Re: [PATCH RFC] intel_pstate: play well with frequency limits set by acpi

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

On 21.07.2015 00:08, Srinivas Pandruvada wrote:
> On Fri, 2015-07-17 at 07:36 +0300, Konstantin Khlebnikov wrote:
>> On Fri, Jul 17, 2015 at 1:08 AM, Srinivas Pandruvada
>> <srinivas.pandruvada@linux.intel.com> wrote:
>>> On Thu, 2015-07-16 at 21:17 +0300, Konstantin Khlebnikov wrote:
>>>> IPMI can control CPU P-states remotely: configuration is reported via
>>>> common ACPI interface (_PPC/_PSS/etc). This patch adds required minimal
>>>> support in intel_pstate to receive and use these P-state limits.
>>>>
>>>> * ignore limit of top state in _PPC: it lower than turbo boost frequency
>>>> * register intel_pstate in acpi-processor to get states from _PSS
>>>> * link acpi_processor_get_bios_limit: this adds attribute "bios_limit"
>>>>
>>>> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
>>>> ---
>>>>   drivers/acpi/processor_perflib.c |    3 +-
>>>>   drivers/cpufreq/intel_pstate.c   |   57 ++++++++++++++++++++++++++++++++++++++
>>>>   2 files changed, 59 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/acpi/processor_perflib.c b/drivers/acpi/processor_perflib.c
>>>> index cfc8aba72f86..781e328c9d5f 100644
>>>> --- a/drivers/acpi/processor_perflib.c
>>>> +++ b/drivers/acpi/processor_perflib.c
>>>> @@ -98,7 +98,8 @@ static int acpi_processor_ppc_notifier(struct notifier_block *nb,
>>>>
>>>>        ppc = (unsigned int)pr->performance_platform_limit;
>>>>
>>>> -     if (ppc >= pr->performance->state_count)
>>>> +     /* Ignore limit of top state: it lower than turbo boost frequency */
>>>> +     if (!ppc || ppc >= pr->performance->state_count)
>>> Why? Isn't the previous check enough?
>>
>> Zero _PPC state must be top performance state but as I see frequency in
>> _PSS is lower than maximum possible turbo frequency. So, in this case
>> intel_pstate cannnot get "100%"  for max bound even it there is no limit set.
>>
>> For example: I saw _PSS[0] = 2601 Mhz, PSS[1] = 2600 Mhz while turbo
>> state is 3400 Mhz.
>>
> Have you tested dynamic _PPC modification with acpi cpufreq with this
> change (after boot)? Suppose _PPC is changed from 3 to 0, then
> cpufreq_verify_within_limits will not be called to change to new max
> turbo performance state.
>

I haven't checked that but as I see acpi_processor_ppc_notifier()
can only reduce maximum frequency. So, there should be no problem
in this case.

-- 
Konstantin