *policy)
327 24 {
328 25 struct em_data_callback em_cb = EM_DATA_CB(est_power);
329 26 struct device *cpu_dev;
330 27 int nr_opp;
331 28
332 29 cpu_dev = get_cpu_device(cpumask_first(policy->cpus));
333 30
334 31 /* Find the number of OPPs for this policy */
335 32 nr_opp = foo_get_nr_opp(policy);
336 33
337 34 /* And register the new performance domain */
338 35 em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus,
339 36 true);
340 37 }
341 38
342 39 static struct cpufreq_driver foo_cpufreq_driver = {
343 40 .register_em = foo_cpufreq_register_em,
344 41 };
345
346
3473.2 Example driver with EM modification
348^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
349
350This section provides a simple example of a thermal driver modifying the EM.
351The driver implements a foo_thermal_em_update() function. The driver is woken
352up periodically to check the temperature and modify the EM data::
353
354 -> drivers/soc/example/example_em_mod.c
355
356 01 static void foo_get_new_em(struct foo_context *ctx)
357 02 {
358 03 struct em_perf_table __rcu *em_table;
359 04 struct em_perf_state *table, *new_table;
360 05 struct device *dev = ctx->dev;
361 06 struct em_perf_domain *pd;
362 07 unsigned long freq;
363 08 int i, ret;
364 09
365 10 pd = em_pd_get(dev);
366 11 if (!pd)
367 12 return;
368 13
369 14 em_table = em_table_alloc(pd);
370 15 if (!em_table)
371 16 return;
372 17
373 18 new_table = em_table->state;
374 19
375 20 rcu_read_lock();
376 21 table = em_perf_state_from_pd(pd);
377 22 for (i = 0; i < pd->nr_perf_states; i++) {
378 23 freq = table[i].frequency;
379 24 foo_get_power_perf_values(dev, freq, &new_table[i]);
380 25 }
381 26 rcu_read_unlock();
382 27
383 28 /* Calculate 'cost' values for EAS */
384 29 ret = em_dev_compute_costs(dev, table, pd->nr_perf_states);
385 30 if (ret) {
386 31 dev_warn(dev, "EM: compute costs failed %d\n", ret);
387 32 em_free_table(em_table);
388 33 return;
389 34 }
390 35
391 36 ret = em_dev_update_perf_domain(dev, em_table);
392 37 if (ret) {
393 38 dev_warn(dev, "EM: update failed %d\n", ret);
394 39 em_free_table(em_table);
395 40 return;
396 41 }
397 42
398 43 /*
399 44 * Since it's one-time-update drop the usage counter.
400 45 * The EM framework will later free the table when needed.
401 46 */
402 47 em_table_free(em_table);
403 48 }
404 49
405 50 /*
406 51 * Function called periodically to check the temperature and
407 52 * update the EM if needed
408 53 */
409 54 static void foo_thermal_em_update(struct foo_context *ctx)
410 55 {
411 56 struct device *dev = ctx->dev;
412 57 int cpu;
413 58
414 59 ctx->temperature = foo_get_temp(dev, ctx);
415 60 if (ctx->temperature < FOO_EM_UPDATE_TEMP_THRESHOLD)
416 61 return;
417 62
418 63 foo_get_new_em(ctx);
419 64 }
1.. SPDX-License-Identifier: GPL-2.0
2
3=======================
4Energy Model of devices
5=======================
6
71. Overview
8-----------
9
10The Energy Model (EM) framework serves as an interface between drivers knowing
11the power consumed by devices at various performance levels, and the kernel
12subsystems willing to use that information to make energy-aware decisions.
13
14The source of the information about the power consumed by devices can vary greatly
15from one platform to another. These power costs can be estimated using
16devicetree data in some cases. In others, the firmware will know better.
17Alternatively, userspace might be best positioned. And so on. In order to avoid
18each and every client subsystem to re-implement support for each and every
19possible source of information on its own, the EM framework intervenes as an
20abstraction layer which standardizes the format of power cost tables in the
21kernel, hence enabling to avoid redundant work.
22
23The power values might be expressed in micro-Watts or in an 'abstract scale'.
24Multiple subsystems might use the EM and it is up to the system integrator to
25check that the requirements for the power value scale types are met. An example
26can be found in the Energy-Aware Scheduler documentation
27Documentation/scheduler/sched-energy.rst. For some subsystems like thermal or
28powercap power values expressed in an 'abstract scale' might cause issues.
29These subsystems are more interested in estimation of power used in the past,
30thus the real micro-Watts might be needed. An example of these requirements can
31be found in the Intelligent Power Allocation in
32Documentation/driver-api/thermal/power_allocator.rst.
33Kernel subsystems might implement automatic detection to check whether EM
34registered devices have inconsistent scale (based on EM internal flag).
35Important thing to keep in mind is that when the power values are expressed in
36an 'abstract scale' deriving real energy in micro-Joules would not be possible.
37
38The figure below depicts an example of drivers (Arm-specific here, but the
39approach is applicable to any architecture) providing power costs to the EM
40framework, and interested clients reading the data from it::
41
42 +---------------+ +-----------------+ +---------------+
43 | Thermal (IPA) | | Scheduler (EAS) | | Other |
44 +---------------+ +-----------------+ +---------------+
45 | | em_cpu_energy() |
46 | | em_cpu_get() |
47 +---------+ | +---------+
48 | | |
49 v v v
50 +---------------------+
51 | Energy Model |
52 | Framework |
53 +---------------------+
54 ^ ^ ^
55 | | | em_dev_register_perf_domain()
56 +----------+ | +---------+
57 | | |
58 +---------------+ +---------------+ +--------------+
59 | cpufreq-dt | | arm_scmi | | Other |
60 +---------------+ +---------------+ +--------------+
61 ^ ^ ^
62 | | |
63 +--------------+ +---------------+ +--------------+
64 | Device Tree | | Firmware | | ? |
65 +--------------+ +---------------+ +--------------+
66
67In case of CPU devices the EM framework manages power cost tables per
68'performance domain' in the system. A performance domain is a group of CPUs
69whose performance is scaled together. Performance domains generally have a
701-to-1 mapping with CPUFreq policies. All CPUs in a performance domain are
71required to have the same micro-architecture. CPUs in different performance
72domains can have different micro-architectures.
73
74
752. Core APIs
76------------
77
782.1 Config options
79^^^^^^^^^^^^^^^^^^
80
81CONFIG_ENERGY_MODEL must be enabled to use the EM framework.
82
83
842.2 Registration of performance domains
85^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
86
87Registration of 'advanced' EM
88~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
89
90The 'advanced' EM gets it's name due to the fact that the driver is allowed
91to provide more precised power model. It's not limited to some implemented math
92formula in the framework (like it's in 'simple' EM case). It can better reflect
93the real power measurements performed for each performance state. Thus, this
94registration method should be preferred in case considering EM static power
95(leakage) is important.
96
97Drivers are expected to register performance domains into the EM framework by
98calling the following API::
99
100 int em_dev_register_perf_domain(struct device *dev, unsigned int nr_states,
101 struct em_data_callback *cb, cpumask_t *cpus, bool microwatts);
102
103Drivers must provide a callback function returning <frequency, power> tuples
104for each performance state. The callback function provided by the driver is free
105to fetch data from any relevant location (DT, firmware, ...), and by any mean
106deemed necessary. Only for CPU devices, drivers must specify the CPUs of the
107performance domains using cpumask. For other devices than CPUs the last
108argument must be set to NULL.
109The last argument 'microwatts' is important to set with correct value. Kernel
110subsystems which use EM might rely on this flag to check if all EM devices use
111the same scale. If there are different scales, these subsystems might decide
112to return warning/error, stop working or panic.
113See Section 3. for an example of driver implementing this
114callback, or Section 2.4 for further documentation on this API
115
116Registration of EM using DT
117~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
118
119The EM can also be registered using OPP framework and information in DT
120"operating-points-v2". Each OPP entry in DT can be extended with a property
121"opp-microwatt" containing micro-Watts power value. This OPP DT property
122allows a platform to register EM power values which are reflecting total power
123(static + dynamic). These power values might be coming directly from
124experiments and measurements.
125
126Registration of 'artificial' EM
127~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
128
129There is an option to provide a custom callback for drivers missing detailed
130knowledge about power value for each performance state. The callback
131.get_cost() is optional and provides the 'cost' values used by the EAS.
132This is useful for platforms that only provide information on relative
133efficiency between CPU types, where one could use the information to
134create an abstract power model. But even an abstract power model can
135sometimes be hard to fit in, given the input power value size restrictions.
136The .get_cost() allows to provide the 'cost' values which reflect the
137efficiency of the CPUs. This would allow to provide EAS information which
138has different relation than what would be forced by the EM internal
139formulas calculating 'cost' values. To register an EM for such platform, the
140driver must set the flag 'microwatts' to 0, provide .get_power() callback
141and provide .get_cost() callback. The EM framework would handle such platform
142properly during registration. A flag EM_PERF_DOMAIN_ARTIFICIAL is set for such
143platform. Special care should be taken by other frameworks which are using EM
144to test and treat this flag properly.
145
146Registration of 'simple' EM
147~~~~~~~~~~~~~~~~~~~~~~~~~~~
148
149The 'simple' EM is registered using the framework helper function
150cpufreq_register_em_with_opp(). It implements a power model which is tight to
151math formula::
152
153 Power = C * V^2 * f
154
155The EM which is registered using this method might not reflect correctly the
156physics of a real device, e.g. when static power (leakage) is important.
157
158
1592.3 Accessing performance domains
160^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
161
162There are two API functions which provide the access to the energy model:
163em_cpu_get() which takes CPU id as an argument and em_pd_get() with device
164pointer as an argument. It depends on the subsystem which interface it is
165going to use, but in case of CPU devices both functions return the same
166performance domain.
167
168Subsystems interested in the energy model of a CPU can retrieve it using the
169em_cpu_get() API. The energy model tables are allocated once upon creation of
170the performance domains, and kept in memory untouched.
171
172The energy consumed by a performance domain can be estimated using the
173em_cpu_energy() API. The estimation is performed assuming that the schedutil
174CPUfreq governor is in use in case of CPU device. Currently this calculation is
175not provided for other type of devices.
176
177More details about the above APIs can be found in ``<linux/energy_model.h>``
178or in Section 2.4
179
180
1812.4 Description details of this API
182^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
183.. kernel-doc:: include/linux/energy_model.h
184 :internal:
185
186.. kernel-doc:: kernel/power/energy_model.c
187 :export:
188
189
1903. Example driver
191-----------------
192
193The CPUFreq framework supports dedicated callback for registering
194the EM for a given CPU(s) 'policy' object: cpufreq_driver::register_em().
195That callback has to be implemented properly for a given driver,
196because the framework would call it at the right time during setup.
197This section provides a simple example of a CPUFreq driver registering a
198performance domain in the Energy Model framework using the (fake) 'foo'
199protocol. The driver implements an est_power() function to be provided to the
200EM framework::
201
202 -> drivers/cpufreq/foo_cpufreq.c
203
204 01 static int est_power(struct device *dev, unsigned long *mW,
205 02 unsigned long *KHz)
206 03 {
207 04 long freq, power;
208 05
209 06 /* Use the 'foo' protocol to ceil the frequency */
210 07 freq = foo_get_freq_ceil(dev, *KHz);
211 08 if (freq < 0);
212 09 return freq;
213 10
214 11 /* Estimate the power cost for the dev at the relevant freq. */
215 12 power = foo_estimate_power(dev, freq);
216 13 if (power < 0);
217 14 return power;
218 15
219 16 /* Return the values to the EM framework */
220 17 *mW = power;
221 18 *KHz = freq;
222 19
223 20 return 0;
224 21 }
225 22
226 23 static void foo_cpufreq_register_em(struct cpufreq_policy *policy)
227 24 {
228 25 struct em_data_callback em_cb = EM_DATA_CB(est_power);
229 26 struct device *cpu_dev;
230 27 int nr_opp;
231 28
232 29 cpu_dev = get_cpu_device(cpumask_first(policy->cpus));
233 30
234 31 /* Find the number of OPPs for this policy */
235 32 nr_opp = foo_get_nr_opp(policy);
236 33
237 34 /* And register the new performance domain */
238 35 em_dev_register_perf_domain(cpu_dev, nr_opp, &em_cb, policy->cpus,
239 36 true);
240 37 }
241 38
242 39 static struct cpufreq_driver foo_cpufreq_driver = {
243 40 .register_em = foo_cpufreq_register_em,
244 41 };