[PATCH] intel_idle: Fix max_cstate for processor models without C-state tables

[Chen Yu]

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 might get wrong. Fix this by
subtracting the index by 1 when checking max cstate via _CST.
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>
---
 drivers/idle/intel_idle.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index 9a810e4a7946..dbd4be1c271b 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -1236,7 +1236,7 @@ static void __init intel_idle_init_cstates_acpi(struct cpuidle_driver *drv)
 		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];
-- 
2.17.1