linux-kernel - Re: [PATCH v1 2/4] cpufreq: CPPC: Avoid using CPUFREQ

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Message-ID: <11e2cb33-05cd-4ef3-86e6-ef57e5f1222f@amd.com>
Date: Thu, 25 Sep 2025 11:36:43 -0500
From: Mario Limonciello <mario.limonciello@....com>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
 Linux PM <linux-pm@...r.kernel.org>
Cc: Shawn Guo <shawnguo@...nel.org>, Qais Yousef <qyousef@...alina.io>,
 LKML <linux-kernel@...r.kernel.org>, Viresh Kumar <viresh.kumar@...aro.org>,
 Prashanth Prakash <pprakash@...eaurora.org>,
 Pierre Gondois <pierre.gondois@....com>,
 Linux ACPI <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v1 2/4] cpufreq: CPPC: Avoid using CPUFREQ_ETERNAL as
 transition delay



On 9/25/2025 10:44 AM, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> If cppc_get_transition_latency() returns CPUFREQ_ETERNAL to indicate a
> failure to retrieve the transition latency value from the platform
> firmware, the CPPC cpufreq driver will use that value (converted to
> microseconds) as the policy transition delay, but it is way too large
> for any practical use.
> 
> Address this by making the driver use the cpufreq's default
> transition latency value (in microseconds) as the transition delay
> if CPUFREQ_ETERNAL is returned by cppc_get_transition_latency().
> 
> Fixes: d4f3388afd48 ("cpufreq / CPPC: Set platform specific transition_delay_us")
> Cc: 5.19+ <stable@...r.kernel.org> # 5.19
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Reviewed-by: Mario Limonciello (AMD) <superm1@...nel.org>> ---
>   drivers/cpufreq/cppc_cpufreq.c |   14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
> 
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -308,6 +308,16 @@ static int cppc_verify_policy(struct cpu
>   	return 0;
>   }
>   
> +static unsigned int get_transition_latency_from_cppc(unsigned int cpu)
> +{
> +	unsigned int transition_latency_ns = cppc_get_transition_latency(cpu);
> +
> +	if (transition_latency_ns == CPUFREQ_ETERNAL)
> +		return CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS / NSEC_PER_USEC;
> +
> +	return transition_latency_ns / NSEC_PER_USEC;
> +}
> +
>   /*
>    * The PCC subspace describes the rate at which platform can accept commands
>    * on the shared PCC channel (including READs which do not count towards freq
> @@ -330,12 +340,12 @@ static unsigned int cppc_cpufreq_get_tra
>   			return 10000;
>   		}
>   	}
> -	return cppc_get_transition_latency(cpu) / NSEC_PER_USEC;
> +	return get_transition_latency_from_cppc(cpu);
>   }
>   #else
>   static unsigned int cppc_cpufreq_get_transition_delay_us(unsigned int cpu)
>   {
> -	return cppc_get_transition_latency(cpu) / NSEC_PER_USEC;
> +	return get_transition_latency_from_cppc(cpu);
>   }
>   #endif
>   
> 
> 
>