hwmon: (pmbus) delta-ahe50dc-fan: work around hardware quirk

CLEAR_FAULTS commands can apparently sometimes trigger catastrophic
power output glitches on the ahe-50dc, so block them from being sent
at all.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20220427035109.3819-1-zev@bewilderbeest.net
Fixes: d387d88 ("hwmon: (pmbus) Add Delta AHE-50DC fan control module driver")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

drivers/hwmon/pmbus/delta-ahe50dc-fan.c

@@ -14,6 +14,21 @@
 
 #define AHE50DC_PMBUS_READ_TEMP4 0xd0
 
+static int ahe50dc_fan_write_byte(struct i2c_client *client, int page, u8 value)
+{
+	/*
+	 * The CLEAR_FAULTS operation seems to sometimes (unpredictably, perhaps
+	 * 5% of the time or so) trigger a problematic phenomenon in which the
+	 * fan speeds surge momentarily and at least some (perhaps all?) of the
+	 * system's power outputs experience a glitch.
+	 *
+	 * However, according to Delta it should be OK to simply not send any
+	 * CLEAR_FAULTS commands (the device doesn't seem to be capable of
+	 * reporting any faults anyway), so just blackhole them unconditionally.
+	 */
+	return value == PMBUS_CLEAR_FAULTS ? -EOPNOTSUPP : -ENODATA;
+}
+
 static int ahe50dc_fan_read_word_data(struct i2c_client *client, int page, int phase, int reg)
 {
 	/* temp1 in (virtual) page 1 is remapped to mfr-specific temp4 */
@@ -68,6 +83,7 @@ static struct pmbus_driver_info ahe50dc_fan_info = {
 		PMBUS_HAVE_VIN | PMBUS_HAVE_FAN12 | PMBUS_HAVE_FAN34 |
 		PMBUS_HAVE_STATUS_FAN12 | PMBUS_HAVE_STATUS_FAN34 | PMBUS_PAGE_VIRTUAL,
 	.func[1] = PMBUS_HAVE_TEMP | PMBUS_PAGE_VIRTUAL,
+	.write_byte = ahe50dc_fan_write_byte,
 	.read_word_data = ahe50dc_fan_read_word_data,
 };