regulator: rpi_panel_v2: Add delay on I2C reads

As with many microcontrollers, the panel wants to use clock
stretching during a read so that the appropriate return value
can be generated and programmed into the hardware.
With Pi0-3 really not supporting clock stretching, this resulted
in the panel firmware dying.

Insert a delay between the write and the read to give the firmware
a chance to generate the relevant return value.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>

Author: 6by9

drivers/regulator/rpi-panel-v2-regulator.c

@@ -87,6 +87,39 @@ static const struct backlight_ops rpi_panel_v2_bl = {
 	.update_status	= rpi_panel_v2_update_status,
 };
 
+static int rpi_panel_v2_i2c_read(struct i2c_client *client, u8 reg, unsigned int *buf)
+{
+	struct i2c_msg msgs[1];
+	u8 addr_buf[1] = { reg };
+	u8 data_buf[1] = { 0, };
+	int ret;
+
+	/* Write register address */
+	msgs[0].addr = client->addr;
+	msgs[0].flags = 0;
+	msgs[0].len = ARRAY_SIZE(addr_buf);
+	msgs[0].buf = addr_buf;
+
+	ret = i2c_transfer(client->adapter, msgs, ARRAY_SIZE(msgs));
+	if (ret != ARRAY_SIZE(msgs))
+		return -EIO;
+
+	usleep_range(5000, 10000);
+
+	/* Read data from register */
+	msgs[0].addr = client->addr;
+	msgs[0].flags = I2C_M_RD;
+	msgs[0].len = 1;
+	msgs[0].buf = data_buf;
+
+	ret = i2c_transfer(client->adapter, msgs, ARRAY_SIZE(msgs));
+	if (ret != ARRAY_SIZE(msgs))
+		return -EIO;
+
+	*buf = data_buf[0];
+	return 0;
+}
+
 /*
  * I2C driver interface functions
  */
@@ -114,7 +147,7 @@ static int rpi_panel_v2_i2c_probe(struct i2c_client *i2c)
 		goto error;
 	}
 
-	ret = regmap_read(regmap, REG_ID, &data);
+	ret = rpi_panel_v2_i2c_read(i2c, REG_ID, &data);
 	if (ret < 0) {
 		dev_err(&i2c->dev, "Failed to read REG_ID reg: %d\n", ret);
 		goto error;