[PATCH v7] cpufreq: intel_pstate: Implement passive mode with HWP enabled

Rafael J. Wysocki
2020-08-06T17:34:35Z

From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Allow intel_pstate to work in the passive mode with HWP enabled and
make it set the HWP minimum performance limit (HWP floor) to the
P-state value given by the target frequency supplied by the cpufreq
governor, so as to prevent the HWP algorithm and the CPU scheduler
from working against each other, at least when the schedutil governor
is in use, and update the intel_pstate documentation accordingly.

Among other things, this allows utilization clamps to be taken
into account, at least to a certain extent, when intel_pstate is
in use and makes it more likely that sufficient capacity for
deadline tasks will be provided.

After this change, the resulting behavior of an HWP system with
intel_pstate in the passive mode should be close to the behavior
of the analogous non-HWP system with intel_pstate in the passive
mode, except that in the frequency range below the base frequency
(ie. the frequency retured by the base_frequency cpufreq attribute
in sysfs on HWP systems) the HWP algorithm is allowed to make the
CPU run at a frequency above the floor P-state set by intel_pstate,
with or without hardware coordination of P-states among CPUs in the
same package.

[If P-states of the CPUs in the same package are coordinated at the
hardware level, a non-HWP processor may choose a P-state above the
target one like a processor with HWP enabled may choose a P-state
above the HWP floor, so the HWP behavior is analogous to the non-HWP
one in that case.

Also note that the HWP floor may not be taken into account by
the processor in the range of P-states above the base frequency,
referred to as the turbo range, where the processor has a license to
choose any P-state, either below or above the HWP floor, just like a
non-HWP processor in the case when the target P-state falls into the
turbo range.]

With this change applied, intel_pstate in the passive mode
assumes complete control over the HWP request MSR and concurrent
changes of that MSR (eg. via the direct MSR access interface) are
overridden by it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

Sending the right patch this time, sorry for the confusion.

This is based on the current mainline.

v1 -> v2:
* Avoid a race condition when updating the HWP request register while
setting a new EPP value via sysfs.

v2 -> v3:
* Rebase.

v3 -> v4:
* Avoid exposing the hwp_dynamic_boost sysfs switch in the passive mode.

v4 -> v5:
* Do not acquire intel_pstate_driver_lock in
store_energy_performance_preference(), because it runs under
policy->rwsem, so intel_pstate_driver cannot change while it is running.
* Rearrange the changelog a bit to avoid confusion.

v5 -> v6:
* Fix the problem with the EPP setting via sysfs not working with the
performance and powersave governors by stopping and restarting the
governor around the sysfs-based EPP updates in the passive mode.
* Because of that, use the epp_cached field just for avoiding the above
if the new EPP value for the given CPU is the same as the old one.
* Export cpufreq_start/stop_governor() from the core (for the above).

v6 -> v7:
* Cosmetic changes in store_energy_performance_prefernce() to reduce the
LoC number and make it a bit easier to read. No intentional functional
impact.

---
Documentation/admin-guide/pm/intel_pstate.rst | 89 ++++-----
drivers/cpufreq/cpufreq.c | 6
drivers/cpufreq/intel_pstate.c | 245 +++++++++++++++++++-------
include/linux/cpufreq.h | 2
4 files changed, 229 insertions(+), 113 deletions(-)

Index: linux-pm/drivers/cpufreq/intel_pstate.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/intel_pstate.c
+++ linux-pm/drivers/cpufreq/intel_pstate.c
@@ -36,6 +36,7 @@
#define INTEL_PSTATE_SAMPLING_INTERVAL (10 * NSEC_PER_MSEC)

#define INTEL_CPUFREQ_TRANSITION_LATENCY 20000
+#define INTEL_CPUFREQ_TRANSITION_DELAY_HWP 5000
#define INTEL_CPUFREQ_TRANSITION_DELAY 500

#ifdef CONFIG_ACPI
@@ -220,6 +221,7 @@ struct global_params {
* preference/bias
* @epp_saved: Saved EPP/EPB during system suspend or CPU offline
* operation
+ * @epp_cached Cached HWP energy-performance preference value
* @hwp_req_cached: Cached value of the last HWP Request MSR
* @hwp_cap_cached: Cached value of the last HWP Capabilities MSR
* @last_io_update: Last time when IO wake flag was set
@@ -257,6 +259,7 @@ struct cpudata {
s16 epp_policy;
s16 epp_default;
s16 epp_saved;
+ s16 epp_cached;
u64 hwp_req_cached;
u64 hwp_cap_cached;
u64 last_io_update;
@@ -639,6 +642,26 @@ static int intel_pstate_get_energy_pref_
return index;
}

+static int intel_pstate_set_epp(struct cpudata *cpu, u32 epp)
+{
+ /*
+ * Use the cached HWP Request MSR value, because in the active mode the
+ * register itself may be updated by intel_pstate_hwp_boost_up() or
+ * intel_pstate_hwp_boost_down() at any time.
+ */
+ u64 value = READ_ONCE(cpu->hwp_req_cached);
+
+ value &= ~GENMASK_ULL(31, 24);
+ value |= (u64)epp << 24;
+ /*
+ * The only other updater of hwp_req_cached in the active mode,
+ * intel_pstate_hwp_set(), is called under the same lock as this
+ * function, so it cannot run in parallel with the update below.
+ */
+ WRITE_ONCE(cpu->hwp_req_cached, value);
+ return wrmsrl_on_cpu(cpu->cpu, MSR_HWP_REQUEST, value);
+}
+
static int intel_pstate_set_energy_pref_index(struct cpudata *cpu_data,
int pref_index, bool use_raw,
u32 raw_epp)
@@ -650,28 +673,12 @@ static int intel_pstate_set_energy_pref_
epp = cpu_data->epp_default;

if (boot_cpu_has(X86_FEATURE_HWP_EPP)) {
- /*
- * Use the cached HWP Request MSR value, because the register
- * itself may be updated by intel_pstate_hwp_boost_up() or
- * intel_pstate_hwp_boost_down() at any time.
- */
- u64 value = READ_ONCE(cpu_data->hwp_req_cached);
-
- value &= ~GENMASK_ULL(31, 24);
-
if (use_raw)
epp = raw_epp;
else if (epp == -EINVAL)
epp = epp_values[pref_index - 1];

- value |= (u64)epp << 24;
- /*
- * The only other updater of hwp_req_cached in the active mode,
- * intel_pstate_hwp_set(), is called under the same lock as this
- * function, so it cannot run in parallel with the update below.
- */
- WRITE_ONCE(cpu_data->hwp_req_cached, value);
- ret = wrmsrl_on_cpu(cpu_data->cpu, MSR_HWP_REQUEST, value);
+ ret = intel_pstate_set_epp(cpu_data, epp);
} else {
if (epp == -EINVAL)
epp = (pref_index - 1) << 2;
@@ -697,10 +704,12 @@ static ssize_t show_energy_performance_a

cpufreq_freq_attr_ro(energy_performance_available_preferences);

+static struct cpufreq_driver intel_pstate;
+
static ssize_t store_energy_performance_preference(
struct cpufreq_policy *policy, const char *buf, size_t count)
{
- struct cpudata *cpu_data = all_cpu_data[policy->cpu];
+ struct cpudata *cpu = all_cpu_data[policy->cpu];
char str_preference[21];
bool raw = false;
ssize_t ret;
@@ -725,15 +734,44 @@ static ssize_t store_energy_performance_
raw = true;
}

+ /*
+ * This function runs with the policy R/W semaphore held, which
+ * guarantees that the driver pointer will not change while it is
+ * running.
+ */
+ if (!intel_pstate_driver)
+ return -EAGAIN;
+
mutex_lock(&intel_pstate_limits_lock);

- ret = intel_pstate_set_energy_pref_index(cpu_data, ret, raw, epp);
- if (!ret)
- ret = count;
+ if (intel_pstate_driver == &intel_pstate) {
+ ret = intel_pstate_set_energy_pref_index(cpu, ret, raw, epp);
+ } else {
+ /*
+ * In the passive mode the governor needs to be stopped on the
+ * target CPU before the EPP update and restarted after it,
+ * which is super-heavy-weight, so make sure it is worth doing
+ * upfront.
+ */
+ if (!raw)
+ epp = ret ? epp_values[ret - 1] : cpu->epp_default;
+
+ if (cpu->epp_cached != epp) {
+ int err;
+
+ cpufreq_stop_governor(policy);
+ ret = intel_pstate_set_epp(cpu, epp);
+ err = cpufreq_start_governor(policy);
+ if (!ret) {
+ cpu->epp_cached = epp;
+ ret = err;
+ }
+ }
+ }

mutex_unlock(&intel_pstate_limits_lock);

- return ret;
+ return ret ?: count;
}

static ssize_t show_energy_performance_preference(
@@ -1145,8 +1183,6 @@ static ssize_t store_no_turbo(struct kob
return count;
}

-static struct cpufreq_driver intel_pstate;
-
static void update_qos_request(enum freq_qos_req_type type)
{
int max_state, turbo_max, freq, i, perf_pct;
@@ -1330,9 +1366,10 @@ static const struct attribute_group inte

static const struct x86_cpu_id intel_pstate_cpu_ee_disable_ids[];

+static struct kobject *intel_pstate_kobject;
+
static void __init intel_pstate_sysfs_expose_params(void)
{
- struct kobject *intel_pstate_kobject;
int rc;

intel_pstate_kobject = kobject_create_and_add("intel_pstate",
@@ -1357,17 +1394,31 @@ static void __init intel_pstate_sysfs_ex
rc = sysfs_create_file(intel_pstate_kobject, &min_perf_pct.attr);
WARN_ON(rc);

- if (hwp_active) {
- rc = sysfs_create_file(intel_pstate_kobject,
- &hwp_dynamic_boost.attr);
- WARN_ON(rc);
- }
-
if (x86_match_cpu(intel_pstate_cpu_ee_disable_ids)) {
rc = sysfs_create_file(intel_pstate_kobject, &energy_efficiency.attr);
WARN_ON(rc);
}
}
+
+static void intel_pstate_sysfs_expose_hwp_dynamic_boost(void)
+{
+ int rc;
+
+ if (!hwp_active)
+ return;
+
+ rc = sysfs_create_file(intel_pstate_kobject, &hwp_dynamic_boost.attr);
+ WARN_ON_ONCE(rc);
+}
+
+static void intel_pstate_sysfs_hide_hwp_dynamic_boost(void)
+{
+ if (!hwp_active)
+ return;
+
+ sysfs_remove_file(intel_pstate_kobject, &hwp_dynamic_boost.attr);
+}
+
/************************** sysfs end ************************/

static void intel_pstate_hwp_enable(struct cpudata *cpudata)
@@ -2246,7 +2297,10 @@ static int intel_pstate_verify_policy(st

static void intel_cpufreq_stop_cpu(struct cpufreq_policy *policy)
{
- intel_pstate_set_min_pstate(all_cpu_data[policy->cpu]);
+ if (hwp_active)
+ intel_pstate_hwp_force_min_perf(policy->cpu);
+ else
+ intel_pstate_set_min_pstate(all_cpu_data[policy->cpu]);
}

static void intel_pstate_stop_cpu(struct cpufreq_policy *policy)
@@ -2254,12 +2308,10 @@ static void intel_pstate_stop_cpu(struct
pr_debug("CPU %d exiting\n", policy->cpu);

intel_pstate_clear_update_util_hook(policy->cpu);
- if (hwp_active) {
+ if (hwp_active)
intel_pstate_hwp_save_state(policy);
- intel_pstate_hwp_force_min_perf(policy->cpu);
- } else {
- intel_cpufreq_stop_cpu(policy);
- }
+
+ intel_cpufreq_stop_cpu(policy);
}

static int intel_pstate_cpu_exit(struct cpufreq_policy *policy)
@@ -2389,13 +2441,71 @@ static void intel_cpufreq_trace(struct c
fp_toint(cpu->iowait_boost * 100));
}

+static void intel_cpufreq_adjust_hwp(struct cpudata *cpu, u32 target_pstate,
+ bool fast_switch)
+{
+ u64 prev = READ_ONCE(cpu->hwp_req_cached), value = prev;
+
+ value &= ~HWP_MIN_PERF(~0L);
+ value |= HWP_MIN_PERF(target_pstate);
+
+ /*
+ * The entire MSR needs to be updated in order to update the HWP min
+ * field in it, so opportunistically update the max too if needed.
+ */
+ value &= ~HWP_MAX_PERF(~0L);
+ value |= HWP_MAX_PERF(cpu->max_perf_ratio);
+
+ if (value == prev)
+ return;
+
+ WRITE_ONCE(cpu->hwp_req_cached, value);
+ if (fast_switch)
+ wrmsrl(MSR_HWP_REQUEST, value);
+ else
+ wrmsrl_on_cpu(cpu->cpu, MSR_HWP_REQUEST, value);
+}
+
+static void intel_cpufreq_adjust_perf_ctl(struct cpudata *cpu,
+ u32 target_pstate, bool fast_switch)
+{
+ if (fast_switch)
+ wrmsrl(MSR_IA32_PERF_CTL,
+ pstate_funcs.get_val(cpu, target_pstate));
+ else
+ wrmsrl_on_cpu(cpu->cpu, MSR_IA32_PERF_CTL,
+ pstate_funcs.get_val(cpu, target_pstate));
+}
+
+static int intel_cpufreq_update_pstate(struct cpudata *cpu, int target_pstate,
+ bool fast_switch)
+{
+ int old_pstate = cpu->pstate.current_pstate;
+
+ target_pstate = intel_pstate_prepare_request(cpu, target_pstate);
+ if (target_pstate != old_pstate) {
+ cpu->pstate.current_pstate = target_pstate;
+ if (hwp_active)
+ intel_cpufreq_adjust_hwp(cpu, target_pstate,
+ fast_switch);
+ else
+ intel_cpufreq_adjust_perf_ctl(cpu, target_pstate,
+ fast_switch);
+ }
+
+ intel_cpufreq_trace(cpu, fast_switch ? INTEL_PSTATE_TRACE_FAST_SWITCH :
+ INTEL_PSTATE_TRACE_TARGET, old_pstate);
+
+ return target_pstate;
+}
+
static int intel_cpufreq_target(struct cpufreq_policy *policy,
unsigned int target_freq,
unsigned int relation)
{
struct cpudata *cpu = all_cpu_data[policy->cpu];
struct cpufreq_freqs freqs;
- int target_pstate, old_pstate;
+ int target_pstate;

update_turbo_state();

@@ -2403,6 +2513,7 @@ static int intel_cpufreq_target(struct c
freqs.new = target_freq;

cpufreq_freq_transition_begin(policy, &freqs);
+
switch (relation) {
case CPUFREQ_RELATION_L:
target_pstate = DIV_ROUND_UP(freqs.new, cpu->pstate.scaling);
@@ -2414,15 +2525,11 @@ static int intel_cpufreq_target(struct c
target_pstate = DIV_ROUND_CLOSEST(freqs.new, cpu->pstate.scaling);
break;
}
- target_pstate = intel_pstate_prepare_request(cpu, target_pstate);
- old_pstate = cpu->pstate.current_pstate;
- if (target_pstate != cpu->pstate.current_pstate) {
- cpu->pstate.current_pstate = target_pstate;
- wrmsrl_on_cpu(policy->cpu, MSR_IA32_PERF_CTL,
- pstate_funcs.get_val(cpu, target_pstate));
- }
+
+ target_pstate = intel_cpufreq_update_pstate(cpu, target_pstate, false);
+
freqs.new = target_pstate * cpu->pstate.scaling;
- intel_cpufreq_trace(cpu, INTEL_PSTATE_TRACE_TARGET, old_pstate);
+
cpufreq_freq_transition_end(policy, &freqs, false);

return 0;
@@ -2432,15 +2539,14 @@ static unsigned int intel_cpufreq_fast_s
unsigned int target_freq)
{
struct cpudata *cpu = all_cpu_data[policy->cpu];
- int target_pstate, old_pstate;
+ int target_pstate;

update_turbo_state();

target_pstate = DIV_ROUND_UP(target_freq, cpu->pstate.scaling);
- target_pstate = intel_pstate_prepare_request(cpu, target_pstate);
- old_pstate = cpu->pstate.current_pstate;
- intel_pstate_update_pstate(cpu, target_pstate);
- intel_cpufreq_trace(cpu, INTEL_PSTATE_TRACE_FAST_SWITCH, old_pstate);
+
+ target_pstate = intel_cpufreq_update_pstate(cpu, target_pstate, true);
+
return target_pstate * cpu->pstate.scaling;
}

@@ -2460,7 +2566,6 @@ static int intel_cpufreq_cpu_init(struct
return ret;

policy->cpuinfo.transition_latency = INTEL_CPUFREQ_TRANSITION_LATENCY;
- policy->transition_delay_us = INTEL_CPUFREQ_TRANSITION_DELAY;
/* This reflects the intel_pstate_get_cpu_pstates() setting. */
policy->cur = policy->cpuinfo.min_freq;

@@ -2472,10 +2577,18 @@ static int intel_cpufreq_cpu_init(struct

cpu = all_cpu_data[policy->cpu];

- if (hwp_active)
+ if (hwp_active) {
+ u64 value;
+
intel_pstate_get_hwp_max(policy->cpu, &turbo_max, &max_state);
- else
+ policy->transition_delay_us = INTEL_CPUFREQ_TRANSITION_DELAY_HWP;
+ rdmsrl_on_cpu(cpu->cpu, MSR_HWP_REQUEST, &value);
+ WRITE_ONCE(cpu->hwp_req_cached, value);
+ cpu->epp_cached = (value & GENMASK_ULL(31, 24)) >> 24;
+ } else {
turbo_max = cpu->pstate.turbo_pstate;
+ policy->transition_delay_us = INTEL_CPUFREQ_TRANSITION_DELAY;
+ }

min_freq = DIV_ROUND_UP(turbo_max * global.min_perf_pct, 100);
min_freq *= cpu->pstate.scaling;
@@ -2552,6 +2665,10 @@ static void intel_pstate_driver_cleanup(
}
}
put_online_cpus();
+
+ if (intel_pstate_driver == &intel_pstate)
+ intel_pstate_sysfs_hide_hwp_dynamic_boost();
+
intel_pstate_driver = NULL;
}

@@ -2559,6 +2676,9 @@ static int intel_pstate_register_driver(
{
int ret;

+ if (driver == &intel_pstate)
+ intel_pstate_sysfs_expose_hwp_dynamic_boost();
+
memset(&global, 0, sizeof(global));
global.max_perf_pct = 100;

@@ -2576,9 +2696,6 @@ static int intel_pstate_register_driver(

static int intel_pstate_unregister_driver(void)
{
- if (hwp_active)
- return -EBUSY;
-
cpufreq_unregister_driver(intel_pstate_driver);
intel_pstate_driver_cleanup();

@@ -2834,7 +2951,10 @@ static int __init intel_pstate_init(void
hwp_active++;
hwp_mode_bdw = id->driver_data;
intel_pstate.attr = hwp_cpufreq_attrs;
- default_driver = &intel_pstate;
+ intel_cpufreq.attr = hwp_cpufreq_attrs;
+ if (!default_driver)
+ default_driver = &intel_pstate;
+
goto hwp_cpu_matched;
}
} else {
@@ -2905,14 +3025,13 @@ static int __init intel_pstate_setup(cha
if (!str)
return -EINVAL;

- if (!strcmp(str, "disable")) {
+ if (!strcmp(str, "disable"))
no_load = 1;
- } else if (!strcmp(str, "active")) {
+ else if (!strcmp(str, "active"))
default_driver = &intel_pstate;
- } else if (!strcmp(str, "passive")) {
+ else if (!strcmp(str, "passive"))
default_driver = &intel_cpufreq;
- no_hwp = 1;
- }
+
if (!strcmp(str, "no_hwp")) {
pr_info("HWP disabled\n");
no_hwp = 1;
Index: linux-pm/Documentation/admin-guide/pm/intel_pstate.rst
===================================================================
--- linux-pm.orig/Documentation/admin-guide/pm/intel_pstate.rst
+++ linux-pm/Documentation/admin-guide/pm/intel_pstate.rst
@@ -54,10 +54,13 @@ registered (see `below <status_attr_>`_)
Operation Modes
===============

-``intel_pstate`` can operate in three different modes: in the active mode with
-or without hardware-managed P-states support and in the passive mode. Which of
-them will be in effect depends on what kernel command line options are used and
-on the capabilities of the processor.
+``intel_pstate`` can operate in two different modes, active or passive. In the
+active mode, it uses its own internal preformance scaling governor algorithm or
+allows the hardware to do preformance scaling by itself, while in the passive
+mode it responds to requests made by a generic ``CPUFreq`` governor implementing
+a certain performance scaling algorithm. Which of them will be in effect
+depends on what kernel command line options are used and on the capabilities of
+the processor.

Active Mode
-----------
@@ -194,10 +197,11 @@ This is the default operation mode of ``
hardware-managed P-states (HWP) support. It is always used if the
``intel_pstate=passive`` argument is passed to the kernel in the command line
regardless of whether or not the given processor supports HWP. [Note that the
-``intel_pstate=no_hwp`` setting implies ``intel_pstate=passive`` if it is used
-without ``intel_pstate=active``.] Like in the active mode without HWP support,
-in this mode ``intel_pstate`` may refuse to work with processors that are not
-recognized by it.
+``intel_pstate=no_hwp`` setting causes the driver to start in the passive mode
+if it is not combined with ``intel_pstate=active``.] Like in the active mode
+without HWP support, in this mode ``intel_pstate`` may refuse to work with
+processors that are not recognized by it if HWP is prevented from being enabled
+through the kernel command line.

If the driver works in this mode, the ``scaling_driver`` policy attribute in
``sysfs`` for all ``CPUFreq`` policies contains the string "intel_cpufreq".
@@ -318,10 +322,9 @@ manuals need to be consulted to get to i

For this reason, there is a list of supported processors in ``intel_pstate`` and
the driver initialization will fail if the detected processor is not in that
-list, unless it supports the `HWP feature <Active Mode_>`_. [The interface to
-obtain all of the information listed above is the same for all of the processors
-supporting the HWP feature, which is why they all are supported by
-``intel_pstate``.]
+list, unless it supports the HWP feature. [The interface to obtain all of the
+information listed above is the same for all of the processors supporting the
+HWP feature, which is why ``intel_pstate`` works with all of them.]

User Space Interface in ``sysfs``
@@ -425,22 +428,16 @@ argument is passed to the kernel in the
as well as the per-policy ones) are then reset to their default
values, possibly depending on the target operation mode.]

- That only is supported in some configurations, though (for example, if
- the `HWP feature is enabled in the processor <Active Mode With HWP_>`_,
- the operation mode of the driver cannot be changed), and if it is not
- supported in the current configuration, writes to this attribute will
- fail with an appropriate error.
-
``energy_efficiency``
- This attribute is only present on platforms, which have CPUs matching
- Kaby Lake or Coffee Lake desktop CPU model. By default
- energy efficiency optimizations are disabled on these CPU models in HWP
- mode by this driver. Enabling energy efficiency may limit maximum
- operating frequency in both HWP and non HWP mode. In non HWP mode,
- optimizations are done only in the turbo frequency range. In HWP mode,
- optimizations are done in the entire frequency range. Setting this
- attribute to "1" enables energy efficiency optimizations and setting
- to "0" disables energy efficiency optimizations.
+ This attribute is only present on platforms with CPUs matching the Kaby
+ Lake or Coffee Lake desktop CPU model. By default, energy-efficiency
+ optimizations are disabled on these CPU models if HWP is enabled.
+ Enabling energy-efficiency optimizations may limit maximum operating
+ frequency with or without the HWP feature. With HWP enabled, the
+ optimizations are done only in the turbo frequency range. Without it,
+ they are done in the entire available frequency range. Setting this
+ attribute to "1" enables the energy-efficiency optimizations and setting
+ to "0" disables them.

Interpretation of Policy Attributes
-----------------------------------
@@ -484,8 +481,8 @@ Next, the following policy attributes ha
policy for the time interval between the last two invocations of the
driver's utilization update callback by the CPU scheduler for that CPU.

-One more policy attribute is present if the `HWP feature is enabled in the
-processor <Active Mode With HWP_>`_:
+One more policy attribute is present if the HWP feature is enabled in the
+processor:

``base_frequency``
Shows the base frequency of the CPU. Any frequency above this will be
@@ -526,11 +523,11 @@ on the following rules, regardless of th

3. The global and per-policy limits can be set independently.

-If the `HWP feature is enabled in the processor <Active Mode With HWP_>`_, the
-resulting effective values are written into its registers whenever the limits
-change in order to request its internal P-state selection logic to always set
-P-states within these limits. Otherwise, the limits are taken into account by
-scaling governors (in the `passive mode <Passive Mode_>`_) and by the driver
+In the `active mode with the HWP feature enabled <Active Mode With HWP_>`_, the
+resulting effective values are written into hardware registers whenever the
+limits change in order to request its internal P-state selection logic to always
+set P-states within these limits. Otherwise, the limits are taken into account
+by scaling governors (in the `passive mode <Passive Mode_>`_) and by the driver
every time before setting a new P-state for a CPU.

Additionally, if the ``intel_pstate=per_cpu_perf_limits`` command line argument
@@ -541,12 +538,11 @@ at all and the only way to set the limit
Energy vs Performance Hints
---------------------------

-If ``intel_pstate`` works in the `active mode with the HWP feature enabled
-<Active Mode With HWP_>`_ in the processor, additional attributes are present
-in every ``CPUFreq`` policy directory in ``sysfs``. They are intended to allow
-user space to help ``intel_pstate`` to adjust the processor's internal P-state
-selection logic by focusing it on performance or on energy-efficiency, or
-somewhere between the two extremes:
+If the hardware-managed P-states (HWP) is enabled in the processor, additional
+attributes, intended to allow user space to help ``intel_pstate`` to adjust the
+processor's internal P-state selection logic by focusing it on performance or on
+energy-efficiency, or somewhere between the two extremes, are present in every
+``CPUFreq`` policy directory in ``sysfs``. They are :

``energy_performance_preference``
Current value of the energy vs performance hint for the given policy
@@ -650,12 +646,14 @@ of them have to be prepended with the ``
Do not register ``intel_pstate`` as the scaling driver even if the
processor is supported by it.

+``active``
+ Register ``intel_pstate`` in the `active mode <Active Mode_>`_ to start
+ with.
+
``passive``
Register ``intel_pstate`` in the `passive mode <Passive Mode_>`_ to
start with.

- This option implies the ``no_hwp`` one described below.
-
``force``
Register ``intel_pstate`` as the scaling driver instead of
``acpi-cpufreq`` even if the latter is preferred on the given system.
@@ -670,13 +668,12 @@ of them have to be prepended with the ``
driver is used instead of ``acpi-cpufreq``.

``no_hwp``
- Do not enable the `hardware-managed P-states (HWP) feature
- <Active Mode With HWP_>`_ even if it is supported by the processor.
+ Do not enable the hardware-managed P-states (HWP) feature even if it is
+ supported by the processor.

``hwp_only``
Register ``intel_pstate`` as the scaling driver only if the
- `hardware-managed P-states (HWP) feature <Active Mode With HWP_>`_ is
- supported by the processor.
+ hardware-managed P-states (HWP) feature is supported by the processor.

``support_acpi_ppc``
Take ACPI ``_PPC`` performance limits into account.
Index: linux-pm/drivers/cpufreq/cpufreq.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq.c
+++ linux-pm/drivers/cpufreq/cpufreq.c
@@ -73,8 +73,6 @@ static inline bool has_target(void)
static unsigned int __cpufreq_get(struct cpufreq_policy *policy);
static int cpufreq_init_governor(struct cpufreq_policy *policy);
static void cpufreq_exit_governor(struct cpufreq_policy *policy);
-static int cpufreq_start_governor(struct cpufreq_policy *policy);
-static void cpufreq_stop_governor(struct cpufreq_policy *policy);
static void cpufreq_governor_limits(struct cpufreq_policy *policy);
static int cpufreq_set_policy(struct cpufreq_policy *policy,
struct cpufreq_governor *new_gov,
@@ -2266,7 +2264,7 @@ static void cpufreq_exit_governor(struct
module_put(policy->governor->owner);
}

-static int cpufreq_start_governor(struct cpufreq_policy *policy)
+int cpufreq_start_governor(struct cpufreq_policy *policy)
{
int ret;

@@ -2293,7 +2291,7 @@ static int cpufreq_start_governor(struct
return 0;
}

-static void cpufreq_stop_governor(struct cpufreq_policy *policy)
+void cpufreq_stop_governor(struct cpufreq_policy *policy)
{
if (cpufreq_suspended || !policy->governor)
return;
Index: linux-pm/include/linux/cpufreq.h
===================================================================
--- linux-pm.orig/include/linux/cpufreq.h
+++ linux-pm/include/linux/cpufreq.h
@@ -576,6 +576,8 @@ unsigned int cpufreq_driver_resolve_freq
unsigned int cpufreq_policy_transition_delay_us(struct cpufreq_policy *policy);
int cpufreq_register_governor(struct cpufreq_governor *governor);
void cpufreq_unregister_governor(struct cpufreq_governor *governor);
+int cpufreq_start_governor(struct cpufreq_policy *policy);
+void cpufreq_stop_governor(struct cpufreq_policy *policy);

#define cpufreq_governor_init(__governor) \
static int __init __governor##_init(void) \