regulator/rpi-panel-attiny: Use two transactions for I2C read

The I2C to the Atmel is very fussy, and locks up easily on
Pi0-3 particularly on reads.
If running at 100kHz on Pi3, reading the ID register generally
locks up the Atmel, but splitting the register select write and
read into two transactions is reliable.
Signed-off-by: NDave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: NDetlev Casanova <detlev.casanova@collabora.com>
Link: https://lore.kernel.org/r/20220124220129.158891-10-detlev.casanova@collabora.comSigned-off-by: NMark Brown <broonie@kernel.org>

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

@@ -250,6 +250,39 @@ static void attiny_gpio_set(struct gpio_chip *gc, unsigned int off, int val)
 	mutex_unlock(&state->lock);
 }
 
+static int attiny_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
  */
@@ -280,7 +313,7 @@ static int attiny_i2c_probe(struct i2c_client *i2c,
 		goto error;
 	}
 
-	ret = regmap_read(regmap, REG_ID, &data);
+	ret = attiny_i2c_read(i2c, REG_ID, &data);
 	if (ret < 0) {
 		dev_err(&i2c->dev, "Failed to read REG_ID reg: %d\n", ret);
 		goto error;