cpufreq: amd-pstate: Fix invalid write to MSR_AMD_CPPC_REQ

`amd_pstate_set_epp` function uses `cppc_req_cached` and `epp` variable
to update the MSR_AMD_CPPC_REQ register for AMD MSR systems. The recent
commit 7cca9a98 ("cpufreq: amd-pstate: avoid uninitialized variable
use") changed the sequence of updating cppc_req_cached and writing the
MSR_AMD_CPPC_REQ. Therefore while switching from powersave to
performance governor and vice-versa in active mode MSR_AMD_CPPC_REQ is
set with the previous cached value. To fix this: first update the
`cppc_req_cached` variable and then call `amd_pstate_set_epp` function.

 - Before commit 7cca9a98 ("cpufreq: amd-pstate: avoid uninitialized
   variable use"):

With powersave governor:
[    1.652743] amd_pstate_epp_init: writing to cppc_req_cached = 0x1eff
[    1.652744] amd_pstate_set_epp: writing cppc_req_cached = 0x1eff
[    1.652746] amd_pstate_set_epp: writing min_perf = 30, des_perf = 0, max_perf = 255, epp = 0

Changing to performance governor:
[  300.493842] amd_pstate_epp_init: writing to cppc_req_cached = 0xffff
[  300.493846] amd_pstate_set_epp: writing cppc_req_cached = 0xffff
[  300.493847] amd_pstate_set_epp: writing min_perf = 255, des_perf = 0, max_perf = 255, epp = 0

 - After commit 7cca9a98 ("cpufreq: amd-pstate: avoid uninitialized
   variable use"):

With powersave governor:
[    1.646037] amd_pstate_set_epp: writing cppc_req_cached = 0xffff
[    1.646038] amd_pstate_set_epp: writing min_perf = 255, des_perf = 0, max_perf = 255, epp = 0
[    1.646042] amd_pstate_epp_init: writing to cppc_req_cached = 0x1eff

Changing to performance governor:
[  687.117401] amd_pstate_set_epp: writing cppc_req_cached = 0x1eff
[  687.117405] amd_pstate_set_epp: writing min_perf = 30, des_perf = 0, max_perf = 255, epp = 0
[  687.117419] amd_pstate_epp_init: writing to cppc_req_cached = 0xffff

 - After this fix:

With powersave governor:
[    2.525717] amd_pstate_epp_init: writing to cppc_req_cached = 0x1eff
[    2.525720] amd_pstate_set_epp: writing cppc_req_cached = 0x1eff
[    2.525722] amd_pstate_set_epp: writing min_perf = 30, des_perf = 0, max_perf = 255, epp = 0

Changing to performance governor:
[ 3440.152468] amd_pstate_epp_init: writing to cppc_req_cached = 0xffff
[ 3440.152473] amd_pstate_set_epp: writing cppc_req_cached = 0xffff
[ 3440.152474] amd_pstate_set_epp: writing min_perf = 255, des_perf = 0, max_perf = 255, epp = 0

Fixes: 7cca9a98 ("cpufreq: amd-pstate: avoid uninitialized variable use")
Signed-off-by: Wyes Karny <wyes.karny@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/amd-pstate.c

@@ -1057,27 +1057,28 @@ static void amd_pstate_epp_init(unsigned int cpu)
 
 	cpudata->epp_policy = cpudata->policy;
 
-	if (cpudata->policy == CPUFREQ_POLICY_PERFORMANCE) {
-		epp = amd_pstate_get_epp(cpudata, value);
-		if (epp < 0)
-			goto skip_epp;
-		/* force the epp value to be zero for performance policy */
-		epp = 0;
-	} else {
-		/* Get BIOS pre-defined epp value */
-		epp = amd_pstate_get_epp(cpudata, value);
-		if (epp)
-			goto skip_epp;
+	/* Get BIOS pre-defined epp value */
+	epp = amd_pstate_get_epp(cpudata, value);
+	if (epp < 0) {
+		/**
+		 * This return value can only be negative for shared_memory
+		 * systems where EPP register read/write not supported.
+		 */
+		goto skip_epp;
 	}
+
+	if (cpudata->policy == CPUFREQ_POLICY_PERFORMANCE)
+		epp = 0;
+
 	/* Set initial EPP value */
 	if (boot_cpu_has(X86_FEATURE_CPPC)) {
 		value &= ~GENMASK_ULL(31, 24);
 		value |= (u64)epp << 24;
 	}
 
+	WRITE_ONCE(cpudata->cppc_req_cached, value);
 	amd_pstate_set_epp(cpudata, epp);
 skip_epp:
-	WRITE_ONCE(cpudata->cppc_req_cached, value);
 	cpufreq_cpu_put(policy);
 }