Re: [PATCH v6 1/2] ACPI: CPPC: Add ospm_nominal_perf support
From: Sumit Gupta
Date: Tue Aug 04 2026 - 12:21:32 EST
Hi Zhongqiu,
On 28/07/26 18:54, Zhongqiu Han wrote:
External email: Use caution opening links or attachments
Hello Sumit,
On 7/18/2026 5:53 AM, Sumit Gupta wrote:
Expose the OSPM Nominal Performance register (ACPI 6.6, Section
8.4.6.1.2.6), which conveys the desired nominal performance level
at which the platform may run. Unlike the existing read-only
Nominal Performance register, it is writable and lets OSPM
request a lower nominal level than the platform-reported nominal.
The platform classifies performance above this level as boosted
and below as throttled for its power/thermal decisions.
It is exposed as a per-policy cpufreq sysfs attribute in kHz, to
match the cpufreq sysfs unit convention:
/sys/devices/system/cpu/cpuX/cpufreq/ospm_nominal_freq
The attribute is documented in
Documentation/ABI/testing/sysfs-devices-system-cpu.
Writes are converted to perf via cppc_khz_to_perf(), validated
against [Lowest Performance, Nominal Performance], and applied to
the policy->cpu. The register is assumed shared across the
policy->cpus.
On read, the current register value is returned, or
"<unsupported>" if the platform does not implement the register.
Also add the register to the OSPM-set register save/restore
table, so its value survives CPU hotplug and reverts to the
firmware value on driver unload, like the other registers in
the table.
Signed-off-by: Sumit Gupta <sumitg@xxxxxxxxxx>
---
.../ABI/testing/sysfs-devices-system-cpu | 26 ++++++++++
drivers/acpi/cppc_acpi.c | 32 +++++++++++++
drivers/cpufreq/cppc_cpufreq.c | 47 +++++++++++++++++++
include/acpi/cppc_acpi.h | 10 ++++
4 files changed, 115 insertions(+)
diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 82d10d556cc8..a8d592c08823 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -346,6 +346,32 @@ Description: Performance Limited
This file is only present if the cppc-cpufreq driver is in use.
+What: /sys/devices/system/cpu/cpuX/cpufreq/ospm_nominal_freq
+Date: May 2026
+Contact: linux-pm@xxxxxxxxxxxxxxx
+Description: OSPM Nominal Performance (kHz)
+
+ OSPM uses this attribute to request a nominal performance
+ level lower than the platform-reported nominal. The
+ platform treats performance above this level as boost
+ and below as throttle for power and thermal decisions.
+
+ Read returns the current value in kHz, or "<unsupported>"
+ if the platform does not implement the register. Write a
+ kHz value in the range [lowest_freq, nominal_freq].
+
+ Note that tasks may be migrated from one CPU to another
+ by the scheduler's load-balancing algorithm, and if
+ different OSPM Nominal Performance values are set for
+ those CPUs (through different cpufreq policies), that may
+ lead to undesirable outcomes. To avoid such issues it is
+ better to set the same value across all policies, or to
+ pin every task potentially sensitive to it to a specific
+ CPU.
+
+ This file is only present if the cppc-cpufreq driver is
+ in use.
+
What: /sys/devices/system/cpu/cpu*/cache/index3/cache_disable_{0,1}
Date: August 2008
KernelVersion: 2.6.27
diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index a7fec6c93178..681d4fd40c11 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1685,6 +1685,38 @@ int cppc_set_epp(int cpu, u64 epp_val)
}
EXPORT_SYMBOL_GPL(cppc_set_epp);
+/**
+ * cppc_set_ospm_nominal_perf() - Write OSPM Nominal Performance register.
+ * @cpu: CPU on which to write register.
+ * @ospm_nominal_perf: Value to write to the OSPM Nominal Performance register.
+ *
+ * OSPM Nominal Performance conveys the desired nominal performance level
+ * at which the platform may run. Per ACPI 6.6, s8.4.6.1.2.6, the value
+ * must lie within [Lowest Performance, Nominal Performance] and may be
+ * set independently of Minimum, Maximum and Desired performance. The
+ * caller is responsible for validating the range.
+ *
+ * Return: 0 on success or negative error code.
+ */
+int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf)
+{
+ return cppc_set_reg_val(cpu, OSPM_NOMINAL_PERF, ospm_nominal_perf);
+}
+EXPORT_SYMBOL_GPL(cppc_set_ospm_nominal_perf);
+
+/**
+ * cppc_get_ospm_nominal_perf() - Read OSPM Nominal Performance register.
+ * @cpu: CPU from which to read register.
+ * @ospm_nominal_perf: Pointer to store the OSPM Nominal Performance value.
+ *
+ * Return: 0 on success or negative error code.
+ */
+int cppc_get_ospm_nominal_perf(int cpu, u64 *ospm_nominal_perf)
+{
+ return cppc_get_reg_val(cpu, OSPM_NOMINAL_PERF, ospm_nominal_perf);
+}
+EXPORT_SYMBOL_GPL(cppc_get_ospm_nominal_perf);
+
/**
* cppc_get_auto_act_window() - Read autonomous activity window register.
* @cpu: CPU from which to read register.
diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index 9c88512d635c..eb6746810fa6 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -37,6 +37,7 @@ enum cppc_saved_reg_id {
CPPC_SAVED_AUTO_SEL,
CPPC_SAVED_EPP,
CPPC_SAVED_AUTO_ACT_WINDOW,
+ CPPC_SAVED_OSPM_NOMINAL_PERF,
CPPC_NR_SAVED_REGS,
};
@@ -55,6 +56,9 @@ static const struct cppc_saved_reg cppc_saved_regs[CPPC_NR_SAVED_REGS] = {
[CPPC_SAVED_AUTO_ACT_WINDOW] = {
cppc_get_auto_act_window, cppc_set_auto_act_window,
},
+ [CPPC_SAVED_OSPM_NOMINAL_PERF] = {
+ cppc_get_ospm_nominal_perf, cppc_set_ospm_nominal_perf,
+ },
};
/*
@@ -1166,11 +1170,53 @@ static int cppc_get_perf_limited_filtered(int cpu, u64 *perf_limited)
CPPC_CPUFREQ_ATTR_RW_U64(perf_limited, cppc_get_perf_limited_filtered,
cppc_set_perf_limited)
+static ssize_t show_ospm_nominal_freq(struct cpufreq_policy *policy, char *buf)
+{
+ struct cppc_cpudata *cpu_data = policy->driver_data;
+ u64 perf;
+ int ret;
+
+ ret = cppc_get_ospm_nominal_perf(policy->cpu, &perf);
+ if (ret == -EOPNOTSUPP)
+ return sysfs_emit(buf, "<unsupported>\n");
+ if (ret)
+ return ret;
+
+ return sysfs_emit(buf, "%u\n",
+ cppc_perf_to_khz(&cpu_data->perf_caps, perf));
In patch 2/2, cppc_cpufreq_get_effective_nominal() reads the OSPM
Nominal into ospm_nominal via the output pointer of
cppc_get_ospm_nominal_perf(policy->cpu, &ospm_nominal), then checks
ospm_nominal for zero ("A zero value means OSPM has not selected a
nominal level") and substitutes the platform Nominal in that case.
However, show_ospm_nominal_freq() passes the value read through the same
output pointer straight into cppc_perf_to_khz() without the equivalent
zero check. This looks a bit inconsistent — is it intentional, or
should show() apply the same "zero means not selected" handling?
Thanks for pointing this out.
In v7, I will make ospm_nominal_freq as write-only and remove sysfs
show() and cppc_get_ospm_nominal_perf(), eliminating this inconsistency.
The effective nominal calculation will use the last successfully
written value, falling back to the platform Nominal Performance if none
has been written.
Thanks,
Sumit
+}
+
+static ssize_t store_ospm_nominal_freq(struct cpufreq_policy *policy,
+ const char *buf, size_t count)
+{
+ struct cppc_cpudata *cpu_data = policy->driver_data;
+ unsigned int freq_khz;
+ u32 perf;
+ int ret;
+
+ ret = kstrtouint(buf, 0, &freq_khz);
+ if (ret)
+ return ret;
+
+ perf = cppc_khz_to_perf(&cpu_data->perf_caps, freq_khz);
+ if (perf < cpu_data->perf_caps.lowest_perf ||
+ perf > cpu_data->perf_caps.nominal_perf)
+ return -EINVAL;
+
+ /* The register is assumed shared across the policy's CPUs. */
+ ret = cppc_set_ospm_nominal_perf(policy->cpu, perf);
+ if (ret)
+ return ret;
+
+ return count;
+}
+
cpufreq_freq_attr_ro(freqdomain_cpus);
cpufreq_freq_attr_rw(auto_select);
cpufreq_freq_attr_rw(auto_act_window);
cpufreq_freq_attr_rw(energy_performance_preference_val);
cpufreq_freq_attr_rw(perf_limited);
+cpufreq_freq_attr_rw(ospm_nominal_freq);
static struct freq_attr *cppc_cpufreq_attr[] = {
&freqdomain_cpus,
@@ -1178,6 +1224,7 @@ static struct freq_attr *cppc_cpufreq_attr[] = {
&auto_act_window,
&energy_performance_preference_val,
&perf_limited,
+ &ospm_nominal_freq,
NULL,
};
diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
index cd07e1e92bf4..4c9e59643cc0 100644
--- a/include/acpi/cppc_acpi.h
+++ b/include/acpi/cppc_acpi.h
@@ -180,6 +180,8 @@ extern int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val);
extern int cppc_get_epp_perf(int cpunum, u64 *epp_perf);
extern int cppc_set_epp_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls, bool enable);
extern int cppc_set_epp(int cpu, u64 epp_val);
+extern int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf);
+extern int cppc_get_ospm_nominal_perf(int cpu, u64 *ospm_nominal_perf);
extern int cppc_get_auto_act_window(int cpu, u64 *auto_act_window);
extern int cppc_set_auto_act_window(int cpu, u64 auto_act_window);
extern int cppc_get_auto_sel(int cpu, bool *enable);
@@ -268,6 +270,14 @@ static inline int cppc_set_epp(int cpu, u64 epp_val)
{
return -EOPNOTSUPP;
}
+static inline int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf)
+{
+ return -EOPNOTSUPP;
+}
+static inline int cppc_get_ospm_nominal_perf(int cpu, u64 *ospm_nominal_perf)
+{
+ return -EOPNOTSUPP;
+}
static inline int cppc_get_auto_act_window(int cpu, u64 *auto_act_window)
{
return -EOPNOTSUPP;
--
Thx and BRs,
Zhongqiu Han