Input: st1232 - do not read more bytes than needed

st1232_ts_read_data() already reads ts->read_buf_len bytes (8 or 20
bytes) from the touchscreen controller.  This was fine when it was used
to read touch point coordinates only, but is overkill for reading the
touchscreen resolution, which just needs 3 bytes.

Optimize transfers by passing the wanted number of bytes.

Fixes: 3a54a215 ("Input: st1232 - add support resolution reading")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20201229162601.2154566-3-geert+renesas@glider.beSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/touchscreen/st1232.c

@@ -47,7 +47,8 @@ struct st1232_ts_data {
 	u8 *read_buf;
 };
 
-static int st1232_ts_read_data(struct st1232_ts_data *ts, u8 reg)
+static int st1232_ts_read_data(struct st1232_ts_data *ts, u8 reg,
+			       unsigned int n)
 {
 	struct i2c_client *client = ts->client;
 	struct i2c_msg msg[] = {
@@ -59,7 +60,7 @@ static int st1232_ts_read_data(struct st1232_ts_data *ts, u8 reg)
 		{
 			.addr	= client->addr,
 			.flags	= I2C_M_RD | I2C_M_DMA_SAFE,
-			.len	= ts->read_buf_len,
+			.len	= n,
 			.buf	= ts->read_buf,
 		}
 	};
@@ -79,7 +80,7 @@ static int st1232_ts_read_resolution(struct st1232_ts_data *ts, u16 *max_x,
 	int error;
 
 	/* select resolution register */
-	error = st1232_ts_read_data(ts, REG_XY_RESOLUTION);
+	error = st1232_ts_read_data(ts, REG_XY_RESOLUTION, 3);
 	if (error)
 		return error;
 
@@ -140,7 +141,7 @@ static irqreturn_t st1232_ts_irq_handler(int irq, void *dev_id)
 	int count;
 	int error;
 
-	error = st1232_ts_read_data(ts, REG_XY_COORDINATES);
+	error = st1232_ts_read_data(ts, REG_XY_COORDINATES, ts->read_buf_len);
 	if (error)
 		goto out;