[PATCH v5 06/11] ACPI: CPPC: add APIs and sysfs interface for perf_limited

Sumit Gupta
2025-12-23T12:14:39Z

Add sysfs interface to read/write the Performance Limited register.

The Performance Limited register indicates to the OS that an
unpredictable event (like thermal throttling) has limited processor
performance. It contains two sticky bits set by the platform:
- Bit 0 (Desired_Excursion): Set when delivered performance is
constrained below desired performance. Not used when Autonomous
Selection is enabled.
- Bit 1 (Minimum_Excursion): Set when delivered performance is
constrained below minimum performance.

These bits remain set until OSPM explicitly clears them. The write
operation accepts a bitmask of bits to clear:
- Write 1 to clear bit 0
- Write 2 to clear bit 1
- Write 3 to clear both bits

This enables users to detect if platform throttling impacted a workload.
Users clear the register before execution, run the workload, then check
afterward - if set, hardware throttling occurred during that time window.

The interface is exposed as:
/sys/devices/system/cpu/cpuX/cpufreq/perf_limited

Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
---
drivers/acpi/cppc_acpi.c | 56 ++++++++++++++++++++++++++++++++++
drivers/cpufreq/cppc_cpufreq.c | 31 +++++++++++++++++++
include/acpi/cppc_acpi.h | 15 +++++++++
3 files changed, 102 insertions(+)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 9f28c20d902d..ffd866c1c00d 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1786,6 +1786,62 @@ int cppc_set_max_perf(int cpu, u32 max_perf)
}
EXPORT_SYMBOL_GPL(cppc_set_max_perf);

+/**
+ * cppc_get_perf_limited - Get the Performance Limited register value.
+ * @cpu: CPU from which to get Performance Limited register.
+ * @perf_limited: Pointer to store the Performance Limited value.
+ *
+ * The returned value contains sticky status bits indicating platform-imposed
+ * performance limitations.
+ *
+ * Return: 0 for success, -EIO on failure, -EOPNOTSUPP if not supported.
+ */
+int cppc_get_perf_limited(int cpu, u64 *perf_limited)
+{
+ return cppc_get_reg_val(cpu, PERF_LIMITED, perf_limited);
+}
+EXPORT_SYMBOL_GPL(cppc_get_perf_limited);
+
+/**
+ * cppc_set_perf_limited() - Clear bits in the Performance Limited register.
+ * @cpu: CPU on which to write register.
+ * @bits_to_clear: Bitmask of bits to clear in the perf_limited register.
+ *
+ * The Performance Limited register contains two sticky bits set by platform:
+ * - Bit 0 (Desired_Excursion): Set when delivered performance is constrained
+ * below desired performance. Not used when Autonomous Selection is enabled.
+ * - Bit 1 (Minimum_Excursion): Set when delivered performance is constrained
+ * below minimum performance.
+ *
+ * These bits are sticky and remain set until OSPM explicitly clears them.
+ * This function only allows clearing bits (the platform sets them).
+ *
+ * Return: 0 for success, -EINVAL for invalid bits, -EIO on register
+ * access failure, -EOPNOTSUPP if not supported.
+ */
+int cppc_set_perf_limited(int cpu, u64 bits_to_clear)
+{
+ u64 current_val, new_val;
+ int ret;
+
+ /* Only bits 0 and 1 are valid */
+ if (bits_to_clear & ~CPPC_PERF_LIMITED_MASK)
+ return -EINVAL;
+
+ if (!bits_to_clear)
+ return 0;
+
+ ret = cppc_get_perf_limited(cpu, &current_val);
+ if (ret)
+ return ret;
+
+ /* Clear the specified bits */
+ new_val = current_val & ~bits_to_clear;
+
+ return cppc_set_reg_val(cpu, PERF_LIMITED, new_val);
+}
+EXPORT_SYMBOL_GPL(cppc_set_perf_limited);
+
/**
* cppc_set_enable - Set to enable CPPC on the processor by writing the
* Continuous Performance Control package EnableRegister field.
diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index 1e282dfabc76..1f8825006940 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -1052,12 +1052,42 @@ static ssize_t store_max_perf(struct cpufreq_policy *policy, const char *buf,
return count;
}

+/**
+ * show_perf_limited - Show Performance Limited register status
+ * @policy: cpufreq policy
+ * @buf: buffer to write the value to
+ *
+ * Read the Performance Limited register to check if platform throttling
+ * (thermal/power/current limits) occurred.
+ */
+static ssize_t show_perf_limited(struct cpufreq_policy *policy, char *buf)
+{
+ return cppc_cpufreq_sysfs_show_u64(policy->cpu,
+ cppc_get_perf_limited, buf);
+}
+
+/**
+ * store_perf_limited - Clear Performance Limited register bits
+ * @policy: cpufreq policy
+ * @buf: buffer containing the bitmask of bits to clear
+ * @count: number of bytes in buf
+ *
+ * Write 1 to clear bit 0, 2 to clear bit 1, or 3 to clear both.
+ */
+static ssize_t store_perf_limited(struct cpufreq_policy *policy,
+ const char *buf, size_t count)
+{
+ return cppc_cpufreq_sysfs_store_u64(policy->cpu,
+ cppc_set_perf_limited, buf, 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(min_perf);
cpufreq_freq_attr_rw(max_perf);
+cpufreq_freq_attr_rw(perf_limited);

static struct freq_attr *cppc_cpufreq_attr[] = {
&freqdomain_cpus,
@@ -1066,6 +1096,7 @@ static struct freq_attr *cppc_cpufreq_attr[] = {
&energy_performance_preference_val,
&min_perf,
&max_perf,
+ &perf_limited,
NULL,
};

diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
index a49b50bddaf9..57e04326a4b6 100644
--- a/include/acpi/cppc_acpi.h
+++ b/include/acpi/cppc_acpi.h
@@ -42,6 +42,11 @@
#define CPPC_EPP_PERFORMANCE_PREF 0x00
#define CPPC_EPP_ENERGY_EFFICIENCY_PREF 0xFF

+#define CPPC_PERF_LIMITED_DESIRED_EXCURSION BIT(0)
+#define CPPC_PERF_LIMITED_MINIMUM_EXCURSION BIT(1)
+#define CPPC_PERF_LIMITED_MASK (CPPC_PERF_LIMITED_DESIRED_EXCURSION | \
+ CPPC_PERF_LIMITED_MINIMUM_EXCURSION)
+
/* Each register has the folowing format. */
struct cpc_reg {
u8 descriptor;
@@ -177,6 +182,8 @@ extern int cppc_get_min_perf(int cpu, u64 *min_perf);
extern int cppc_set_min_perf(int cpu, u32 min_perf);
extern int cppc_get_max_perf(int cpu, u64 *max_perf);
extern int cppc_set_max_perf(int cpu, u32 max_perf);
+extern int cppc_get_perf_limited(int cpu, u64 *perf_limited);
+extern int cppc_set_perf_limited(int cpu, u64 perf_limited);
extern int amd_get_highest_perf(unsigned int cpu, u32 *highest_perf);
extern int amd_get_boost_ratio_numerator(unsigned int cpu, u64 *numerator);
extern int amd_detect_prefcore(bool *detected);
@@ -285,6 +292,14 @@ static inline int cppc_set_max_perf(int cpu, u32 max_perf)
{
return -EOPNOTSUPP;
}
+static inline int cppc_get_perf_limited(int cpu, u64 *perf_limited)
+{
+ return -EOPNOTSUPP;
+}
+static inline int cppc_set_perf_limited(int cpu, u64 perf_limited)
+{
+ return -EOPNOTSUPP;
+}
static inline int amd_get_highest_perf(unsigned int cpu, u32 *highest_perf)
{
return -ENODEV;
--
2.34.1