cpufreq/amd-pstate: Refactor max frequency calculation

JIRA: https://issues.redhat.com/browse/RHEL-75923

commit 857a61c
Author: Naresh Solanki <[email protected]>
Date:   Fri Dec 20 01:48:32 2024 +0530

    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]>

Signed-off-by: David Arcari <[email protected]>

Author: darcari

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);