[PATCH v2 0/2] ACPI: CPPC: Add CPPC v4 support (ACPI 6.6)

[PATCH v2 0/2] ACPI: CPPC: Add CPPC v4 support (ACPI 6.6)

[Sumit Gupta]

Add initial kernel support for CPPC v4 (ACPI 6.6, Section 8.4.6),
which extends the _CPC package from 23 to 25 entries with two
optional fields:

  - OSPM Nominal Performance (8.4.6.1.2.6): register used by OSPM
    to tell the platform what it considers nominal. The platform
    classifies performance above this as boost and below as
    throttle for power/thermal decisions.

  - Resource Priority (8.4.6.1.2.7): Package of Resource Priority
    Register Descriptor sub-packages. Full parsing is not yet
    implemented; such entries are marked as unsupported.

Patch 1: Add v4 _CPC parsing - validate the 25-entry layout,
handle the Resource Priority package, and mark the two new
registers optional.

Patch 2: Add acpi_cppc/ospm_nominal_perf as a read-write sysfs
attribute, and initialize it to the platform nominal value
during cppc_cpufreq policy init.

---
v1[1] -> v2:
- Patch 1: added Reviewed-by from Mario Limonciello.
- Patch 2:
  - Make ospm_nominal_perf sysfs read-write; cache last write in
    cpc_desc and skip redundant register writes.
  - Validate input in cppc_set_ospm_nominal_perf.

Sumit Gupta (2):
  ACPI: CPPC: Add support for CPPC v4
  ACPI: CPPC: Add ospm_nominal_perf support

 drivers/acpi/cppc_acpi.c       | 93 +++++++++++++++++++++++++++++++---
 drivers/cpufreq/cppc_cpufreq.c | 10 ++++
 include/acpi/cppc_acpi.h       | 14 ++++-
 3 files changed, 109 insertions(+), 8 deletions(-)

[1] https://lore.kernel.org/lkml/20260427051823.280419-1-sumitg@nvidia.com/

-- 
2.34.1

[PATCH v2 1/2] ACPI: CPPC: Add support for CPPC v4

[Sumit Gupta]

CPPC v4 (ACPI 6.6, Section 8.4.6) adds two optional entries to the
_CPC package:

1. OSPM Nominal Performance (8.4.6.1.2.6): A write-only register that
   lets OSPM inform the platform what it considers nominal performance.
   The platform classifies performance above this level as boost and
   below as throttle for its power/thermal decisions.

2. Resource Priority (8.4.6.1.2.7): A Package of Resource Priority
   Register Descriptor sub-packages that allow OSPM to set relative
   priority among processors for shared resources (boost, throttle,
   L2/L3 cache, memory bandwidth). Parsing the full structure is not
   yet supported; such entries are marked as unsupported.

Add v4 _CPC table parsing (25 entries) and update REG_OPTIONAL to
mark the two new registers as optional.

Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
---
 drivers/acpi/cppc_acpi.c | 24 ++++++++++++++++++------
 include/acpi/cppc_acpi.h |  8 ++++++--
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 2e91c5a97761..a1c91ce20cc8 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -134,7 +134,7 @@ static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
  * cpc_regs[] with the corresponding index. 0 means mandatory and 1
  * means optional.
  */
-#define REG_OPTIONAL (0x1FC7D0)
+#define REG_OPTIONAL (0x7FC7D0)
 
 /*
  * Use the index of the register in per-cpu cpc_regs[] to check if
@@ -751,18 +751,19 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
 	/*
 	 * Disregard _CPC if the number of entries in the return package is not
 	 * as expected, but support future revisions being proper supersets of
-	 * the v3 and only causing more entries to be returned by _CPC.
+	 * the v4 and only causing more entries to be returned by _CPC.
 	 */
 	if ((cpc_rev == CPPC_V2_REV && num_ent != CPPC_V2_NUM_ENT) ||
 	    (cpc_rev == CPPC_V3_REV && num_ent != CPPC_V3_NUM_ENT) ||
-	    (cpc_rev > CPPC_V3_REV && num_ent <= CPPC_V3_NUM_ENT)) {
+	    (cpc_rev == CPPC_V4_REV && num_ent != CPPC_V4_NUM_ENT) ||
+	    (cpc_rev > CPPC_V4_REV && num_ent <= CPPC_V4_NUM_ENT)) {
 		pr_debug("Unexpected number of _CPC return package entries (%d) for CPU:%d\n",
 			 num_ent, pr->id);
 		goto out_free;
 	}
-	if (cpc_rev > CPPC_V3_REV) {
-		num_ent = CPPC_V3_NUM_ENT;
-		cpc_rev = CPPC_V3_REV;
+	if (cpc_rev > CPPC_V4_REV) {
+		num_ent = CPPC_V4_NUM_ENT;
+		cpc_rev = CPPC_V4_REV;
 	}
 
 	cpc_ptr->num_entries = num_ent;
@@ -845,6 +846,17 @@ int acpi_cppc_processor_probe(struct acpi_processor *pr)
 
 			cpc_ptr->cpc_regs[i-2].type = ACPI_TYPE_BUFFER;
 			memcpy(&cpc_ptr->cpc_regs[i-2].cpc_entry.reg, gas_t, sizeof(*gas_t));
+		} else if (cpc_obj->type == ACPI_TYPE_PACKAGE) {
+			/*
+			 * ACPI 6.6, s8.4.6.1.2.7 defines Resource Priority
+			 * as a Package of Resource Priority Register Descriptor
+			 * sub-packages.  Parsing the full structure is not yet
+			 * supported; mark the register as unsupported for now.
+			 */
+			pr_debug("CPU:%d entry %d: package type not supported\n",
+				 pr->id, i);
+			cpc_ptr->cpc_regs[i-2].type = ACPI_TYPE_INTEGER;
+			cpc_ptr->cpc_regs[i-2].cpc_entry.int_value = 0;
 		} else {
 			pr_debug("Invalid entry type (%d) in _CPC for CPU:%d\n",
 				 i, pr->id);
diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
index d1f02ceec4f9..8693890a7275 100644
--- a/include/acpi/cppc_acpi.h
+++ b/include/acpi/cppc_acpi.h
@@ -17,16 +17,18 @@
 #include <acpi/pcc.h>
 #include <acpi/processor.h>
 
-/* CPPCv2 and CPPCv3 support */
+/* CPPCv2, CPPCv3 and CPPCv4 support */
 #define CPPC_V2_REV	2
 #define CPPC_V3_REV	3
+#define CPPC_V4_REV	4
 #define CPPC_V2_NUM_ENT	21
 #define CPPC_V3_NUM_ENT	23
+#define CPPC_V4_NUM_ENT	25
 
 #define PCC_CMD_COMPLETE_MASK	(1 << 0)
 #define PCC_ERROR_MASK		(1 << 2)
 
-#define MAX_CPC_REG_ENT 21
+#define MAX_CPC_REG_ENT 23
 
 /* CPPC specific PCC commands. */
 #define	CMD_READ 0
@@ -109,6 +111,8 @@ enum cppc_regs {
 	REFERENCE_PERF,
 	LOWEST_FREQ,
 	NOMINAL_FREQ,
+	OSPM_NOMINAL_PERF,
+	RESOURCE_PRIORITY,
 };
 
 /*
-- 
2.34.1

Re: [PATCH v2 1/2] ACPI: CPPC: Add support for CPPC v4

[Pierre Gondois]

Hello Sumit,

On 4/30/26 16:24, Sumit Gupta wrote:
> CPPC v4 (ACPI 6.6, Section 8.4.6) adds two optional entries to the
> _CPC package:
>
> 1. OSPM Nominal Performance (8.4.6.1.2.6): A write-only register that
>     lets OSPM inform the platform what it considers nominal performance.
>     The platform classifies performance above this level as boost and
>     below as throttle for its power/thermal decisions.
>
> 2. Resource Priority (8.4.6.1.2.7): A Package of Resource Priority
>     Register Descriptor sub-packages that allow OSPM to set relative
>     priority among processors for shared resources (boost, throttle,
>     L2/L3 cache, memory bandwidth). Parsing the full structure is not
>     yet supported; such entries are marked as unsupported.
>
> Add v4 _CPC table parsing (25 entries) and update REG_OPTIONAL to
> mark the two new registers as optional.
>
> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
> Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>

Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>

[PATCH v2 2/2] ACPI: CPPC: Add ospm_nominal_perf support

[Sumit Gupta]

Add acpi_cppc/ospm_nominal_perf sysfs attribute (read-write) and
cppc_set_ospm_nominal_perf() API for the OSPM Nominal Performance
register (ACPI 6.6, Section 8.4.6.1.2.6).

The register conveys the desired nominal performance level at which
the platform may run. OSPM can request a lower level than platform
nominal. Valid range is [Lowest Performance, Nominal Performance].
The value tells the platform what OSPM considers nominal. The
platform classifies performance above this as boosted and below as
throttled. It uses that for its power/thermal decisions.

Although the register is write-only per spec, cache the OSPM-written
value in cpc_desc so userspace can observe it via sysfs, and to
skip redundant writes.

Initialize to platform nominal at policy init. Override via sysfs
if needed.

Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
---
 drivers/acpi/cppc_acpi.c       | 69 ++++++++++++++++++++++++++++++++++
 drivers/cpufreq/cppc_cpufreq.c | 10 +++++
 include/acpi/cppc_acpi.h       |  6 +++
 3 files changed, 85 insertions(+)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index a1c91ce20cc8..fbc620adafad 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -155,6 +155,10 @@ static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
 static struct kobj_attribute _name =		\
 __ATTR(_name, 0444, show_##_name, NULL)
 
+#define define_one_cppc_rw(_name)		\
+static struct kobj_attribute _name =		\
+__ATTR(_name, 0644, show_##_name, store_##_name)
+
 #define to_cpc_desc(a) container_of(a, struct cpc_desc, kobj)
 
 #define show_cppc_data(access_fn, struct_name, member_name)		\
@@ -211,6 +215,38 @@ static ssize_t show_feedback_ctrs(struct kobject *kobj,
 }
 define_one_cppc_ro(feedback_ctrs);
 
+static ssize_t show_ospm_nominal_perf(struct kobject *kobj,
+				      struct kobj_attribute *attr, char *buf)
+{
+	struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
+	u64 val = READ_ONCE(cpc_ptr->ospm_nominal_perf);
+
+	if (!val)
+		return -ENODATA;
+
+	return sysfs_emit(buf, "%llu\n", val);
+}
+
+static ssize_t store_ospm_nominal_perf(struct kobject *kobj,
+				       struct kobj_attribute *attr,
+				       const char *buf, size_t count)
+{
+	struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
+	u64 val;
+	int ret;
+
+	ret = kstrtou64(buf, 0, &val);
+	if (ret)
+		return ret;
+
+	ret = cppc_set_ospm_nominal_perf(cpc_ptr->cpu_id, val);
+	if (ret)
+		return ret;
+
+	return count;
+}
+define_one_cppc_rw(ospm_nominal_perf);
+
 static struct attribute *cppc_attrs[] = {
 	&feedback_ctrs.attr,
 	&reference_perf.attr,
@@ -222,6 +258,7 @@ static struct attribute *cppc_attrs[] = {
 	&nominal_perf.attr,
 	&nominal_freq.attr,
 	&lowest_freq.attr,
+	&ospm_nominal_perf.attr,
 	NULL
 };
 ATTRIBUTE_GROUPS(cppc);
@@ -1683,6 +1720,38 @@ 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 allows OSPM to inform the platform of the nominal
+ * performance level it intends to maintain.
+ *
+ * Return: 0 for success, -EINVAL on invalid input, -EOPNOTSUPP if not
+ * supported, -EIO otherwise.
+ */
+int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf)
+{
+	struct cpc_desc *cpc_desc = per_cpu(cpc_desc_ptr, cpu);
+	int ret;
+
+	if (!ospm_nominal_perf || ospm_nominal_perf > U32_MAX)
+		return -EINVAL;
+
+	if (cpc_desc &&
+	    READ_ONCE(cpc_desc->ospm_nominal_perf) == ospm_nominal_perf)
+		return 0;
+
+	ret = cppc_set_reg_val(cpu, OSPM_NOMINAL_PERF, ospm_nominal_perf);
+	if (ret)
+		return ret;
+
+	WRITE_ONCE(cpc_desc->ospm_nominal_perf, ospm_nominal_perf);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(cppc_set_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 7e7f9dfb7a24..d06cba963550 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -715,6 +715,16 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
 		goto out;
 	}
 
+	/*
+	 * Initialize OSPM Nominal Performance to inform firmware of
+	 * OSPM's nominal level. Performance above this value = boost;
+	 * below = throttle. Uses platform nominal by default.
+	 */
+	ret = cppc_set_ospm_nominal_perf(cpu, caps->nominal_perf);
+	if (ret && ret != -EOPNOTSUPP)
+		pr_debug("Failed to set ospm_nominal_perf for CPU%d: %d\n",
+			 cpu, ret);
+
 	cppc_cpufreq_cpu_fie_init(policy);
 	return 0;
 
diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
index 8693890a7275..3771e2ed507d 100644
--- a/include/acpi/cppc_acpi.h
+++ b/include/acpi/cppc_acpi.h
@@ -86,6 +86,7 @@ struct cpc_desc {
 	struct cpc_register_resource cpc_regs[MAX_CPC_REG_ENT];
 	struct acpi_psd_package domain_info;
 	struct kobject kobj;
+	u32 ospm_nominal_perf;
 };
 
 /* These are indexes into the per-cpu cpc_regs[]. Order is important. */
@@ -180,6 +181,7 @@ 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_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);
@@ -266,6 +268,10 @@ 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_auto_act_window(int cpu, u64 *auto_act_window)
 {
 	return -EOPNOTSUPP;
-- 
2.34.1

Re: [PATCH v2 2/2] ACPI: CPPC: Add ospm_nominal_perf support

[Mario Limonciello]

On 4/30/26 09:24, Sumit Gupta wrote:
> Add acpi_cppc/ospm_nominal_perf sysfs attribute (read-write) and
> cppc_set_ospm_nominal_perf() API for the OSPM Nominal Performance
> register (ACPI 6.6, Section 8.4.6.1.2.6).
> 
> The register conveys the desired nominal performance level at which
> the platform may run. OSPM can request a lower level than platform
> nominal. Valid range is [Lowest Performance, Nominal Performance].
> The value tells the platform what OSPM considers nominal. The
> platform classifies performance above this as boosted and below as
> throttled. It uses that for its power/thermal decisions.
> 
> Although the register is write-only per spec, cache the OSPM-written
> value in cpc_desc so userspace can observe it via sysfs, and to
> skip redundant writes.
> 
> Initialize to platform nominal at policy init. Override via sysfs
> if needed.
> 
> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@kernel.org>
> ---
>   drivers/acpi/cppc_acpi.c       | 69 ++++++++++++++++++++++++++++++++++
>   drivers/cpufreq/cppc_cpufreq.c | 10 +++++
>   include/acpi/cppc_acpi.h       |  6 +++
>   3 files changed, 85 insertions(+)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index a1c91ce20cc8..fbc620adafad 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -155,6 +155,10 @@ static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
>   static struct kobj_attribute _name =		\
>   __ATTR(_name, 0444, show_##_name, NULL)
>   
> +#define define_one_cppc_rw(_name)		\
> +static struct kobj_attribute _name =		\
> +__ATTR(_name, 0644, show_##_name, store_##_name)
> +
>   #define to_cpc_desc(a) container_of(a, struct cpc_desc, kobj)
>   
>   #define show_cppc_data(access_fn, struct_name, member_name)		\
> @@ -211,6 +215,38 @@ static ssize_t show_feedback_ctrs(struct kobject *kobj,
>   }
>   define_one_cppc_ro(feedback_ctrs);
>   
> +static ssize_t show_ospm_nominal_perf(struct kobject *kobj,
> +				      struct kobj_attribute *attr, char *buf)
> +{
> +	struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
> +	u64 val = READ_ONCE(cpc_ptr->ospm_nominal_perf);
> +
> +	if (!val)
> +		return -ENODATA;
> +
> +	return sysfs_emit(buf, "%llu\n", val);
> +}
> +
> +static ssize_t store_ospm_nominal_perf(struct kobject *kobj,
> +				       struct kobj_attribute *attr,
> +				       const char *buf, size_t count)
> +{
> +	struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
> +	u64 val;
> +	int ret;
> +
> +	ret = kstrtou64(buf, 0, &val);
> +	if (ret)
> +		return ret;
> +
> +	ret = cppc_set_ospm_nominal_perf(cpc_ptr->cpu_id, val);
> +	if (ret)
> +		return ret;
> +
> +	return count;
> +}
> +define_one_cppc_rw(ospm_nominal_perf);
> +
>   static struct attribute *cppc_attrs[] = {
>   	&feedback_ctrs.attr,
>   	&reference_perf.attr,
> @@ -222,6 +258,7 @@ static struct attribute *cppc_attrs[] = {
>   	&nominal_perf.attr,
>   	&nominal_freq.attr,
>   	&lowest_freq.attr,
> +	&ospm_nominal_perf.attr,
>   	NULL
>   };
>   ATTRIBUTE_GROUPS(cppc);
> @@ -1683,6 +1720,38 @@ 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 allows OSPM to inform the platform of the nominal
> + * performance level it intends to maintain.
> + *
> + * Return: 0 for success, -EINVAL on invalid input, -EOPNOTSUPP if not
> + * supported, -EIO otherwise.
> + */
> +int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf)
> +{
> +	struct cpc_desc *cpc_desc = per_cpu(cpc_desc_ptr, cpu);
> +	int ret;
> +
> +	if (!ospm_nominal_perf || ospm_nominal_perf > U32_MAX)
> +		return -EINVAL;
> +
> +	if (cpc_desc &&
> +	    READ_ONCE(cpc_desc->ospm_nominal_perf) == ospm_nominal_perf)
> +		return 0;
> +
> +	ret = cppc_set_reg_val(cpu, OSPM_NOMINAL_PERF, ospm_nominal_perf);
> +	if (ret)
> +		return ret;
> +
> +	WRITE_ONCE(cpc_desc->ospm_nominal_perf, ospm_nominal_perf);
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(cppc_set_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 7e7f9dfb7a24..d06cba963550 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -715,6 +715,16 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
>   		goto out;
>   	}
>   
> +	/*
> +	 * Initialize OSPM Nominal Performance to inform firmware of
> +	 * OSPM's nominal level. Performance above this value = boost;
> +	 * below = throttle. Uses platform nominal by default.
> +	 */
> +	ret = cppc_set_ospm_nominal_perf(cpu, caps->nominal_perf);
> +	if (ret && ret != -EOPNOTSUPP)
> +		pr_debug("Failed to set ospm_nominal_perf for CPU%d: %d\n",
> +			 cpu, ret);
> +
>   	cppc_cpufreq_cpu_fie_init(policy);
>   	return 0;
>   
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index 8693890a7275..3771e2ed507d 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -86,6 +86,7 @@ struct cpc_desc {
>   	struct cpc_register_resource cpc_regs[MAX_CPC_REG_ENT];
>   	struct acpi_psd_package domain_info;
>   	struct kobject kobj;
> +	u32 ospm_nominal_perf;
>   };
>   
>   /* These are indexes into the per-cpu cpc_regs[]. Order is important. */
> @@ -180,6 +181,7 @@ 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_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);
> @@ -266,6 +268,10 @@ 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_auto_act_window(int cpu, u64 *auto_act_window)
>   {
>   	return -EOPNOTSUPP;

Re: [PATCH v2 2/2] ACPI: CPPC: Add ospm_nominal_perf support

[Pierre Gondois]

Hello Sumit,

On 4/30/26 16:24, Sumit Gupta wrote:
> Add acpi_cppc/ospm_nominal_perf sysfs attribute (read-write) and
> cppc_set_ospm_nominal_perf() API for the OSPM Nominal Performance
> register (ACPI 6.6, Section 8.4.6.1.2.6).
>
> The register conveys the desired nominal performance level at which
> the platform may run. OSPM can request a lower level than platform
> nominal. Valid range is [Lowest Performance, Nominal Performance].
> The value tells the platform what OSPM considers nominal. The
> platform classifies performance above this as boosted and below as
> throttled. It uses that for its power/thermal decisions.
>
> Although the register is write-only per spec, cache the OSPM-written
> value in cpc_desc so userspace can observe it via sysfs, and to
> skip redundant writes.
>
> Initialize to platform nominal at policy init. Override via sysfs
> if needed.
>
> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
> ---
>   drivers/acpi/cppc_acpi.c       | 69 ++++++++++++++++++++++++++++++++++
>   drivers/cpufreq/cppc_cpufreq.c | 10 +++++
>   include/acpi/cppc_acpi.h       |  6 +++
>   3 files changed, 85 insertions(+)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index a1c91ce20cc8..fbc620adafad 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -155,6 +155,10 @@ static DEFINE_PER_CPU(struct cpc_desc *, cpc_desc_ptr);
>   static struct kobj_attribute _name =		\
>   __ATTR(_name, 0444, show_##_name, NULL)
>   
> +#define define_one_cppc_rw(_name)		\
> +static struct kobj_attribute _name =		\
> +__ATTR(_name, 0644, show_##_name, store_##_name)
> +
>   #define to_cpc_desc(a) container_of(a, struct cpc_desc, kobj)
>   
>   #define show_cppc_data(access_fn, struct_name, member_name)		\
> @@ -211,6 +215,38 @@ static ssize_t show_feedback_ctrs(struct kobject *kobj,
>   }
>   define_one_cppc_ro(feedback_ctrs);
>   
> +static ssize_t show_ospm_nominal_perf(struct kobject *kobj,
> +				      struct kobj_attribute *attr, char *buf)
> +{
> +	struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
> +	u64 val = READ_ONCE(cpc_ptr->ospm_nominal_perf);
> +
> +	if (!val)
> +		return -ENODATA;
> +
> +	return sysfs_emit(buf, "%llu\n", val);
> +}
> +
> +static ssize_t store_ospm_nominal_perf(struct kobject *kobj,
> +				       struct kobj_attribute *attr,
> +				       const char *buf, size_t count)
> +{
> +	struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
> +	u64 val;
> +	int ret;
> +
> +	ret = kstrtou64(buf, 0, &val);
> +	if (ret)
> +		return ret;
> +
> +	ret = cppc_set_ospm_nominal_perf(cpc_ptr->cpu_id, val);
> +	if (ret)
> +		return ret;
> +
> +	return count;
> +}
> +define_one_cppc_rw(ospm_nominal_perf);
> +
>   static struct attribute *cppc_attrs[] = {
>   	&feedback_ctrs.attr,
>   	&reference_perf.attr,
> @@ -222,6 +258,7 @@ static struct attribute *cppc_attrs[] = {
>   	&nominal_perf.attr,
>   	&nominal_freq.attr,
>   	&lowest_freq.attr,
> +	&ospm_nominal_perf.attr,
>   	NULL
>   };
>   ATTRIBUTE_GROUPS(cppc);
> @@ -1683,6 +1720,38 @@ 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 allows OSPM to inform the platform of the nominal
> + * performance level it intends to maintain.
> + *
> + * Return: 0 for success, -EINVAL on invalid input, -EOPNOTSUPP if not
> + * supported, -EIO otherwise.
> + */
> +int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf)
> +{
> +	struct cpc_desc *cpc_desc = per_cpu(cpc_desc_ptr, cpu);
> +	int ret;
> +
> +	if (!ospm_nominal_perf || ospm_nominal_perf > U32_MAX)
> +		return -EINVAL;
I think the spec also requests to have a value in the range

[lowest:nominal]. As these registers are read-only it should

be ok to read the values here ?

> +
> +	if (cpc_desc &&
> +	    READ_ONCE(cpc_desc->ospm_nominal_perf) == ospm_nominal_perf)
> +		return 0;
> +
> +	ret = cppc_set_reg_val(cpu, OSPM_NOMINAL_PERF, ospm_nominal_perf);
> +	if (ret)
> +		return ret;
> +

Shouldn't we have some protection against concurrent accesses ?

> +	WRITE_ONCE(cpc_desc->ospm_nominal_perf, ospm_nominal_perf);
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(cppc_set_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 7e7f9dfb7a24..d06cba963550 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -715,6 +715,16 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
>   		goto out;
>   	}
>   
> +	/*
> +	 * Initialize OSPM Nominal Performance to inform firmware of
> +	 * OSPM's nominal level. Performance above this value = boost;
> +	 * below = throttle. Uses platform nominal by default.
> +	 */
> +	ret = cppc_set_ospm_nominal_perf(cpu, caps->nominal_perf);
> +	if (ret && ret != -EOPNOTSUPP)
> +		pr_debug("Failed to set ospm_nominal_perf for CPU%d: %d\n",
> +			 cpu, ret);
> +

IIUC, if (ospm_nominal_perf == nominal_perf), the firmware should
not behave differently. Is this really useful ?

------------

Also this seems like there will need some synchronization
mechanism to keep-up with the boost state.

If the ospm_nominal_perf is lowered and boost is disabled,
a freq. update should happen. IMO it looks like this could
be handled with (another) freq_qos_request.

This new freq_qos_request, if we name it ospm_nominal_freq_req,
should only be taken into account if boost is disabled.
Otherwise, if boost is enabled, ospm_nominal_freq_req
should be ignored.

------------

Also, the function seems to set the ospm_nominal_freq for
a single CPU when the policy might be common for multiple
CPUs right ?

The issues this field raises seems similar to the auto_sel
ones. I.e. :

- concurrency accesses + need for a scratch value

- what should happen when unloading the driver

- the value can be set for single CPUs but we might
want to have the same value for the whole policy

Maybe a common solution should be found.
(I m not suggesting anything right now unfortunately).


>   	cppc_cpufreq_cpu_fie_init(policy);
>   	return 0;
>   
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index 8693890a7275..3771e2ed507d 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -86,6 +86,7 @@ struct cpc_desc {
>   	struct cpc_register_resource cpc_regs[MAX_CPC_REG_ENT];
>   	struct acpi_psd_package domain_info;
>   	struct kobject kobj;
> +	u32 ospm_nominal_perf;
>   };
>   
>   /* These are indexes into the per-cpu cpc_regs[]. Order is important. */
> @@ -180,6 +181,7 @@ 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_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);
> @@ -266,6 +268,10 @@ 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_auto_act_window(int cpu, u64 *auto_act_window)
>   {
>   	return -EOPNOTSUPP;

Re: [PATCH v2 2/2] ACPI: CPPC: Add ospm_nominal_perf support

[Sumit Gupta]

On 30/04/26 21:55, Pierre Gondois wrote:
> External email: Use caution opening links or attachments
>
>
> Hello Sumit,
>
> On 4/30/26 16:24, Sumit Gupta wrote:
>> Add acpi_cppc/ospm_nominal_perf sysfs attribute (read-write) and
>> cppc_set_ospm_nominal_perf() API for the OSPM Nominal Performance
>> register (ACPI 6.6, Section 8.4.6.1.2.6).
>>
>> The register conveys the desired nominal performance level at which
>> the platform may run. OSPM can request a lower level than platform
>> nominal. Valid range is [Lowest Performance, Nominal Performance].
>> The value tells the platform what OSPM considers nominal. The
>> platform classifies performance above this as boosted and below as
>> throttled. It uses that for its power/thermal decisions.
>>
>> Although the register is write-only per spec, cache the OSPM-written
>> value in cpc_desc so userspace can observe it via sysfs, and to
>> skip redundant writes.
>>
>> Initialize to platform nominal at policy init. Override via sysfs
>> if needed.
>>
>> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
>> ---
>>   drivers/acpi/cppc_acpi.c       | 69 ++++++++++++++++++++++++++++++++++
>>   drivers/cpufreq/cppc_cpufreq.c | 10 +++++
>>   include/acpi/cppc_acpi.h       |  6 +++
>>   3 files changed, 85 insertions(+)
>>
>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>> index a1c91ce20cc8..fbc620adafad 100644
>> --- a/drivers/acpi/cppc_acpi.c
>> +++ b/drivers/acpi/cppc_acpi.c
>> @@ -155,6 +155,10 @@ static DEFINE_PER_CPU(struct cpc_desc *, 
>> cpc_desc_ptr);
>>   static struct kobj_attribute _name =                \
>>   __ATTR(_name, 0444, show_##_name, NULL)
>>
>> +#define define_one_cppc_rw(_name)            \
>> +static struct kobj_attribute _name =         \
>> +__ATTR(_name, 0644, show_##_name, store_##_name)
>> +
>>   #define to_cpc_desc(a) container_of(a, struct cpc_desc, kobj)
>>
>>   #define show_cppc_data(access_fn, struct_name, member_name)         \
>> @@ -211,6 +215,38 @@ static ssize_t show_feedback_ctrs(struct kobject 
>> *kobj,
>>   }
>>   define_one_cppc_ro(feedback_ctrs);
>>
>> +static ssize_t show_ospm_nominal_perf(struct kobject *kobj,
>> +                                   struct kobj_attribute *attr, char 
>> *buf)
>> +{
>> +     struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
>> +     u64 val = READ_ONCE(cpc_ptr->ospm_nominal_perf);
>> +
>> +     if (!val)
>> +             return -ENODATA;
>> +
>> +     return sysfs_emit(buf, "%llu\n", val);
>> +}
>> +
>> +static ssize_t store_ospm_nominal_perf(struct kobject *kobj,
>> +                                    struct kobj_attribute *attr,
>> +                                    const char *buf, size_t count)
>> +{
>> +     struct cpc_desc *cpc_ptr = to_cpc_desc(kobj);
>> +     u64 val;
>> +     int ret;
>> +
>> +     ret = kstrtou64(buf, 0, &val);
>> +     if (ret)
>> +             return ret;
>> +
>> +     ret = cppc_set_ospm_nominal_perf(cpc_ptr->cpu_id, val);
>> +     if (ret)
>> +             return ret;
>> +
>> +     return count;
>> +}
>> +define_one_cppc_rw(ospm_nominal_perf);
>> +
>>   static struct attribute *cppc_attrs[] = {
>>       &feedback_ctrs.attr,
>>       &reference_perf.attr,
>> @@ -222,6 +258,7 @@ static struct attribute *cppc_attrs[] = {
>>       &nominal_perf.attr,
>>       &nominal_freq.attr,
>>       &lowest_freq.attr,
>> +     &ospm_nominal_perf.attr,
>>       NULL
>>   };
>>   ATTRIBUTE_GROUPS(cppc);
>> @@ -1683,6 +1720,38 @@ 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 allows OSPM to inform the platform of 
>> the nominal
>> + * performance level it intends to maintain.
>> + *
>> + * Return: 0 for success, -EINVAL on invalid input, -EOPNOTSUPP if not
>> + * supported, -EIO otherwise.
>> + */
>> +int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf)
>> +{
>> +     struct cpc_desc *cpc_desc = per_cpu(cpc_desc_ptr, cpu);
>> +     int ret;
>> +
>> +     if (!ospm_nominal_perf || ospm_nominal_perf > U32_MAX)
>> +             return -EINVAL;
> I think the spec also requests to have a value in the range
>
> [lowest:nominal]. As these registers are read-only it should
>
> be ok to read the values here ?

Will add the [lowest_perf, nominal_perf] range check in v3,
fetching the bounds via cppc_get_perf_caps().


>
>> +
>> +     if (cpc_desc &&
>> +         READ_ONCE(cpc_desc->ospm_nominal_perf) == ospm_nominal_perf)
>> +             return 0;
>> +
>> +     ret = cppc_set_reg_val(cpu, OSPM_NOMINAL_PERF, ospm_nominal_perf);
>> +     if (ret)
>> +             return ret;
>> +
>
> Shouldn't we have some protection against concurrent accesses ?
>> + WRITE_ONCE(cpc_desc->ospm_nominal_perf, ospm_nominal_perf);
>> +     return 0;
>> +}
>> +EXPORT_SYMBOL_GPL(cppc_set_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 7e7f9dfb7a24..d06cba963550 100644
>> --- a/drivers/cpufreq/cppc_cpufreq.c
>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>> @@ -715,6 +715,16 @@ static int cppc_cpufreq_cpu_init(struct 
>> cpufreq_policy *policy)
>>               goto out;
>>       }
>>
>> +     /*
>> +      * Initialize OSPM Nominal Performance to inform firmware of
>> +      * OSPM's nominal level. Performance above this value = boost;
>> +      * below = throttle. Uses platform nominal by default.
>> +      */
>> +     ret = cppc_set_ospm_nominal_perf(cpu, caps->nominal_perf);
>> +     if (ret && ret != -EOPNOTSUPP)
>> +             pr_debug("Failed to set ospm_nominal_perf for CPU%d: 
>> %d\n",
>> +                      cpu, ret);
>> +
>
> IIUC, if (ospm_nominal_perf == nominal_perf), the firmware should
> not behave differently. Is this really useful ?
>

Right, it's a no-op from the firmware's side. The init was only so that
sysfs would show a value (platform nominal) before any userspace write.
Will drop it in v3 and return 0 from sysfs until userspace writes a value.


> ------------
>
> Also this seems like there will need some synchronization
> mechanism to keep-up with the boost state.
>
> If the ospm_nominal_perf is lowered and boost is disabled,
> a freq. update should happen. IMO it looks like this could
> be handled with (another) freq_qos_request.
>
> This new freq_qos_request, if we name it ospm_nominal_freq_req,
> should only be taken into account if boost is disabled.
> Otherwise, if boost is enabled, ospm_nominal_freq_req
> should be ignored.
>

Agreed, will add the new freq_qos_request in a follow-up patch.

> ------------
>
> Also, the function seems to set the ospm_nominal_freq for
> a single CPU when the policy might be common for multiple
> CPUs right ?

In v3, after dropping the change from cppc_cpufreq_cpu_init,
the problem won't come in this specific instance.


>
> The issues this field raises seems similar to the auto_sel
> ones. I.e. :
>
> - concurrency accesses + need for a scratch value
>
> - what should happen when unloading the driver
>
> - the value can be set for single CPUs but we might
> want to have the same value for the whole policy
>
> Maybe a common solution should be found.
> (I m not suggesting anything right now unfortunately).
>

One way to address this is to move the sysfs from per-CPU acpi_cppc to
a per-policy node under cpufreq (ospm_nominal_perf_freq, kHz).
In the sysfs callback, we can convert kHz to perf and write the register
on every CPU in policy->cpus.
Concurrency is already covered by policy->rwsem at the cpufreq layer.
This is similar to how we were handling min/max_perf in earlier version.
Does this approach make sense?

Thank you,
Sumit Gupta

....

Re: [PATCH v2 0/2] ACPI: CPPC: Add CPPC v4 support (ACPI 6.6)

[Rafael J. Wysocki]

On Thu, Apr 30, 2026 at 4:25 PM Sumit Gupta <sumitg@nvidia.com> wrote:
>
> Add initial kernel support for CPPC v4 (ACPI 6.6, Section 8.4.6),
> which extends the _CPC package from 23 to 25 entries with two
> optional fields:
>
>   - OSPM Nominal Performance (8.4.6.1.2.6): register used by OSPM
>     to tell the platform what it considers nominal. The platform
>     classifies performance above this as boost and below as
>     throttle for power/thermal decisions.
>
>   - Resource Priority (8.4.6.1.2.7): Package of Resource Priority
>     Register Descriptor sub-packages. Full parsing is not yet
>     implemented; such entries are marked as unsupported.
>
> Patch 1: Add v4 _CPC parsing - validate the 25-entry layout,
> handle the Resource Priority package, and mark the two new
> registers optional.
>
> Patch 2: Add acpi_cppc/ospm_nominal_perf as a read-write sysfs
> attribute, and initialize it to the platform nominal value
> during cppc_cpufreq policy init.
>
> ---
> v1[1] -> v2:
> - Patch 1: added Reviewed-by from Mario Limonciello.
> - Patch 2:
>   - Make ospm_nominal_perf sysfs read-write; cache last write in
>     cpc_desc and skip redundant register writes.
>   - Validate input in cppc_set_ospm_nominal_perf.
>
> Sumit Gupta (2):
>   ACPI: CPPC: Add support for CPPC v4
>   ACPI: CPPC: Add ospm_nominal_perf support
>
>  drivers/acpi/cppc_acpi.c       | 93 +++++++++++++++++++++++++++++++---
>  drivers/cpufreq/cppc_cpufreq.c | 10 ++++
>  include/acpi/cppc_acpi.h       | 14 ++++-
>  3 files changed, 109 insertions(+), 8 deletions(-)
>
> [1] https://lore.kernel.org/lkml/20260427051823.280419-1-sumitg@nvidia.com/
>
> --

Can you please see the sashiko.dev feedback on this set:

https://sashiko.dev/#/patchset/20260430142430.755437-1-sumitg%40nvidia.com

and let me know what you think?  Especially regarding the second patch?

Re: [PATCH v2 0/2] ACPI: CPPC: Add CPPC v4 support (ACPI 6.6)

[Sumit Gupta]

Hi Rafael,


On 09/05/26 00:31, Rafael J. Wysocki wrote:
> External email: Use caution opening links or attachments
>
>
> On Thu, Apr 30, 2026 at 4:25 PM Sumit Gupta <sumitg@nvidia.com> wrote:
>> Add initial kernel support for CPPC v4 (ACPI 6.6, Section 8.4.6),
>> which extends the _CPC package from 23 to 25 entries with two
>> optional fields:
>>
>>    - OSPM Nominal Performance (8.4.6.1.2.6): register used by OSPM
>>      to tell the platform what it considers nominal. The platform
>>      classifies performance above this as boost and below as
>>      throttle for power/thermal decisions.
>>
>>    - Resource Priority (8.4.6.1.2.7): Package of Resource Priority
>>      Register Descriptor sub-packages. Full parsing is not yet
>>      implemented; such entries are marked as unsupported.
>>
>> Patch 1: Add v4 _CPC parsing - validate the 25-entry layout,
>> handle the Resource Priority package, and mark the two new
>> registers optional.
>>
>> Patch 2: Add acpi_cppc/ospm_nominal_perf as a read-write sysfs
>> attribute, and initialize it to the platform nominal value
>> during cppc_cpufreq policy init.
>>
>> ---
>> v1[1] -> v2:
>> - Patch 1: added Reviewed-by from Mario Limonciello.
>> - Patch 2:
>>    - Make ospm_nominal_perf sysfs read-write; cache last write in
>>      cpc_desc and skip redundant register writes.
>>    - Validate input in cppc_set_ospm_nominal_perf.
>>
>> Sumit Gupta (2):
>>    ACPI: CPPC: Add support for CPPC v4
>>    ACPI: CPPC: Add ospm_nominal_perf support
>>
>>   drivers/acpi/cppc_acpi.c       | 93 +++++++++++++++++++++++++++++++---
>>   drivers/cpufreq/cppc_cpufreq.c | 10 ++++
>>   include/acpi/cppc_acpi.h       | 14 ++++-
>>   3 files changed, 109 insertions(+), 8 deletions(-)
>>
>> [1] https://lore.kernel.org/lkml/20260427051823.280419-1-sumitg@nvidia.com/
>>
>> --
> Can you please see the sashiko.dev feedback on this set:
>
> https://sashiko.dev/#/patchset/20260430142430.755437-1-sumitg%40nvidia.com
>
> and let me know what you think?  Especially regarding the second patch?


Thank you for sharing this.

Patch 1:
- Comments #1 and #2 are pre-existing issues with rare occurrence.
   I will address them in a separate hardening patch.

- Comment #3: In v3, will limit the ACPI_TYPE_PACKAGE handling to the
   RESOURCE_PRIORITY entry. So a Package at any other slot will be
   treated as invalid and abort probe, as it did before this patch.

----------------

Patch 2:
Discussed the changes for v3 in some detail on this thread already
which address most of the points (Please see my reply to Pierre [1]).

Summary of how each point will be addressed below:


 > The commit message states the valid range is [Lowest Performance,
 > Nominal Performance]. Does this code allow writing arbitrary values
 > outside that range by only checking against U32_MAX, without fetching
 > the CPU's capabilities to validate the input?
Will fetch the bounds via cppc_get_perf_caps() and reject values
outside [lowest_perf, nominal_perf] in v3.


 > If the hardware loses state during a logical CPU hotplug or system
 > suspend, but the software cache is not invalidated, will this check
 > prevent the register from being correctly re-initialized when the CPU
 > comes back online?
The redundant write check will be removed in v3, so the stale cache
failure mode won't be possible.


 > Can concurrent sysfs writes permanently desynchronize the software
 > cache from the hardware register?
 > ...
 > Is a lock needed around the read-modify-write cycle?
This will not occur in v3 since concurrent calls for the same
policy are serialized by policy->rwsem at the cpufreq layer (see [1]).


 > Additionally, can a time-of-check to time-of-use race lead to a NULL
 > pointer dereference if cpc_desc_ptr is initialized concurrently?
 > ...
 > Would this cause the WRITE_ONCE() to dereference the locally fetched NULL
 > cpc_desc pointer? Should this explicitly return -ENODEV early if 
!cpc_desc?
Will add the early -ENODEV return at the top of the function in v3,
eliminating the NULL cpc_desc race.


 > For shared cpufreq policies where policy->cpus contains multiple
 > logical cores (such as CPUFREQ_SHARED_TYPE_ANY), does this skip
 > initializing the secondary CPUs in the domain?
 >
 > If they are uninitialized, will their local cache remain 0, causing
 > sysfs reads for those secondary CPUs to incorrectly return -ENODATA?
Will move the rw sysfs from the per-CPU acpi_cppc interface to a
per-policy cpufreq interface in v3, and write the register on every
CPU in policy->cpus/domain.
The -ENODATA on unwritten read path will go away with the per-CPU node,
and the per-policy show returns 0 until user-space writes a value. See [1].


 > Also, since the sysfs attribute is tied to the physical CPU device
 > lifetime and persists independently of cpufreq policy teardowns, will
 > unconditionally setting the nominal performance here silently clobber
 > any persistent userspace configurations when a CPU is taken offline
 > and online?
Will drop the unconditional cpu_init write in v3, so the user-set
value won't be overwritten on CPU hotplug.

[1] 
https://lore.kernel.org/all/9c32f75a-294f-4cea-810e-c011c4dd91ab@nvidia.com/

Thank you,
Sumit Gupta