[RFC PATCH 2/2] hwmon: (pmbus/ucd9000) Throttle SMBus transfers to avoid poor behaviour

[Andrew Jeffery <andrew@aj.id.au>]

Short turn-around times between transfers to e.g. the UCD90320 can lead
to problematic behaviour, including excessive clock stretching, bus
lockups and potential corruption of the device's volatile state.

Introduce transfer throttling for the device with a minimum access
delay of 1ms.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
---
 drivers/hwmon/pmbus/ucd9000.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/hwmon/pmbus/ucd9000.c b/drivers/hwmon/pmbus/ucd9000.c
index 81f4c4f166cd..a0b97d035326 100644
--- a/drivers/hwmon/pmbus/ucd9000.c
+++ b/drivers/hwmon/pmbus/ucd9000.c
@@ -9,6 +9,7 @@
 #include <linux/debugfs.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/moduleparam.h>
 #include <linux/of_device.h>
 #include <linux/init.h>
 #include <linux/err.h>
@@ -18,6 +19,9 @@
 #include <linux/gpio/driver.h>
 #include "pmbus.h"
 
+static unsigned long smbus_delay_us = 1000;
+module_param(smbus_delay_us, ulong, 0664);
+
 enum chips { ucd9000, ucd90120, ucd90124, ucd90160, ucd90320, ucd9090,
 	     ucd90910 };
 
@@ -502,6 +506,8 @@ static int ucd9000_probe(struct i2c_client *client,
 				     I2C_FUNC_SMBUS_BLOCK_DATA))
 		return -ENODEV;
 
+	i2c_smbus_throttle_client(client, smbus_delay_us);
+
 	ret = i2c_smbus_read_block_data(client, UCD9000_DEVICE_ID,
 					block_buffer);
 	if (ret < 0) {
-- 
2.25.1