power: supply: bq27xxx: Retrieve again when busy

Multiple applications may access the battery gauge at the same time, so
the gauge may be busy and EBUSY will be returned. The driver will set a
flag to record the EBUSY state, and this flag will be kept until the next
periodic update. When this flag is set, bq27xxx_battery_get_property()
will just return ENODEV until the flag is updated.

Even if the gauge was busy during the last accessing attempt, returning
ENODEV is not ideal, and can cause confusion in the applications layer.

Instead, retry accessing the I2C to update the flag is as expected, for
the gauge typically recovers from busy state within a few milliseconds.
If still failed to access the gauge, the real error code would be returned
instead of ENODEV (as suggested by Pali Rohár).

Reviewed-by: Pali Rohár <[e-mail]>
Signed-off-by: Jerry Lv <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sebastian Reichel <[e-mail]>

drivers/power/supply/bq27xxx_battery.c

···
 	mutex_unlock(&di->lock);
 
 	if (psp != POWER_SUPPLY_PROP_PRESENT && di->cache.flags < 0)
-		return -ENODEV;
+		return di->cache.flags;
 
 	switch (psp) {
 	case POWER_SUPPLY_PROP_STATUS:

drivers/power/supply/bq27xxx_battery_i2c.c

···
  *	Andrew F. Davis <[e-mail]>
  */
 
+#include <linux/delay.h>
 #include <linux/i2c.h>
 #include <linux/interrupt.h>
 #include <linux/module.h>
···
 	struct i2c_msg msg[2];
 	u8 data[2];
 	int ret;
+	int retry = 0;
 
 	if (!client->adapter)
 		return -ENODEV;
···
 	else
 		msg[1].len = 2;
 
-	ret = i2c_transfer(client->adapter, msg, ARRAY_SIZE(msg));
+	do {
+		ret = i2c_transfer(client->adapter, msg, ARRAY_SIZE(msg));
+		if (ret == -EBUSY && ++retry < 3) {
+			/* sleep 10 milliseconds when busy */
+			usleep_range(10000, 11000);
+			continue;
+		}
+		break;
+	} while (1);
+
 	if (ret < 0)
 		return ret;