1 .. SPDX-License-Identifier: GPL-2.0
2
3 =======================
4 Energy Model of devices
5 =======================
6
7 1. Overview
8 -----------
9
10 The Energy Model (EM) framework serves as an i
11 the power consumed by devices at various perfo
12 subsystems willing to use that information to
13
14 The source of the information about the power
15 from one platform to another. These power cost
16 devicetree data in some cases. In others, the
17 Alternatively, userspace might be best positio
18 each and every client subsystem to re-implemen
19 possible source of information on its own, the
20 abstraction layer which standardizes the forma
21 kernel, hence enabling to avoid redundant work
22
23 The power values might be expressed in micro-W
24 Multiple subsystems might use the EM and it is
25 check that the requirements for the power valu
26 can be found in the Energy-Aware Scheduler doc
27 Documentation/scheduler/sched-energy.rst. For
28 powercap power values expressed in an 'abstrac
29 These subsystems are more interested in estima
30 thus the real micro-Watts might be needed. An
31 be found in the Intelligent Power Allocation i
32 Documentation/driver-api/thermal/power_allocat
33 Kernel subsystems might implement automatic de
34 registered devices have inconsistent scale (ba
35 Important thing to keep in mind is that when t
36 an 'abstract scale' deriving real energy in mi
37
38 The figure below depicts an example of drivers
39 approach is applicable to any architecture) pr
40 framework, and interested clients reading the
41
42 +---------------+ +-----------------+
43 | Thermal (IPA) | | Scheduler (EAS) |
44 +---------------+ +-----------------+
45 | | em_cpu_en
46 | | em_cpu_ge
47 +---------+ | +
48 | | |
49 v v v
50 +---------------------
51 | Energy Model
52 | Framework
53 +---------------------
54 ^ ^ ^
55 | | | e
56 +----------+ | +--
57 | |
58 +---------------+ +---------------+
59 | cpufreq-dt | | arm_scmi |
60 +---------------+ +---------------+
61 ^ ^
62 | |
63 +--------------+ +---------------+
64 | Device Tree | | Firmware |
65 +--------------+ +---------------+
66
67 In case of CPU devices the EM framework manage
68 'performance domain' in the system. A performa
69 whose performance is scaled together. Performa
70 1-to-1 mapping with CPUFreq policies. All CPUs
71 required to have the same micro-architecture.
72 domains can have different micro-architectures
73
74 To better reflect power variation due to stati
75 supports runtime modifications of the power va
76 RCU to free the modifiable EM perf_state table
77 scheduler, also uses RCU to access this memory
78 API for allocating/freeing the new memory for
79 The old memory is freed automatically using RC
80 are no owners anymore for the given EM runtime
81 using kref mechanism. The device driver which
82 should call EM API to free it safely when it's
83 framework will handle the clean-up when it's p
84
85 The kernel code which want to modify the EM va
86 access using a mutex. Therefore, the device dr
87 context when it tries to modify the EM.
88
89 With the runtime modifiable EM we switch from
90 runtime static EM' (system property) design to
91 changed during runtime according e.g. to the w
92 property) design.
93
94 It is possible also to modify the CPU performa
95 performance state. Thus, the full power and pe
96 is an exponential curve) can be changed accord
97 or system property.
98
99
100 2. Core APIs
101 ------------
102
103 2.1 Config options
104 ^^^^^^^^^^^^^^^^^^
105
106 CONFIG_ENERGY_MODEL must be enabled to use the
107
108
109 2.2 Registration of performance domains
110 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
111
112 Registration of 'advanced' EM
113 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
114
115 The 'advanced' EM gets its name due to the fac
116 to provide more precised power model. It's not
117 formula in the framework (like it is in 'simpl
118 the real power measurements performed for each
119 registration method should be preferred in cas
120 (leakage) is important.
121
122 Drivers are expected to register performance d
123 calling the following API::
124
125 int em_dev_register_perf_domain(struct devic
126 struct em_data_callback *cb, c
127
128 Drivers must provide a callback function retur
129 for each performance state. The callback funct
130 to fetch data from any relevant location (DT,
131 deemed necessary. Only for CPU devices, driver
132 performance domains using cpumask. For other d
133 argument must be set to NULL.
134 The last argument 'microwatts' is important to
135 subsystems which use EM might rely on this fla
136 the same scale. If there are different scales,
137 to return warning/error, stop working or panic
138 See Section 3. for an example of driver implem
139 callback, or Section 2.4 for further documenta
140
141 Registration of EM using DT
142 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
143
144 The EM can also be registered using OPP frame
145 "operating-points-v2". Each OPP entry in DT ca
146 "opp-microwatt" containing micro-Watts power v
147 allows a platform to register EM power values
148 (static + dynamic). These power values might b
149 experiments and measurements.
150
151 Registration of 'artificial' EM
152 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
153
154 There is an option to provide a custom callbac
155 knowledge about power value for each performan
156 .get_cost() is optional and provides the 'cost
157 This is useful for platforms that only provide
158 efficiency between CPU types, where one could
159 create an abstract power model. But even an ab
160 sometimes be hard to fit in, given the input p
161 The .get_cost() allows to provide the 'cost' v
162 efficiency of the CPUs. This would allow to pr
163 has different relation than what would be forc
164 formulas calculating 'cost' values. To registe
165 driver must set the flag 'microwatts' to 0, pr
166 and provide .get_cost() callback. The EM frame
167 properly during registration. A flag EM_PERF_D
168 platform. Special care should be taken by othe
169 to test and treat this flag properly.
170
171 Registration of 'simple' EM
172 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
173
174 The 'simple' EM is registered using the framew
175 cpufreq_register_em_with_opp(). It implements
176 math formula::
177
178 Power = C * V^2 * f
179
180 The EM which is registered using this method m
181 physics of a real device, e.g. when static pow
182
183
184 2.3 Accessing performance domains
185 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
186
187 There are two API functions which provide the
188 em_cpu_get() which takes CPU id as an argument
189 pointer as an argument. It depends on the subs
190 going to use, but in case of CPU devices both
191 performance domain.
192
193 Subsystems interested in the energy model of a
194 em_cpu_get() API. The energy model tables are
195 the performance domains, and kept in memory un
196
197 The energy consumed by a performance domain ca
198 em_cpu_energy() API. The estimation is perform
199 CPUfreq governor is in use in case of CPU devi
200 not provided for other type of devices.
201
202 More details about the above APIs can be found
203 or in Section 2.5
204
205
206 2.4 Runtime modifications
207 ^^^^^^^^^^^^^^^^^^^^^^^^^
208
209 Drivers willing to update the EM at runtime sh
210 function to allocate a new instance of the mod
211 below::
212
213 struct em_perf_table __rcu *em_table_alloc(s
214
215 This allows to allocate a structure which cont
216 also RCU and kref needed by the EM framework.
217 contains array 'struct em_perf_state state[]'
218 states in ascending order. That list must be p
219 which wants to update the EM. The list of freq
220 existing EM (created during boot). The content
221 must be populated by the driver as well.
222
223 This is the API which does the EM update, usin
224
225 int em_dev_update_perf_domain(struct device
226 struct em_perf_table _
227
228 Drivers must provide a pointer to the allocate
229 'struct em_perf_table'. That new EM will be sa
230 and will be visible to other sub-systems in th
231 The main design goal for this API is to be fas
232 or memory allocations at runtime. When pre-com
233 device driver, than it should be possible to s
234 performance overhead.
235
236 In order to free the EM, provided earlier by t
237 is unloaded), there is a need to call the API:
238
239 void em_table_free(struct em_perf_table __rc
240
241 It will allow the EM framework to safely remov
242 no other sub-system using it, e.g. EAS.
243
244 To use the power values in other sub-systems (
245 a need to call API which protects the reader a
246 table data::
247
248 struct em_perf_state *em_perf_state_from_pd(
249
250 It returns the 'struct em_perf_state' pointer
251 states in ascending order.
252 This function must be called in the RCU read l
253 rcu_read_lock()). When the EM table is not nee
254 call rcu_real_unlock(). In this way the EM saf
255 and protects the users. It also allows the EM
256 and free it. More details how to use it can be
257 example driver.
258
259 There is dedicated API for device drivers to c
260 values::
261
262 int em_dev_compute_costs(struct device *dev,
263 int nr_states);
264
265 These 'cost' values from EM are used in EAS. T
266 together with the number of entries and device
267 of the cost values is done properly the return
268 The function takes care for right setting of i
269 state as well. It updates em_perf_state::flags
270 Then such prepared new EM can be passed to the
271 function, which will allow to use it.
272
273 More details about the above APIs can be found
274 or in Section 3.2 with an example code showing
275 updating mechanism in a device driver.
276
277
278 2.5 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
287 3. Examples
288 -----------
289
290 3.1 Example driver with EM registration
291 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
292
293 The CPUFreq framework supports dedicated callb
294 the EM for a given CPU(s) 'policy' object: cpu
295 That callback has to be implemented properly f
296 because the framework would call it at the rig
297 This section provides a simple example of a CP
298 performance domain in the Energy Model framewo
299 protocol. The driver implements an est_power()
300 EM framework::
301
302 -> drivers/cpufreq/foo_cpufreq.c
303
304 01 static int est_power(struct device *de
305 02 unsigned long *KHz)
306 03 {
307 04 long freq, power;
308 05
309 06 /* Use the 'foo' protocol to c
310 07 freq = foo_get_freq_ceil(dev,
311 08 if (freq < 0);
312 09 return freq;
313 10
314 11 /* Estimate the power cost for
315 12 power = foo_estimate_power(dev
316 13 if (power < 0);
317 14 return power;
318 15
319 16 /* Return the values to the EM
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(st
327 24 {
328 25 struct em_data_callback em_cb
329 26 struct device *cpu_dev;
330 27 int nr_opp;
331 28
332 29 cpu_dev = get_cpu_device(cpuma
333 30
334 31 /* Find the number of OPPs for
335 32 nr_opp = foo_get_nr_opp(policy
336 33
337 34 /* And register the new perfor
338 35 em_dev_register_perf_domain(cp
339 36 tr
340 37 }
341 38
342 39 static struct cpufreq_driver foo_cpufr
343 40 .register_em = foo_cpufreq_reg
344 41 };
345
346
347 3.2 Example driver with EM modification
348 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
349
350 This section provides a simple example of a th
351 The driver implements a foo_thermal_em_update(
352 up periodically to check the temperature and m
353
354 -> drivers/soc/example/example_em_mod.c
355
356 01 static void foo_get_new_em(struct foo_
357 02 {
358 03 struct em_perf_table __rcu *em
359 04 struct em_perf_state *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(
377 22 for (i = 0; i < pd->nr_perf_st
378 23 freq = table[i].freque
379 24 foo_get_power_perf_val
380 25 }
381 26 rcu_read_unlock();
382 27
383 28 /* Calculate 'cost' values for
384 29 ret = em_dev_compute_costs(dev
385 30 if (ret) {
386 31 dev_warn(dev, "EM: com
387 32 em_free_table(em_table
388 33 return;
389 34 }
390 35
391 36 ret = em_dev_update_perf_domai
392 37 if (ret) {
393 38 dev_warn(dev, "EM: upd
394 39 em_free_table(em_table
395 40 return;
396 41 }
397 42
398 43 /*
399 44 * Since it's one-time-update
400 45 * The EM framework will later
401 46 */
402 47 em_table_free(em_table);
403 48 }
404 49
405 50 /*
406 51 * Function called periodically to che
407 52 * update the EM if needed
408 53 */
409 54 static void foo_thermal_em_update(stru
410 55 {
411 56 struct device *dev = ctx->dev;
412 57 int cpu;
413 58
414 59 ctx->temperature = foo_get_tem
415 60 if (ctx->temperature < FOO_EM_
416 61 return;
417 62
418 63 foo_get_new_em(ctx);
419 64 }
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