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

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

[George Cherian]

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;
+	}
+
+	if (delta_reference)  /* Check to avoid divide-by zero */
+		delivered_perf = (reference_perf * delta_delivered) /
+					delta_reference;
+	else
+		delivered_perf = reference_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",
-- 
2.7.4

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

[Prakash, Prashanth]

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@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 {
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.
> +		/*
> +		 * 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.

> +
> +	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

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

[Jayachandran C]

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.

JC.

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@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 {
> 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

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@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

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

[Prakash, Prashanth]

Hi George,

On 6/20/2018 3:17 AM, George Cherian wrote:
> 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@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 {
>> 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?

I am not sure I followed the above. The gist of my comment was when the counters
are equal we cannot assume that there was a overflow. So change the ">" condition
to ">=" and my concern about assuming overflow when equal should be take care of.

The above change would be required for both reference and delivered counters.