Re: [PATCH v6 1/2] ACPI: CPPC: Add ospm_nominal_perf support
From: Sumit Gupta
Date: Tue Aug 04 2026 - 12:59:04 EST
On 28/07/26 02:53, Christian Loehle wrote:
External email: Use caution opening links or attachments
On 7/27/26 18:49, Christian Loehle wrote:
On 7/27/26 15:01, Christian Loehle wrote:FWIW desired_perf was read/write up until ACPI 6.5 and only 6.6 marked
On 7/17/26 22:53, Sumit Gupta wrote:So just reread Pierre's comments, TBH I don't see the point of ever
Expose the OSPM Nominal Performance register (ACPI 6.6, SectionIt's a write-only register, we need to track everything in the driver.
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);
reading it, even for sysfs reads, but I don't think
reading it for cppc_cpufreq_get_effective_nominal() would be valid
in any case?
it as write-only. I don't think we should be reading it either on
platforms advertising 6.6.
I will clarify and send a patch.
Good catch that ACPI 6.6 makes Desired Performance write-only,
and thanks for the patches preventing those reads.
The same reasoning applies to OSPM Nominal Performance, so I will treat
it as write-only: make ospm_nominal_freq write-only, remove show(), and
drop cppc_get_ospm_nominal_perf().
Boost and policy limits will use the last value the driver successfully
wrote, falling back to the platform reported Nominal Performance when
nothing has been set.
On unload, I will reset it to the platform Nominal Performance only if
the driver wrote it during its lifetime. Otherwise the register is left
untouched, since the pre-driver value cannot be read back.
Thanks,
Sumit