ACPI / CPPC: Fix guaranteed performance handling

As per the ACPI specification, "Guaranteed Performance Register" is
a "Buffer" field and it cannot be "Integer", so treat the "Integer"
type for "Guaranteed Performance Register" field as invalid and
ignore its value in that case.

Also save one cpc_read() call when "Guaranteed Performance Register"
is not present, which means a register defined as:
"Register(SystemMemory, 0, 0, 0, 0)".

Fixes: 29523f095397 ("ACPI / CPPC: Add support for guaranteed performance")
Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: 4.20+ <stable@vger.kernel.org> # 4.20+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
index 1b207fca1420..d4244e7d0e38 100644
--- a/drivers/acpi/cppc_acpi.c
+++ b/drivers/acpi/cppc_acpi.c
@@ -1150,8 +1150,13 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
         cpc_read(cpunum, nominal_reg, &nom);
         perf_caps->nominal_perf = nom;
 
-        cpc_read(cpunum, guaranteed_reg, &guaranteed);
-        perf_caps->guaranteed_perf = guaranteed;
+        if (guaranteed_reg->type != ACPI_TYPE_BUFFER  ||
+            IS_NULL_REG(&guaranteed_reg->cpc_entry.reg)) {
+                perf_caps->guaranteed_perf = 0;
+        } else {
+                cpc_read(cpunum, guaranteed_reg, &guaranteed);
+                perf_caps->guaranteed_perf = guaranteed;
+        }
 
         cpc_read(cpunum, lowest_non_linear_reg, &min_nonlinear);
         perf_caps->lowest_nonlinear_perf = min_nonlinear;