Re: [PATCH v2] cpufreq / CPPC: Add cpuinfo_cur_freq support for CPPC

[George Cherian]

Hi Prakash,

Thanks for the review.

On 06/19/2018 01:51 AM, Prakash, Prashanth wrote:
> External Email
>
> Hi George,
>
> On 6/15/2018 4:03 AM, George Cherian wrote:
> > Per Section 8.4.7.1.3 of ACPI 6.2, The platform provides performance
> > feedback via set of performance counters. To determine the actual
> > performance level delivered over time, OSPM may read a set of
> > performance counters from the Reference Performance Counter Register
> > and the Delivered Performance Counter Register.
> >
> > OSPM calculates the delivered performance over a given time period by
> > taking a beginning and ending snapshot of both the reference and
> > delivered performance counters, and calculating:
> >
> > delivered_perf = reference_perf X (delta of delivered_perf counter / delta of reference_perf counter).
> >
> > Implement the above and hook this to the cpufreq->get method.
> >
> > Signed-off-by: George Cherian <george.cherian@xxxxxxxxxx>
> > Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> > ---
> >   drivers/cpufreq/cppc_cpufreq.c | 71 ++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 71 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> > index 3464580..3fe7625 100644
> > --- a/drivers/cpufreq/cppc_cpufreq.c
> > +++ b/drivers/cpufreq/cppc_cpufreq.c
> > @@ -296,10 +296,81 @@ static int cppc_cpufreq_cpu_init(struct cpufreq_policy *policy)
> >        return ret;
> >   }
> >
> > +static int cppc_get_rate_from_fbctrs(struct cppc_cpudata *cpu,
> > +                                  struct cppc_perf_fb_ctrs fb_ctrs_t0,
> > +                                  struct cppc_perf_fb_ctrs fb_ctrs_t1)
> > +{
> > +     u64 delta_reference, delta_delivered;
> > +     u64 reference_perf, delivered_perf;
> > +
> > +     reference_perf = fb_ctrs_t0.reference_perf;
> > +     if (fb_ctrs_t1.reference > fb_ctrs_t0.reference) {
> > +             delta_reference = fb_ctrs_t1.reference - fb_ctrs_t0.reference;
> > +     } else {
> There should be another if () here to check if the reference counters are equal.
> We cannot assume, there was a overflow when the counters are equal. As I
> mentioned on last patch, the counters *may* pause in idle states.
My Bad... I somehow, over looked that point. In case of delta_reference 
being zero there is actually a check below to avoid divide-by-zero. 
There I returned  reference perf instead of desired perf, same I will 
take care in v3. Isn't that sufficient or is there a need for an 
explicit check here for delta = zero?

Moreover the delta calculation am planning to replace with single
line comparison in v3 for both normal and overflow case.
> > +             /*
> > +              * Counters would have wrapped-around
> > +              * We also need to find whether the low level fw
> > +              * maintains 32 bit or 64 bit counters, to calculate
> > +              * the correct delta.
> > +              */
> > +             if (fb_ctrs_t0.reference > (~(u32)0))
> > +                     delta_reference  = (~((u64)0) - fb_ctrs_t0.reference) +
> > +                                     fb_ctrs_t1.reference;
> > +             else
> > +                     delta_reference  = (~((u32)0) - fb_ctrs_t0.reference) +
> > +                                     fb_ctrs_t1.reference;
> > +     }
> > +
> > +     if (fb_ctrs_t1.delivered > fb_ctrs_t0.delivered) {
> > +             delta_delivered = fb_ctrs_t1.delivered - fb_ctrs_t0.delivered;
> > +     } else {
> > +             /*
> > +              * Counters would have wrapped-around
> > +              * We also need to find whether the low level fw
> > +              * maintains 32 bit or 64 bit counters, to calculate
> > +              * the correct delta.
> > +              */
> > +             if (fb_ctrs_t0.delivered > (~(u32)0))
> > +                     delta_delivered  = (~((u64)0) - fb_ctrs_t0.delivered) +
> > +                                     fb_ctrs_t1.delivered;
> > +             else
> > +                     delta_delivered  = (~((u32)0) - fb_ctrs_t0.delivered) +
> > +                                     fb_ctrs_t1.delivered;
> > +     }
> > +
> > +     if (delta_reference)  /* Check to avoid divide-by zero */
> > +             delivered_perf = (reference_perf * delta_delivered) /
> > +                                     delta_reference;
> > +     else
> > +             delivered_perf = reference_perf;
>
> If we cannot compute delivered performance then we should return
> desired/requested perf and not reference_perf.
Noted!!
> > +
> > +     return cppc_cpufreq_perf_to_khz(cpu, delivered_perf);
> > +}
> > +
> > +static unsigned int cppc_cpufreq_get_rate(unsigned int cpunum)
> > +{
> > +     struct cppc_perf_fb_ctrs fb_ctrs_t0 = {0}, fb_ctrs_t1 = {0};
> > +     struct cppc_cpudata *cpu = all_cpu_data[cpunum];
> > +     int ret;
> > +
> > +     ret = cppc_get_perf_ctrs(cpunum, &fb_ctrs_t0);
> > +     if (ret)
> > +             return ret;
> > +
> > +     udelay(2); /* 2usec delay between sampling */
> > +
> > +     ret = cppc_get_perf_ctrs(cpunum, &fb_ctrs_t1);
> > +     if (ret)
> > +             return ret;
> > +
> > +     return cppc_get_rate_from_fbctrs(cpu, fb_ctrs_t0, fb_ctrs_t1);
> > +}
> > +
> >   static struct cpufreq_driver cppc_cpufreq_driver = {
> >        .flags = CPUFREQ_CONST_LOOPS,
> >        .verify = cppc_verify_policy,
> >        .target = cppc_cpufreq_set_target,
> > +     .get = cppc_cpufreq_get_rate,
> >        .init = cppc_cpufreq_cpu_init,
> >        .stop_cpu = cppc_cpufreq_stop_cpu,
> >        .name = "cppc_cpufreq",
>
> Thanks,
> Prashanth

Thanks,
-George