linux-kernel - Re: [PATCH] cpufreq: intel_pstate: Load when ACPI PCCH is present

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Date:   Tue, 17 Jul 2018 10:23:00 -0700
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Andreas Herrmann <aherrmann@...e.com>
Cc:     "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: intel_pstate: Load when ACPI PCCH is present

On Tue, 2018-07-17 at 18:13 +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Currently, intel_pstate doesn't load if _PSS is not present on
> HP Proliant systems, because it expects the firmware to take over
> CPU performance scaling in that case.  However, if ACPI PCCH is
> present, the firmware expects the kernel to use it for CPU
> performance scaling and the pcc-cpufreq driver is loaded for that.
> 
> Unfortunately, the firmware interface used by that driver is not
> scalable for fundamental reasons, so pcc-cpufreq is way suboptimal
> on systems with more than just a few CPUs.  In fact, it is better to
> avoid using it at all.
> 
> For this reason, modify intel_pstate to look for ACPI PCCH if _PSS
> is not present and load if it is there.
> 
> Fixes: fbbcdc0744da (intel_pstate: skip the driver if ACPI has power
> mgmt option)
> Reported-by: Andreas Herrmann <aherrmann@...e.com>
> Tested-by: Andreas Herrmann <aherrmann@...e.com>
> Reviewed-by: Andreas Herrmann <aherrmann@...e.com>
> Cc: 4.17+ <stable@...r.kernel.org> # 4.17+
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>

But do we need a change as done by the following commit in in pcc-
cpufreq.c?

"
commit 8a61e12e84597b5f8155ac91b44dea866ccfaac2
Author: Yinghai Lu <yinghai@...nel.org>
Date:   Fri Sep 20 10:43:56 2013 -0700

    acpi-cpufreq: skip loading acpi_cpufreq after intel_pstate
    
"

Thanks,
Srinivas

> ---
>  drivers/cpufreq/intel_pstate.c |   17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> Index: linux-pm/drivers/cpufreq/intel_pstate.c
> ===================================================================
> --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> +++ linux-pm/drivers/cpufreq/intel_pstate.c
> @@ -2391,6 +2391,18 @@ static bool __init intel_pstate_no_acpi_
>  	return true;
>  }
>  
> +static bool __init intel_pstate_no_acpi_pcch(void)
> +{
> +	acpi_status status;
> +	acpi_handle handle;
> +
> +	status = acpi_get_handle(NULL, "\\_SB", &handle);
> +	if (ACPI_FAILURE(status))
> +		return true;
> +
> +	return !acpi_has_method(handle, "PCCH");
> +}
> +
>  static bool __init intel_pstate_has_acpi_ppc(void)
>  {
>  	int i;
> @@ -2450,7 +2462,10 @@ static bool __init intel_pstate_platform
>  
>  	switch (plat_info[idx].data) {
>  	case PSS:
> -		return intel_pstate_no_acpi_pss();
> +		if (!intel_pstate_no_acpi_pss())
> +			return false;
> +
> +		return intel_pstate_no_acpi_pcch();
>  	case PPC:
>  		return intel_pstate_has_acpi_ppc() && !force_load;
>  	}
>