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Subject: Re: [v2] cpufreq / CPPC: Add cpuinfo_cur_freq support for CPPC
To:     Jayachandran C <jnair@caviumnetworks.com>,
        George Cherian <george.cherian@cavium.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        viresh.kumar@linaro.org, rjw@rjwysocki.net, pprakash@codeaurora.org
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 <20180619203846.GA6083@jc-sabre>
From:   George Cherian <gcherian@caviumnetworks.com>
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Date:   Wed, 20 Jun 2018 14:59:15 +0530
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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@cavium.com>
>> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
>> ---
>>   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