Re: [PATCH v4 4/8] ACPI: CPPC: add APIs and sysfs interface for min/max_perf
From: Sumit Gupta
Date: Tue Dec 09 2025 - 11:38:44 EST
On 27/11/25 20:24, Pierre Gondois wrote:
External email: Use caution opening links or attachments
On 11/5/25 12:38, Sumit Gupta wrote:
CPPC allows platforms to specify minimum and maximum performance
limits that constrain the operating range for CPU performance scaling
when Autonomous Selection is enabled. These limits can be dynamically
adjusted to implement power management policies or workload-specific
optimizations.
Add cppc_get_min_perf() and cppc_set_min_perf() functions to read and
write the MIN_PERF register, allowing dynamic adjustment of the minimum
performance floor.
Add cppc_get_max_perf() and cppc_set_max_perf() functions to read and
write the MAX_PERF register, enabling dynamic ceiling control for
maximum performance.
Expose these capabilities through cpufreq sysfs attributes that accept
frequency values in kHz (which are converted to/from performance values
internally):
- /sys/.../cpufreq/policy*/min_perf: Read/write min perf as freq (kHz)
- /sys/.../cpufreq/policy*/max_perf: Read/write max perf as freq (kHz)
The frequency-based interface provides a user-friendly abstraction which
is similar to other cpufreq sysfs interfaces, while the driver handles
conversion to hardware performance values.
Also update EPP constants for better clarity:
- Rename CPPC_ENERGY_PERF_MAX to CPPC_EPP_ENERGY_EFFICIENCY_PREF
- Add CPPC_EPP_PERFORMANCE_PREF for the performance-oriented setting
Signed-off-by: Sumit Gupta<sumitg@xxxxxxxxxx>
---
drivers/acpi/cppc_acpi.c | 55 ++++++++++-
drivers/cpufreq/cppc_cpufreq.c | 166 +++++++++++++++++++++++++++++++++
include/acpi/cppc_acpi.h | 23 ++++-
3 files changed, 242 insertions(+), 2 deletions(-)
diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 757e8ce87e9b..ef53eb8a1feb 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1634,7 +1634,7 @@ EXPORT_SYMBOL_GPL(cppc_set_epp_perf);
*/
int cppc_set_epp(int cpu, u64 epp_val)
{
- if (epp_val > CPPC_ENERGY_PERF_MAX)
+ if (epp_val > CPPC_EPP_ENERGY_EFFICIENCY_PREF)
return -EINVAL;
return cppc_set_reg_val(cpu, ENERGY_PERF, epp_val);
@@ -1757,6 +1757,59 @@ int cppc_set_enable(int cpu, bool enable)
return cppc_set_reg_val(cpu, ENABLE, enable);
}
EXPORT_SYMBOL_GPL(cppc_set_enable);
+
+/**
+ * cppc_get_min_perf - Get the min performance register value.
+ * @cpu: CPU from which to get min performance.
+ * @min_perf: Return address.
+ *
+ * Return: 0 for success, -EIO on register access failure,
-EOPNOTSUPP if not supported.
+ */
+int cppc_get_min_perf(int cpu, u64 *min_perf)
+{
+ return cppc_get_reg_val(cpu, MIN_PERF, min_perf);
+}
+EXPORT_SYMBOL_GPL(cppc_get_min_perf);
+
+/**
+ * cppc_set_min_perf() - Write the min performance register.
+ * @cpu: CPU on which to write register.
+ * @min_perf: Value to write to the MIN_PERF register.
+ *
+ * Return: 0 for success, -EIO otherwise.
+ */
+int cppc_set_min_perf(int cpu, u64 min_perf)
+{
+ return cppc_set_reg_val(cpu, MIN_PERF, min_perf);
+}
+EXPORT_SYMBOL_GPL(cppc_set_min_perf);
+
+/**
+ * cppc_get_max_perf - Get the max performance register value.
+ * @cpu: CPU from which to get max performance.
+ * @max_perf: Return address.
+ *
+ * Return: 0 for success, -EIO on register access failure,
-EOPNOTSUPP if not supported.
+ */
+int cppc_get_max_perf(int cpu, u64 *max_perf)
+{
+ return cppc_get_reg_val(cpu, MAX_PERF, max_perf);
+}
+EXPORT_SYMBOL_GPL(cppc_get_max_perf);
+
+/**
+ * cppc_set_max_perf() - Write the max performance register.
+ * @cpu: CPU on which to write register.
+ * @max_perf: Value to write to the MAX_PERF register.
+ *
+ * Return: 0 for success, -EIO otherwise.
+ */
+int cppc_set_max_perf(int cpu, u64 max_perf)
+{
+ return cppc_set_reg_val(cpu, MAX_PERF, max_perf);
+}
+EXPORT_SYMBOL_GPL(cppc_set_max_perf);
+
/**
* cppc_get_perf - Get a CPU's performance controls.
* @cpu: CPU for which to get performance controls.
diff --git a/drivers/cpufreq/cppc_cpufreq.c
b/drivers/cpufreq/cppc_cpufreq.c
index cf3ed6489a4f..cde6202e9c51 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -23,10 +23,12 @@
#include <uapi/linux/sched/types.h>
#include <linux/unaligned.h>
+#include <linux/cleanup.h>
#include <acpi/cppc_acpi.h>
static struct cpufreq_driver cppc_cpufreq_driver;
+static DEFINE_MUTEX(cppc_cpufreq_update_autosel_config_lock);
#ifdef CONFIG_ACPI_CPPC_CPUFREQ_FIE
static enum {
@@ -582,6 +584,68 @@ static void cppc_cpufreq_put_cpu_data(struct
cpufreq_policy *policy)
policy->driver_data = NULL;
}
+/**
+ * cppc_cpufreq_set_mperf_limit - Generic function to set min/max
performance limit
+ * @policy: cpufreq policy
+ * @val: performance value to set
+ * @update_reg: whether to update hardware register
I m not sure I see in which case we might not want to update the
hardware register.
Isn't the min/max_perf values relevant even when autonomous selection is
disabled/absent ?
Explained in reply on 'patch 7/8'. Adding here also brief info.
When disabling auto_sel, only the policy limits are reset, the
min/max_perf registers are preserved.
When re-enabled, these preserved values are restored to both
hardware reg and policy.
+ * @update_policy: whether to update policy constraints
+ * @is_min: true for min_perf, false for max_perf
+ */
+static int cppc_cpufreq_set_mperf_limit(struct cpufreq_policy
*policy, u64 val,
+ bool update_reg, bool
update_policy, bool is_min)
+{
+ struct cppc_cpudata *cpu_data = policy->driver_data;
+ struct cppc_perf_caps *caps = &cpu_data->perf_caps;
+ unsigned int cpu = policy->cpu;
+ struct freq_qos_request *req;
+ unsigned int freq;
+ u32 perf;
+ int ret;
+
+ perf = clamp(val, caps->lowest_perf, caps->highest_perf);
+ freq = cppc_perf_to_khz(caps, perf);
+
+ pr_debug("cpu%d, %s_perf:%llu, update_reg:%d,
update_policy:%d\n", cpu,
+ is_min ? "min" : "max", (u64)perf, update_reg,
update_policy);
+
+ guard(mutex)(&cppc_cpufreq_update_autosel_config_lock);
+
+ if (update_reg) {
+ ret = is_min ? cppc_set_min_perf(cpu, perf) :
cppc_set_max_perf(cpu, perf);
+ if (ret) {
+ if (ret != -EOPNOTSUPP)
+ pr_warn("Failed to set %s_perf (%llu)
on CPU%d (%d)\n",
+ is_min ? "min" : "max",
(u64)perf, cpu, ret);
+ return ret;
+ }
+
+ if (is_min)
+ cpu_data->perf_ctrls.min_perf = perf;
+ else
+ cpu_data->perf_ctrls.max_perf = perf;
+ }
+
+ if (update_policy) {
+ req = is_min ? policy->min_freq_req :
policy->max_freq_req;
+
+ ret = freq_qos_update_request(req, freq);
IIUC, we are adding a qos constraint to the min_freq_req or
max_freq_req. However these constraints should match the
scaling_min/max_freq sysfs interface. So doesn't it mean that if we set
the 'max_perf', we are overwriting the the max_freq_req constraint ?
Yes.
If you have frequencies between 600000:1200000 # Init state:
max_perf:1200000 scaling_max_freq:1200000 # echo 10000000 > max_perf
max_perf:1000000 scaling_max_freq:1000000 # echo 900000 >
scaling_max_freq max_perf:1000000 scaling_max_freq:900000 # echo 1200000
> scaling_max_freq max_perf:1000000 scaling_max_freq:1200000
The 2 values are not in sync. Is it the desired behaviour ?
Making scaling_min/max_freq read-only in auto_sel mode will solve this.
We can do this by setting policy limits to min/max_perf bounds in
cppc_verify_policy() when the auto_sel is enabled.
In autonomous mode, the hardware controls performance within these
bounds, so scaling_min/max_freq is effectively read-only.
Users must use min_perf/max_perf sysfs to change limits.
Please share if you have different thoughts or another approach.
cppc_verify_policy(struct cpufreq_policy_data *policy_data)
{
...
if (caps->auto_sel) {
min_perf = cpu_data->perf_ctrls.min_perf ?:
caps->lowest_nonlinear_perf;
max_perf = cpu_data->perf_ctrls.max_perf ?: caps->nominal_perf;
/* set min/max_perf bounds (read-only behavior) */
policy_data->min = cppc_perf_to_khz(caps, min_perf);
policy_data->max = cppc_perf_to_khz(caps, max_perf);
} else {
cpufreq_verify_within_limits(policy_data, min_freq, max_freq);
}
....
}
+ if (ret < 0) {
+ pr_warn("Failed to update %s_freq constraint
for CPU%d: %d\n",
+ is_min ? "min" : "max", cpu, ret);
+ return ret;
+ }
+ }
+
+ return 0;
+}
+
+#define cppc_cpufreq_set_min_perf(policy, val, update_reg,
update_policy) \
+ cppc_cpufreq_set_mperf_limit(policy, val, update_reg,
update_policy, true)
+
+#define cppc_cpufreq_set_max_perf(policy, val, update_reg,
update_policy) \
+ cppc_cpufreq_set_mperf_limit(policy, val, update_reg,
update_policy, false)
+
static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
{
unsigned int cpu = policy->cpu;
@@ -881,16 +945,118 @@ static ssize_t
store_energy_performance_preference_val(struct cpufreq_policy *po
return cppc_cpufreq_sysfs_store_u64(policy->cpu, cppc_set_epp,
buf, count);
}
+/**
+ * show_min_perf - Show minimum performance as frequency (kHz)
+ *
+ * Reads the MIN_PERF register and converts the performance value to
+ * frequency (kHz) for user-space consumption.
+ */
+static ssize_t show_min_perf(struct cpufreq_policy *policy, char *buf)
+{
+ struct cppc_cpudata *cpu_data = policy->driver_data;
+ u64 perf;
+ int ret;
+
+ ret = cppc_get_min_perf(policy->cpu, &perf);
+ if (ret == -EOPNOTSUPP)
+ return sysfs_emit(buf, "<unsupported>\n");
+ if (ret)
+ return ret;
+
+ /* Convert performance to frequency (kHz) for user */
+ return sysfs_emit(buf, "%u\n",
cppc_perf_to_khz(&cpu_data->perf_caps, perf));
+}
+
+/**
+ * store_min_perf - Set minimum performance from frequency (kHz)
+ *
+ * Converts the user-provided frequency (kHz) to a performance value
+ * and writes it to the MIN_PERF register.
+ */
+static ssize_t store_min_perf(struct cpufreq_policy *policy, const
char *buf, size_t count)
+{
+ struct cppc_cpudata *cpu_data = policy->driver_data;
+ unsigned int freq_khz;
+ u64 perf;
+ int ret;
+
+ ret = kstrtouint(buf, 0, &freq_khz);
+ if (ret)
+ return ret;
+
+ /* Convert frequency (kHz) to performance value */
+ perf = cppc_khz_to_perf(&cpu_data->perf_caps, freq_khz);
+
+ ret = cppc_cpufreq_set_min_perf(policy, perf, true,
cpu_data->perf_caps.auto_sel);
+ if (ret)
+ return ret;
+
+ return count;
+}
+
+/**
+ * show_max_perf - Show maximum performance as frequency (kHz)
+ *
+ * Reads the MAX_PERF register and converts the performance value to
+ * frequency (kHz) for user-space consumption.
+ */
+static ssize_t show_max_perf(struct cpufreq_policy *policy, char *buf)
I think it might collide with the scaling_min/max_freq.
I saw that you answered this point at:
https://lore.kernel.org/lkml/b2bd3258-51bd-462a-ae29-71f1d6f823f3@xxxxxxxxxx/
But I m not sure I understood why it is needed to have 2 interfaces.
Would it be possible to explain it again ?
Separate interface for min/max_perf are kept because we are writing
to different CPPC hardware registers with that name.
I don't see any case where we would like to make a distinction between:
- scaling_max_freq, i.e. the maximal freq. the cpufreq driver is allowed
to set
- max_perf, i.e. the maximal perf. level the firmware will set
------------
Another point is that the min/max_perf interface actually uses freq.
values.
Changed the min/max_perf interfaces from perf to freq to sync their scale
with other cpufreq sysfs interfaces after discussion in [1].
[1]
https://lore.kernel.org/lkml/80e16de0-63e4-4ead-9577-4ebba9b1a02d@xxxxxxxxxx/
Thank you,
Sumit Gupta