cpufreq: amd-pstate: Allow users to write 'default' EPP string

The EPP string for 'default' represents what the firmware had configured
as the default EPP value but once a user changes EPP to another string
they can't reset it back to 'default'.

Cache the firmware EPP value and allow the user to write 'default' using
this value.

Reported-by: Artem S. Tashkinov <aros@gmx.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217931#c61
Reviewed-by: Perry Yuan <perry.yuan@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>

Author: superm1

drivers/cpufreq/amd-pstate.c

@@ -282,10 +282,8 @@ static int amd_pstate_set_energy_pref_index(struct amd_cpudata *cpudata,
 	int epp = -EINVAL;
 	int ret;
 
-	if (!pref_index) {
-		pr_debug("EPP pref_index is invalid\n");
-		return -EINVAL;
-	}
+	if (!pref_index)
+		epp = cpudata->epp_default;
 
 	if (epp == -EINVAL)
 		epp = epp_values[pref_index];
@@ -1441,7 +1439,7 @@ static int amd_pstate_epp_cpu_init(struct cpufreq_policy *policy)
 
 	policy->driver_data = cpudata;
 
-	cpudata->epp_cached = amd_pstate_get_epp(cpudata, 0);
+	cpudata->epp_cached = cpudata->epp_default = amd_pstate_get_epp(cpudata, 0);
 
 	policy->min = policy->cpuinfo.min_freq;
 	policy->max = policy->cpuinfo.max_freq;

drivers/cpufreq/amd-pstate.h

@@ -99,6 +99,7 @@ struct amd_cpudata {
 	u32	policy;
 	u64	cppc_cap1_cached;
 	bool	suspended;
+	s16	epp_default;
 };
 
 #endif /* _LINUX_AMD_PSTATE_H */