cpufreq/amd-pstate: Refactor max frequency calculation

The previous approach introduced roundoff errors during division when
calculating the boost ratio. This, in turn, affected the maximum
frequency calculation, often resulting in reporting lower frequency
values.

For example, on the Glinda SoC based board with the following
parameters:

max_perf = 208
nominal_perf = 100
nominal_freq = 2600 MHz

The Linux kernel previously calculated the frequency as:
freq = ((max_perf * 1024 / nominal_perf) * nominal_freq) / 1024
freq = 5405 MHz  // Integer arithmetic.

With the updated formula:
freq = (max_perf * nominal_freq) / nominal_perf
freq = 5408 MHz

This change ensures more accurate frequency calculations by eliminating
unnecessary shifts and divisions, thereby improving precision.

Signed-off-by: Naresh Solanki <[email protected]>
[ML: trim the changelog from commit message]
Reviewed-by: Mario Limonciello <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mario Limonciello <[email protected]>

Author: nsolanki22

drivers/cpufreq/amd-pstate.c

@@ -908,9 +908,8 @@ static int amd_pstate_init_freq(struct amd_cpudata *cpudata)
 {
 	int ret;
 	u32 min_freq, max_freq;
-	u32 nominal_perf, nominal_freq;
+	u32 highest_perf, nominal_perf, nominal_freq;
 	u32 lowest_nonlinear_perf, lowest_nonlinear_freq;
-	u32 boost_ratio, lowest_nonlinear_ratio;
 	struct cppc_perf_caps cppc_perf;
 
 	ret = cppc_get_perf_caps(cpudata->cpu, &cppc_perf);
@@ -927,16 +926,12 @@ static int amd_pstate_init_freq(struct amd_cpudata *cpudata)
 	else
 		nominal_freq = cppc_perf.nominal_freq;
 
+	highest_perf = READ_ONCE(cpudata->highest_perf);
 	nominal_perf = READ_ONCE(cpudata->nominal_perf);
-
-	boost_ratio = div_u64(cpudata->highest_perf << SCHED_CAPACITY_SHIFT, nominal_perf);
-	max_freq = (nominal_freq * boost_ratio >> SCHED_CAPACITY_SHIFT);
+	max_freq = div_u64((u64)highest_perf * nominal_freq, nominal_perf);
 
 	lowest_nonlinear_perf = READ_ONCE(cpudata->lowest_nonlinear_perf);
-	lowest_nonlinear_ratio = div_u64(lowest_nonlinear_perf << SCHED_CAPACITY_SHIFT,
-					 nominal_perf);
-	lowest_nonlinear_freq = (nominal_freq * lowest_nonlinear_ratio >> SCHED_CAPACITY_SHIFT);
-
+	lowest_nonlinear_freq = div_u64((u64)nominal_freq * lowest_nonlinear_perf, nominal_perf);
 	WRITE_ONCE(cpudata->min_freq, min_freq * 1000);
 	WRITE_ONCE(cpudata->lowest_nonlinear_freq, lowest_nonlinear_freq * 1000);
 	WRITE_ONCE(cpudata->nominal_freq, nominal_freq * 1000);