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

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:

quoted

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 <redacted>
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