[PATCH 1/2 v4] hwmon: (aspeed-pwm-tacho) reduce fan_tach period

[Patrick Venture <venture@google.com>]

Reduce the fan_tach period such that the fan controller uses a shorter
period to measure the rpm.

The original period of 0x1000 was chosen as a conversative value from the
reference implementation.  Through experimentation on the quanta-q71l
board, I was able to drive the number down which ultimately reduced the
time the controller would use to determine the fan_tach.  This value was
recently tested and accepted downstream on the IBM Zaius board which uses
the ast2500.

Future work: It may be worthwhile as this is a tunable parameter to the
system, to allow overriding it through the device tree.

Testing: Tested on an ast2400 sitting on a quanta-q71l and ast2500 on
power9.

Signed-off-by: Patrick Venture <venture@google.com>
---
v4: Added explanation for value.
v3: Added missing change log
v2: Updated commit message language
---
 drivers/hwmon/aspeed-pwm-tacho.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hwmon/aspeed-pwm-tacho.c b/drivers/hwmon/aspeed-pwm-tacho.c
index 86e2ea8287a7..b2ab5612d8a4 100644
--- a/drivers/hwmon/aspeed-pwm-tacho.c
+++ b/drivers/hwmon/aspeed-pwm-tacho.c
@@ -160,7 +160,7 @@
  * 11: reserved.
  */
 #define M_TACH_MODE 0x02 /* 10b */
-#define M_TACH_UNIT 0x1000
+#define M_TACH_UNIT 0x00c0
 #define INIT_FAN_CTRL 0xFF
 
 struct aspeed_pwm_tacho_data {
-- 
2.13.1.611.g7e3b11ae1-goog