Re: [PATCH 2/2] cpufreq/amd-pstate: Add support for the "Requested CPU Min frequency" BIOS option

[Mario Limonciello]

On 4/15/2025 5:21 AM, Dhananjay Ugwekar wrote:
> Initialize lower frequency limit to the "Requested CPU Min frequency"
> BIOS option (if it is set) value as part of the driver->init()
> callback. The BIOS specified value is passed by the PMFW as min_perf in
> CPPC_REQ MSR. To ensure that we don't mistake a stale min_perf value in
> CPPC_REQ value as the "Requested CPU Min frequency" during a kexec wakeup,
> reset the CPPC_REQ.min_perf value back to the BIOS specified one in the
> offline, exit and suspend callbacks.
>
> amd_pstate_target() and amd_pstate_epp_update_limit() which are invoked
> as part of the resume() and online() callbacks will take care of restoring
> the CPPC_REQ back to the latest sane values.
>
> Signed-off-by: Dhananjay Ugwekar <dhananjay.ugwekar@xxxxxxx>

I'm generally fine with this, but I have one nit below.

> ---
>   drivers/cpufreq/amd-pstate.c | 62 ++++++++++++++++++++++++++++--------
>   drivers/cpufreq/amd-pstate.h |  2 ++
>   2 files changed, 51 insertions(+), 13 deletions(-)
>
> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
> index 02de51001eba..d94fd2a38990 100644
> --- a/drivers/cpufreq/amd-pstate.c
> +++ b/drivers/cpufreq/amd-pstate.c
> @@ -389,7 +389,8 @@ static inline int amd_pstate_cppc_enable(struct cpufreq_policy *policy)
>   static int msr_init_perf(struct amd_cpudata *cpudata)
>   {
>   	union perf_cached perf = READ_ONCE(cpudata->perf);
> -	u64 cap1, numerator;
> +	u64 cap1, numerator, cppc_req;
> +	u8 min_perf;
>   
>   	int ret = rdmsrl_safe_on_cpu(cpudata->cpu, MSR_AMD_CPPC_CAP1,
>   				     &cap1);
> @@ -400,6 +401,22 @@ static int msr_init_perf(struct amd_cpudata *cpudata)
>   	if (ret)
>   		return ret;
>   
> +	ret = rdmsrl_on_cpu(cpudata->cpu, MSR_AMD_CPPC_REQ, &cppc_req);
> +	if (ret)
> +		return ret;
> +
> +	WRITE_ONCE(cpudata->cppc_req_cached, cppc_req);
> +	min_perf = FIELD_GET(AMD_CPPC_MIN_PERF_MASK, cppc_req);
> +
> +	/*
> +	 * Clear out the min_perf part to check if the rest of the MSR is 0, if yes, this is an
> +	 * indication that the min_perf value is the one specified through the BIOS option
> +	 */
> +	cppc_req &= ~(AMD_CPPC_MIN_PERF_MASK);
> +
> +	if (!cppc_req && min_perf)
> +		perf.bios_min_perf = min_perf;

To avoid a risk of garbage being in perf.bios_min_perf leading to hard 
to root cause bugs could we initialize this to 0 in the non 
bios_min_perf case?

something like this:

cppc_req &= ~(AMD_CPPC_MIN_PERF_MASK);
perf.bios_min_perf = (!cppc_req && min_perf) ? min_perf : 0;

> +
>   	perf.highest_perf = numerator;
>   	perf.max_limit_perf = numerator;
>   	perf.min_limit_perf = FIELD_GET(AMD_CPPC_LOWEST_PERF_MASK, cap1);
> @@ -580,20 +597,26 @@ static int amd_pstate_verify(struct cpufreq_policy_data *policy_data)
>   {
>   	/*
>   	 * Initialize lower frequency limit (i.e.policy->min) with
> -	 * lowest_nonlinear_frequency which is the most energy efficient
> -	 * frequency. Override the initial value set by cpufreq core and
> -	 * amd-pstate qos_requests.
> +	 * lowest_nonlinear_frequency or the min frequency (if) specified in BIOS,
> +	 * Override the initial value set by cpufreq core and amd-pstate qos_requests.
>   	 */
>   	if (policy_data->min == FREQ_QOS_MIN_DEFAULT_VALUE) {
>   		struct cpufreq_policy *policy __free(put_cpufreq_policy) =
>   					      cpufreq_cpu_get(policy_data->cpu);
>   		struct amd_cpudata *cpudata;
> +		union perf_cached perf;
>   
>   		if (!policy)
>   			return -EINVAL;
>   
>   		cpudata = policy->driver_data;
> -		policy_data->min = cpudata->lowest_nonlinear_freq;
> +		perf = READ_ONCE(cpudata->perf);
> +
> +		if (perf.bios_min_perf)
> +			policy_data->min = perf_to_freq(perf, cpudata->nominal_freq,
> +							perf.bios_min_perf);
> +		else
> +			policy_data->min = cpudata->lowest_nonlinear_freq;
>   	}
>   
>   	cpufreq_verify_within_cpu_limits(policy_data);
> @@ -1041,6 +1064,9 @@ static void amd_pstate_cpu_exit(struct cpufreq_policy *policy)
>   {
>   	struct amd_cpudata *cpudata = policy->driver_data;
>   
> +	/* Reset CPPC_REQ MSR to the BIOS value */
> +	amd_pstate_update_perf(policy, cpudata->perf.bios_min_perf, 0U, 0U, 0U, false);
> +
>   	freq_qos_remove_request(&cpudata->req[1]);
>   	freq_qos_remove_request(&cpudata->req[0]);
>   	policy->fast_switch_possible = false;
> @@ -1428,7 +1454,6 @@ static int amd_pstate_epp_cpu_init(struct cpufreq_policy *policy)
>   	struct amd_cpudata *cpudata;
>   	union perf_cached perf;
>   	struct device *dev;
> -	u64 value;
>   	int ret;
>   
>   	/*
> @@ -1493,12 +1518,6 @@ static int amd_pstate_epp_cpu_init(struct cpufreq_policy *policy)
>   		cpudata->epp_default = AMD_CPPC_EPP_BALANCE_PERFORMANCE;
>   	}
>   
> -	if (cpu_feature_enabled(X86_FEATURE_CPPC)) {
> -		ret = rdmsrl_on_cpu(cpudata->cpu, MSR_AMD_CPPC_REQ, &value);
> -		if (ret)
> -			return ret;
> -		WRITE_ONCE(cpudata->cppc_req_cached, value);
> -	}
>   	ret = amd_pstate_set_epp(policy, cpudata->epp_default);
>   	if (ret)
>   		return ret;
> @@ -1518,6 +1537,9 @@ static void amd_pstate_epp_cpu_exit(struct cpufreq_policy *policy)
>   	struct amd_cpudata *cpudata = policy->driver_data;
>   
>   	if (cpudata) {
> +		/* Reset CPPC_REQ MSR to the BIOS value */
> +		amd_pstate_update_perf(policy, cpudata->perf.bios_min_perf, 0U, 0U, 0U, false);
> +
>   		kfree(cpudata);
>   		policy->driver_data = NULL;
>   	}
> @@ -1575,13 +1597,27 @@ static int amd_pstate_cpu_online(struct cpufreq_policy *policy)
>   
>   static int amd_pstate_cpu_offline(struct cpufreq_policy *policy)
>   {
> -	return 0;
> +	struct amd_cpudata *cpudata = policy->driver_data;
> +
> +	/*
> +	 * Reset CPPC_REQ MSR to the BIOS value, this will allow us to retain the BIOS specified
> +	 * min_perf value across kexec reboots. If this CPU is just onlined normally after this, the
> +	 * limits, epp and desired perf will get reset to the cached values in cpudata struct
> +	 */
> +	return amd_pstate_update_perf(policy, cpudata->perf.bios_min_perf, 0U, 0U, 0U, false);
>   }
>   
>   static int amd_pstate_suspend(struct cpufreq_policy *policy)
>   {
>   	struct amd_cpudata *cpudata = policy->driver_data;
>   
> +	/*
> +	 * Reset CPPC_REQ MSR to the BIOS value, this will allow us to retain the BIOS specified
> +	 * min_perf value across kexec reboots. If this CPU is just resumed back without kexec,
> +	 * the limits, epp and desired perf will get reset to the cached values in cpudata struct
> +	 */
> +	amd_pstate_update_perf(policy, cpudata->perf.bios_min_perf, 0U, 0U, 0U, false);
> +
>   	/* invalidate to ensure it's rewritten during resume */
>   	cpudata->cppc_req_cached = 0;
>   
> diff --git a/drivers/cpufreq/amd-pstate.h b/drivers/cpufreq/amd-pstate.h
> index fbe1c08d3f06..2f7ae364d331 100644
> --- a/drivers/cpufreq/amd-pstate.h
> +++ b/drivers/cpufreq/amd-pstate.h
> @@ -30,6 +30,7 @@
>    * @lowest_perf: the absolute lowest performance level of the processor
>    * @min_limit_perf: Cached value of the performance corresponding to policy->min
>    * @max_limit_perf: Cached value of the performance corresponding to policy->max
> + * @bios_min_perf: Cached perf value corresponding to the "Requested CPU Min Frequency" BIOS option
>    */
>   union perf_cached {
>   	struct {
> @@ -39,6 +40,7 @@ union perf_cached {
>   		u8	lowest_perf;
>   		u8	min_limit_perf;
>   		u8	max_limit_perf;
> +		u8	bios_min_perf;
>   	};
>   	u64	val;
>   };