Re: [PATCH v3] cpufreq / CPPC: Add cpuinfo_cur_freq support for CPPC

[George Cherian]

Hi Prakash,


On 07/09/2018 10:12 PM, Prakash, Prashanth wrote:
> Hi George,
>
>
> On 7/9/2018 4:10 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 | 44 ++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 44 insertions(+)
> >
> > diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> > index a9d3eec..61132e8 100644
> > --- a/drivers/cpufreq/cppc_cpufreq.c
> > +++ b/drivers/cpufreq/cppc_cpufreq.c
> > @@ -296,10 +296,54 @@ 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;
> > +
> > +     delta_reference = (u32)fb_ctrs_t1.reference -
> > +                             (u32)fb_ctrs_t0.reference;
> > +     delta_delivered = (u32)fb_ctrs_t1.delivered -
> > +                             (u32)fb_ctrs_t0.delivered;
> Why (u32)? These registers can be 64bits and that's why cppc_perf_fb_ctrs
> have 64b fields for reference and delivered counters.
>
> Moreover, the integer math is incorrect. You can run into a scenario where
> t1.ref/del < t0.ref/del,  thus setting a negative number to u64! The likelihood
> of this is very high especially when you throw away the higher 32bits.
Because of binary representation, unsigned subtraction will work even if
t1.ref/del < t0.ref/del. So essentially, the code should look like
this,

static inline u64 get_delta(u64 t1, u64 t0)
{
        if (t1 > t0 || t0 > ~(u32)0)
                return t1 - t0;

        return (u32)t1 - (u32)t0;
}

As a further optimization, I used (u32) since that also works,
as long as the momentary delta at any point is not greater than 2 ^ 32.
I don't foresee any reason for any platform to increment the counters at
an interval greater than 2 ^ 32.

> To keep things simple, do something like below:
>
> if (t1.reference <= t0.reference || t1.delivered <= t0.delivered) {
>       /* Atleast one of them should have overflowed */
>       return desired_perf;
> }
> else {
>       compute the delivered perf using the counters.
> }

No need to do like this as this is tested and found working across 
counter overruns in our platform.
> > +
> > +     /* Check to avoid divide-by zero */
> > +     if (delta_reference || delta_delivered)
> > +             delivered_perf = (reference_perf * delta_delivered) /
> > +                                     delta_reference;
> > +     else
> > +             delivered_perf = cpu->perf_ctrls.desired_perf;
> > +
> > +     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",