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

[George Cherian]

Hi JC,

Thanks for the review.


On 06/20/2018 02:09 AM, Jayachandran C wrote:
> Hi George,
>
> Few comments on your patch:
>
> On Fri, Jun 15, 2018 at 03:03:15AM -0700, 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 {
> > +		/*
> > +		 * 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;
> > +	}
>
> Having this code repeated twice does not look great. Also the math here
> is not correct, since (~0 - val2 + val1) is off by one. Because of
> binary representation, unsigned subtraction will work even if
> val2 < val1. So cleaner way would be to do:
>
> static inline u64 ts_sub(u64 t1, u64 t0)
> {
> 	if (t1 > t0 || t0 > ~(u32)0)
> 		return t1 - t0;
>
> 	return (u32)t1 - (u32)t0;
> }
>
> And then use ts_sub in both places above.

I was actually thinking to replace the whole comparison with a single
line irrespective of rollover or not.
It will look something like this.

delta = (u32)(((1UL << 32) - t0) + t1);

This will also take care of the value being off by one.
> JC.

Regards,
-George