[PATCH 5.9 335/391] intel_idle: Fix max_cstate for processor models without C-state tables

[Greg Kroah-Hartman]

From: Chen Yu <yu.c.chen@xxxxxxxxx>

commit 4e0ba5577dba686f96c1c10ef4166380667fdec7 upstream.

Currently intel_idle driver gets the c-state information from ACPI
_CST if the processor model is not recognized by it. However the
c-state in _CST starts with index 1 which is different from the
index in intel_idle driver's internal c-state table.

While intel_idle_max_cstate_reached() was previously introduced to
deal with intel_idle driver's internal c-state table, re-using
this function directly on _CST is incorrect.

Fix this by subtracting 1 from the index when checking max_cstate
in the _CST case.

For example, append intel_idle.max_cstate=1 in boot command line,
Before the patch:
grep . /sys/devices/system/cpu/cpu0/cpuidle/state*/name
POLL
After the patch:
grep . /sys/devices/system/cpu/cpu0/cpuidle/state*/name
/sys/devices/system/cpu/cpu0/cpuidle/state0/name:POLL
/sys/devices/system/cpu/cpu0/cpuidle/state1/name:C1_ACPI

Fixes: 18734958e9bf ("intel_idle: Use ACPI _CST for processor models without C-state tables")
Reported-by: Pengfei Xu <pengfei.xu@xxxxxxxxx>
Cc: 5.6+ <stable@xxxxxxxxxxxxxxx> # 5.6+
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/idle/intel_idle.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -1235,7 +1235,7 @@ static void __init intel_idle_init_cstat
 		struct acpi_processor_cx *cx;
 		struct cpuidle_state *state;
 
-		if (intel_idle_max_cstate_reached(cstate))
+		if (intel_idle_max_cstate_reached(cstate - 1))
 			break;
 
 		cx = &acpi_state_table.states[cstate];