[PATCH] cpufreq/cppc: fix perf_to_khz conversion exception

[liwei]

When the nominal_freq recorded by the kernel is equal to lowest_freq and
the frequency reduction operation is triggered externally, there is a
conversion logic error in cppc_perf_to_khz(), causing the kernel to be
unable to feedback the true frequency.

Fix this by adding the branch processing logic when nominal_freq is equal
to lowest_freq.

Fixes: ec1c7ad47664 ("cpufreq: CPPC: Fix performance/frequency conversion")
Signed-off-by: liwei <liwei728@xxxxxxxxxx>
---
 drivers/acpi/cppc_acpi.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index a40b6f3946ef..92aac6974e0e 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1869,9 +1869,15 @@ unsigned int cppc_perf_to_khz(struct cppc_perf_caps *caps, unsigned int perf)
 	u64 mul, div;
 
 	if (caps->lowest_freq && caps->nominal_freq) {
-		mul = caps->nominal_freq - caps->lowest_freq;
+		/* Avoid the special case when nominal_freq is equal to lowest_freq */
+		if (caps->lowest_freq == caps->nominal_freq) {
+			mul = caps->nominal_freq;
+			div = caps->nominal_perf;
+		} else {
+			mul = caps->nominal_freq - caps->lowest_freq;
+			div = caps->nominal_perf - caps->lowest_perf;
+		}
 		mul *= KHZ_PER_MHZ;
-		div = caps->nominal_perf - caps->lowest_perf;
 		offset = caps->nominal_freq * KHZ_PER_MHZ -
 			 div64_u64(caps->nominal_perf * mul, div);
 	} else {
-- 
2.25.1