Re: [PATCH 1/1] cpufreq: Set policy->min and max as real QoS constraints

Zhongqiu Han
2026-05-06T12:45:56Z

On 4/30/2026 9:41 PM, Pierre Gondois wrote:
> Hello Zhongqiu,
>
> On 4/29/26 15:00, Zhongqiu Han wrote:
>> On 4/23/2026 4:47 PM, Pierre Gondois wrote:
>>> cpufreq_set_policy() will ultimately override the policy min/max
>>> values written in the .init() callback through:
>>> cpufreq_policy_online()
>>> \-cpufreq_init_policy()
>>>    \-cpufreq_set_policy()
>>>      \-/* Set policy->min/max */
>>> Thus the policy min/max values provided are only temporary.
>>>
>>> There is an exception if CPUFREQ_NEED_INITIAL_FREQ_CHECK is set and:
>>> cpufreq_policy_online()
>>> \-cpufreq_init_policy()
>>>    \-__cpufreq_driver_target()
>>>      \-cpufreq_driver->target()
>>> is called. To avoid any regression, set policy->min/max in cpufreq.c
>>> if the values were not initialized.
>>>
>>> In this patch:
>>> - Setting policy->min or max value in driver .init() cb is
>>>    interpreted as setting a QoS constraint.
>>> - Remove policy->min/max initialization in drivers if the values
>>>    are similar to policy->cpuinfo.min_freq/max_freq.
>>>    The only drivers where these values are different are:
>>>    - gx-suspmod.c
>>>    - cppc-cpufreq.c
>>>    - longrun.c
>>> - For the cppc-cpufreq driver, the lowest non-linear freq. is
>>>    used as a min QoS constraint as suggested at:
>>> https://lore.kernel.org/lkml/20260213100633.15413-1-
>>> zhangpengjie2@huawei.com/
>>>
>>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
>>
>>
>> Hi Pierre,
>> Thanks for the patch. I have a few additional inline comments/questions
>> below.
>>
>>
>>> ---
>>>   drivers/cpufreq/amd-pstate.c      | 16 ++++++++--------
>>>   drivers/cpufreq/cppc_cpufreq.c    | 11 +++++++----
>>>   drivers/cpufreq/cpufreq-nforce2.c |  4 ++--
>>>   drivers/cpufreq/cpufreq.c         | 19 +++++++++++++++++--
>>>   drivers/cpufreq/freq_table.c      |  7 +++----
>>>   drivers/cpufreq/gx-suspmod.c      |  9 +++++----
>>>   drivers/cpufreq/intel_pstate.c    |  3 ---
>>>   drivers/cpufreq/pcc-cpufreq.c     |  8 ++++----
>>>   drivers/cpufreq/pxa3xx-cpufreq.c  |  4 ++--
>>>   drivers/cpufreq/sh-cpufreq.c      |  4 ++--
>>>   drivers/cpufreq/virtual-cpufreq.c |  5 +----
>>>   11 files changed, 51 insertions(+), 39 deletions(-)
>>>
>>> diff --git a/drivers/cpufreq/amd-pstate.c b/drivers/cpufreq/amd-pstate.c
>>> index 453084c67327f..1ed4bcdcc957f 100644
>>> --- a/drivers/cpufreq/amd-pstate.c
>>> +++ b/drivers/cpufreq/amd-pstate.c
>>> @@ -1090,10 +1090,10 @@ static int amd_pstate_cpu_init(struct
>>> cpufreq_policy *policy)
>>>         perf = READ_ONCE(cpudata->perf);
>>>   -    policy->cpuinfo.min_freq = policy->min = perf_to_freq(perf,
>>> -                                  cpudata->nominal_freq,
>>> -                                  perf.lowest_perf);
>>> -    policy->cpuinfo.max_freq = policy->max = cpudata->max_freq;
>>> +    policy->cpuinfo.min_freq = perf_to_freq(perf,
>>> +                        cpudata->nominal_freq,
>>> +                        perf.lowest_perf);
>>> +    policy->cpuinfo.max_freq = cpudata->max_freq;
>>
>>
>> It is better to update doc as well to avoid new dirver developmenter set
>> policy->min / policy->max again?
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/
>> tree/Documentation/cpu-freq/cpu-drivers.rst#n102
>
> Yes sure, does the following works for you:
> (tabs might not be conserved in the mail)
>
> diff --git a/Documentation/cpu-freq/cpu-drivers.rst b/Documentation/cpu-
> freq/cpu-drivers.rst
> index c5635ac3de547..12dc4dcbdd5a6 100644
> --- a/Documentation/cpu-freq/cpu-drivers.rst
> +++ b/Documentation/cpu-freq/cpu-drivers.rst
> @@ -114,8 +114,14 @@ Then, the driver must fill in the following values:
>  |policy->cur                       | The current operating frequency
> of   |
>  |                                  | this CPU (if appropriate)         |
>  +-----------------------------------
> +--------------------------------------+
> -|policy->min,                      |             |
> -|policy->max,                      |             |
> +|policy->min                       | Minimum frequency QoS constraint.
>   |
> +|                                  | Can be overwritten by writing to
>    |
> +|                                  | scaling_min sysfs file.         |
> ++-----------------------------------
> +--------------------------------------+
> +|policy->max                       | Maximum frequency QoS constraint.
>   |
> +|                                  | Can be overwritten by writing to
>    |
> +|                                  | scaling_max sysfs file.         |
> ++-----------------------------------
> +--------------------------------------+
>  |policy->policy and, if necessary,  |            |
>  |policy->governor                  | must contain the "default policy"
> for|
>  |                                  | this CPU. A few moments later,    |
>

Thanks Pierre,

Just shared a minor nit and used 'If' to indicate that it's optional.
For example, if you think it makes sense, you may consider:

+|policy->min | If set by the driver in ->init(), used as the |
+| | initial minimum frequency QoS request. |
++-------------------------------------------------------------------+
+|policy->max | If set by the driver in ->init(), used as the |
+| | initial maximum frequency QoS request. |

>>
>>>         policy->driver_data = cpudata;
>>>       ret = amd_pstate_cppc_enable(policy);
>>> @@ -1907,10 +1907,10 @@ static int amd_pstate_epp_cpu_init(struct
>>> cpufreq_policy *policy)
>>>         perf = READ_ONCE(cpudata->perf);
>>>   -    policy->cpuinfo.min_freq = policy->min = perf_to_freq(perf,
>>> -                                  cpudata->nominal_freq,
>>> -                                  perf.lowest_perf);
>>> -    policy->cpuinfo.max_freq = policy->max = cpudata->max_freq;
>>> +    policy->cpuinfo.min_freq = perf_to_freq(perf,
>>> +                        cpudata->nominal_freq,
>>> +                        perf.lowest_perf);
>>> +    policy->cpuinfo.max_freq = cpudata->max_freq;
>>>       policy->driver_data = cpudata;
>>>         ret = amd_pstate_cppc_enable(policy);
>>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/
>>> cppc_cpufreq.c
>>> index 7e7f9dfb7a24c..c6fcecdbbab0c 100644
>>> --- a/drivers/cpufreq/cppc_cpufreq.c
>>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>>> @@ -645,6 +645,7 @@ static int cppc_cpufreq_cpu_init(struct
>>> cpufreq_policy *policy)
>>>       unsigned int cpu = policy->cpu;
>>>       struct cppc_cpudata *cpu_data;
>>>       struct cppc_perf_caps *caps;
>>> +    unsigned int min, max;
>>>       int ret;
>>>         cpu_data = cppc_cpufreq_get_cpu_data(cpu);
>>> @@ -655,13 +656,15 @@ static int cppc_cpufreq_cpu_init(struct
>>> cpufreq_policy *policy)
>>>       caps = &cpu_data->perf_caps;
>>>       policy->driver_data = cpu_data;
>>>   +    min = cppc_perf_to_khz(caps, caps->lowest_nonlinear_perf);
>>> +    max = cppc_perf_to_khz(caps, policy->boost_enabled ?
>>> +                   caps->highest_perf : caps->nominal_perf);
>>> +
>>>       /*
>>>        * Set min to lowest nonlinear perf to avoid any efficiency
>>> penalty (see
>>>        * Section 8.4.7.1.1.5 of ACPI 6.1 spec)
>>>        */
>>> -    policy->min = cppc_perf_to_khz(caps, caps->lowest_nonlinear_perf);
>>> -    policy->max = cppc_perf_to_khz(caps, policy->boost_enabled ?
>>> -                        caps->highest_perf : caps->nominal_perf);
>>> +    policy->min = min;
>>>         /*
>>>        * Set cpuinfo.min_freq to Lowest to make the full range of
>>> performance
>>> @@ -669,7 +672,7 @@ static int cppc_cpufreq_cpu_init(struct
>>> cpufreq_policy *policy)
>>>        * nonlinear perf
>>>        */
>>>       policy->cpuinfo.min_freq = cppc_perf_to_khz(caps, caps-
>>> >lowest_perf);
>>> -    policy->cpuinfo.max_freq = policy->max;
>>> +    policy->cpuinfo.max_freq = max;
>>>         policy->transition_delay_us =
>>> cppc_cpufreq_get_transition_delay_us(cpu);
>>>       policy->shared_type = cpu_data->shared_type;
>>> diff --git a/drivers/cpufreq/cpufreq-nforce2.c b/drivers/cpufreq/
>>> cpufreq-nforce2.c
>>> index fbbbe501cf2dc..831102522ad64 100644
>>> --- a/drivers/cpufreq/cpufreq-nforce2.c
>>> +++ b/drivers/cpufreq/cpufreq-nforce2.c
>>> @@ -355,8 +355,8 @@ static int nforce2_cpu_init(struct cpufreq_policy
>>> *policy)
>>>           min_fsb = NFORCE2_MIN_FSB;
>>>         /* cpuinfo and default policy values */
>>> -    policy->min = policy->cpuinfo.min_freq = min_fsb * fid * 100;
>>> -    policy->max = policy->cpuinfo.max_freq = max_fsb * fid * 100;
>>> +    policy->cpuinfo.min_freq = min_fsb * fid * 100;
>>> +    policy->cpuinfo.max_freq = max_fsb * fid * 100;
>>>         return 0;
>>>   }
>>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>>> index 44eb1b7e7fc1b..b30bfa3e27daa 100644
>>> --- a/drivers/cpufreq/cpufreq.c
>>> +++ b/drivers/cpufreq/cpufreq.c
>>> @@ -1453,6 +1453,14 @@ static int cpufreq_policy_online(struct
>>> cpufreq_policy *policy,
>>>       cpumask_and(policy->cpus, policy->cpus, cpu_online_mask);
>>>         if (new_policy) {
>>> +        unsigned int min, max;
>>> +
>>> +        /* Use policy->min/max set by the driver as QoS requests. */
>>> +        min = max(FREQ_QOS_MIN_DEFAULT_VALUE, policy->min);
>>> +        if (policy->max)
>>> +            max = min(FREQ_QOS_MAX_DEFAULT_VALUE, policy->max);
>>> +        else
>>> +            max = FREQ_QOS_MAX_DEFAULT_VALUE;
>>
>>
>> Nit: Using local variables named min/max is confusing here since they
>> shadow the common min()/max() macros; renaming them (e.g. min_freq
>> / max_freq) would improve readability and maintainability.
>>
> Ok yes sure
>>
>>>           for_each_cpu(j, policy->related_cpus) {
>>>               per_cpu(cpufreq_cpu_data, j) = policy;
>>>               add_cpu_dev_symlink(policy, j, get_cpu_device(j));
>>> @@ -1469,18 +1477,25 @@ static int cpufreq_policy_online(struct
>>> cpufreq_policy *policy,
>>>             ret = freq_qos_add_request(&policy->constraints,
>>>                          &policy->min_freq_req, FREQ_QOS_MIN,
>>> -                       FREQ_QOS_MIN_DEFAULT_VALUE);
>>> +                       min);
>>
>> It seems that the current patch is not merely a superficial cleanup; it
>> also changes the policy->min value in the GX driver, setting it to the
>> 5% value expected by the driver. If so, we should document it in the
>> commit message.
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/
>> tree/drivers/cpufreq/gx-suspmod.c#n137
>>
>> /* gx-suspmod.c constants:
>>  *   POLICY_MIN_DIV = 20
>>  *   max_duration   = 255
>>  *   maxfreq = e.g. 300000 kHz (300 MHz)
>>  */
>>
>> cpufreq_policy_online()
>>   cpufreq_driver->init(policy)      /* cpufreq_gx_cpu_init() */
>>     policy->min = maxfreq/20        /* 15000 kHz, 5% */
>>     cpuinfo.min_freq = maxfreq/255  /* 1176 kHz, 0.39% */
>>
>>   /* Before current patch: 0, not policy->min */
>>   freq_qos_add_request(..., FREQ_QOS_MIN, 0)
>>
>>   cpufreq_init_policy()
>>     cpufreq_set_policy()
>>       /* reads QoS=0, discards init()'s 15000 */
>>       new_data.min = freq_qos_read_value(FREQ_QOS_MIN)
>>       cpufreq_gx_verify()
>>         cpufreq_verify_within_cpu_limits()
>>           /* 0 < 1176: clamp to hw floor */
>>           new_data.min = cpuinfo.min_freq  /* 1176 kHz */
>>       WRITE_ONCE(policy->min, 1176)  /* 0.39%, not 5% */
>>
>> After current patch:
>> freq_qos_add_request(..., FREQ_QOS_MIN, policy->min)
>>   => new_data.min stays 15000, no clamping, policy->min = 15000
>>
> Prior to [1], the policy->min/max values were used as QoS constraint.
> This patch effectively set a min QoS constraint, but it should be no
> different from what the driver was setting initially.
>
> I will add a reference to the patch + explanation in the commit
> message to avoid any ambiguities.
>
> [1] 521223d8b3ec ("cpufreq: Fix initialization of min and max frequency
> QoS requests")
>
>

Thanks, that makes sense to me.

>>
>>>           if (ret < 0)
>>>               goto out_destroy_policy;
>>>             ret = freq_qos_add_request(&policy->constraints,
>>>                          &policy->max_freq_req, FREQ_QOS_MAX,
>>> -                       FREQ_QOS_MAX_DEFAULT_VALUE);
>>> +                       max);
>>>           if (ret < 0)
>>>               goto out_destroy_policy;
>>> blocking_notifier_call_chain(&cpufreq_policy_notifier_list,
>>>                   CPUFREQ_CREATE_POLICY, policy);
>>> +
>>> +        /*
>>> +         * If the driver didn't set QoS constraints, policy->min/max
>>> still
>>> +         * need to be set as they are used to clamp frequency requests.
>>> +         */
>>> +        policy->min = policy->min ? policy->min : policy-
>>> >cpuinfo.min_freq;
>>> +        policy->max = policy->max ? policy->max : policy-
>>> >cpuinfo.max_freq;
>>
>>
>> Does it make sense to set policy->min / policy->max before the
>> CPUFREQ_CREATE_POLICY notifier, since some drivers may use them in their
>> callbacks?
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git/
>> tree/Documentation/cpu-freq/core.rst#n58
>>
>>
> Yes right indeed, this would be better.
>
> Thanks for the review
>

--
Thx and BRs,
Zhongqiu Han