linux-kernel - Re: [PATCH 2/2] cpufreq: intel_pstate: Enable HWP without EPP feature

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Message-ID: <CAJZ5v0gwXiRTo_Lri3rduA-RqDhwgK4ymwX3ttxqhTw9W6d=fQ@mail.gmail.com>
Date: Fri, 5 Sep 2025 23:02:10 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc: rafael@...nel.org, viresh.kumar@...aro.org, linux-pm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] cpufreq: intel_pstate: Enable HWP without EPP feature

On Thu, Sep 4, 2025 at 2:06 AM Srinivas Pandruvada
<srinivas.pandruvada@...ux.intel.com> wrote:
>
> When EPP feature is not available (CPUID CPUID.06H:EAX[10] is not set),
> intel_pstate will not enable HWP.
>
> Some processors support DEC feature (Dynamic Efficiency Control). But in
> this case HWP must be enabled.
>
> So, enable HWP even if EPP feature is not available but DEC feature is
> present.
>
> When EPP feature is not available don't publish sysfs attributes
> "energy_performance_available_preferences" and
> "energy_performance_preference", but continue to enable HWP.
>
> Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> ---
>  drivers/cpufreq/intel_pstate.c | 35 +++++++++++++++++++++++++++++++---
>  1 file changed, 32 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> index c28454b16723..d74abe909fbc 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -904,6 +904,11 @@ static struct freq_attr *hwp_cpufreq_attrs[] = {
>         NULL,
>  };
>
> +static struct freq_attr *hwp_cpufreq_default_attrs[] = {
> +       &base_frequency,
> +       NULL,
> +};
> +
>  static bool no_cas __ro_after_init;
>
>  static struct cpudata *hybrid_max_perf_cpu __read_mostly;
> @@ -1370,6 +1375,9 @@ static void intel_pstate_hwp_offline(struct cpudata *cpu)
>  #define POWER_CTL_EE_ENABLE    1
>  #define POWER_CTL_EE_DISABLE   2
>
> +/* Enable bit for Dynamic Efficiency Control (DEC) */
> +#define POWER_CTL_DEC_ENABLE   27
> +
>  static int power_ctl_ee_state;
>
>  static void set_power_ctl_ee_state(bool input)
> @@ -3761,6 +3769,17 @@ static const struct x86_cpu_id intel_hybrid_scaling_factor[] = {
>         {}
>  };
>
> +static bool dec_enabled(void)
> +{
> +       u64 power_ctl;
> +
> +       rdmsrq(MSR_IA32_POWER_CTL, power_ctl);
> +       if (power_ctl & BIT(POWER_CTL_DEC_ENABLE))
> +               return true;
> +
> +       return false;
> +}
> +
>  static int __init intel_pstate_init(void)
>  {
>         static struct cpudata **_all_cpu_data;
> @@ -3793,15 +3812,24 @@ static int __init intel_pstate_init(void)
>                  * Avoid enabling HWP for processors without EPP support,
>                  * because that means incomplete HWP implementation which is a
>                  * corner case and supporting it is generally problematic.
> +                * But when DEC enable bit is set (MSR 0x1FC bit 27), continue
> +                * to enable HWP.
>                  *
>                  * If HWP is enabled already, though, there is no choice but to
>                  * deal with it.
>                  */
> -               if ((!no_hwp && boot_cpu_has(X86_FEATURE_HWP_EPP)) || hwp_forced) {
> +               if (!no_hwp || hwp_forced) {
> +                       if (boot_cpu_has(X86_FEATURE_HWP_EPP)) {
> +                               intel_pstate.attr = hwp_cpufreq_attrs;
> +                               intel_cpufreq.attr = hwp_cpufreq_attrs;
> +                       } else if (dec_enabled()) {
> +                               intel_pstate.attr = hwp_cpufreq_default_attrs;
> +                               intel_cpufreq.attr = hwp_cpufreq_default_attrs;
> +                       } else {
> +                               goto skip_hwp_enable;
> +                       }
>                         hwp_active = true;
>                         hwp_mode_bdw = id->driver_data;
> -                       intel_pstate.attr = hwp_cpufreq_attrs;
> -                       intel_cpufreq.attr = hwp_cpufreq_attrs;
>                         intel_cpufreq.flags |= CPUFREQ_NEED_UPDATE_LIMITS;
>                         intel_cpufreq.adjust_perf = intel_cpufreq_adjust_perf;
>                         if (!default_driver)
> @@ -3811,6 +3839,7 @@ static int __init intel_pstate_init(void)
>
>                         goto hwp_cpu_matched;
>                 }
> +skip_hwp_enable:
>                 pr_info("HWP not enabled\n");
>         } else {
>                 if (no_load)
> --

I think that this would work, but then it looks super ad hoc and I'd
like to completely rearrange it.

My (totally untested) version is attached.  Please let me know if this
can be made work for you and if so, I'll turn it into a proper patch.

View attachment "cpufreq-intel_pstate-Enable-HWP-without-EPP-feature.patch" of type "text/x-patch" (2856 bytes)