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

Rafael J. Wysocki
2018-07-17T17:28:41Z

On Tue, Jul 17, 2018 at 7:23 PM, Srinivas Pandruvada
<srinivas.pandruvada@linux.intel.com> wrote:
> On Tue, 2018-07-17 at 18:13 +0200, Rafael J. Wysocki wrote:
>> From: Rafael J. Wysocki <rafael.j.wysocki@intel.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@suse.com>
>> Tested-by: Andreas Herrmann <aherrmann@suse.com>
>> Reviewed-by: Andreas Herrmann <aherrmann@suse.com>
>> Cc: 4.17+ <stable@vger.kernel.org> # 4.17+
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.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@kernel.org>
> Date: Fri Sep 20 10:43:56 2013 -0700
>
> acpi-cpufreq: skip loading acpi_cpufreq after intel_pstate
>
> "

Quite likely yes, good point!

>> ---
>> 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;
>> }
>>