Re: [PATCH v8 1/3] ACPI: CPPC: Add ospm_nominal_perf support

[Christian Loehle <christian.loehle@arm.com>]

On 10/30/26 17:27, Sumit Gupta wrote:
> Expose the OSPM Nominal Performance register (ACPI 6.6, Section
> 8.4.6.1.2.6), which conveys the nominal performance level at which the
> platform may run. Unlike the read-only Nominal Performance register, it
> is writable, so OSPM can ask for a nominal level below the
> platform-reported one, which then becomes the boundary between boosted
> and throttled operation.
> 
> Add cppc_set_ospm_nominal_perf() to write the register. Per the spec,
> the value must lie in [Lowest Performance, Nominal Performance], which
> the caller is responsible for validating.
> 
> Add cppc_ospm_nominal_perf_supported() to report whether the platform
> implements the register as writable, since a write-only register cannot
> be probed by reading it. It uses the same cpc_is_writable() test as the
> setter, so support is never reported for a register that a write would
> reject, and takes cpc_desc_lock while looking the descriptor up.
> 
> The register also joins the writable set that the SystemMemory overlap
> validation uses, as the driver now writes it and can compete with
> another control sharing an access unit.
> 
> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>

Reviewed-by: Christian Loehle <christian.loehle@arm.com>

> ---
>  drivers/acpi/cppc_acpi.c | 44 ++++++++++++++++++++++++++++++++++++++++
>  include/acpi/cppc_acpi.h | 10 +++++++++
>  2 files changed, 54 insertions(+)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 5a1f84fddb7f..3a0ecdd1ab11 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -463,6 +463,7 @@ static bool cpc_reg_is_writable(unsigned int reg_idx)
>  	case AUTO_SEL_ENABLE:
>  	case AUTO_ACT_WINDOW:
>  	case ENERGY_PERF:
> +	case OSPM_NOMINAL_PERF:
>  		return true;
>  	default:
>  		return false;
> @@ -3232,6 +3233,49 @@ 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, -EOPNOTSUPP if the platform does not implement the
> + * register as writable, or another 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_ospm_nominal_perf_supported() - Check OSPM Nominal Performance support.
> + * @cpu: CPU to query.
> + *
> + * The OSPM Nominal Performance register is write-only, so its value
> + * cannot be read back. This only reports whether the platform implements
> + * it as a writable register.
> + *
> + * Context: Process context.
> + * Return: true if the register is supported, false otherwise.
> + */
> +bool cppc_ospm_nominal_perf_supported(int cpu)
> +{
> +	struct cpc_desc *cpc_desc;
> +
> +	guard(mutex)(&cpc_desc_lock);
> +	cpc_desc = per_cpu(cpc_desc_ptr, cpu);
> +
> +	return cpc_desc &&
> +	       cpc_is_writable(&cpc_desc->cpc_regs[OSPM_NOMINAL_PERF]);
> +}
> +EXPORT_SYMBOL_GPL(cppc_ospm_nominal_perf_supported);
> +
>  /**
>   * cppc_get_auto_act_window() - Read autonomous activity window register.
>   * @cpu: CPU from which to read register.
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index a5545fd3f8bd..2cec54b4d2e2 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -195,6 +195,8 @@ 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 bool cppc_ospm_nominal_perf_supported(int cpu);
>  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, u64 *enable);
> @@ -290,6 +292,14 @@ 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 bool cppc_ospm_nominal_perf_supported(int cpu)
> +{
> +	return false;
> +}
>  static inline int cppc_get_auto_act_window(int cpu, u64 *auto_act_window)
>  {
>  	return -EOPNOTSUPP;