iio: imu: st_lsm6dsx: pre-allocate read buffer at bootstrap

Allocate device read buffer at bootstrap and do not put it on the stack
since it is pretty big (~200B) and its size will increase adding support
to device hw timestamp.
Moreover this patch fixes following sparse warnings:
drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c:250:17: warning: Variable length
array is used.
drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c:283:55: error: cannot size
expression

Fixes: 290a6ce1 ("iio: imu: add support to lsm6dsx driver")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx.h

@@ -27,6 +27,7 @@ enum st_lsm6dsx_hw_id {
 	ST_LSM6DSX_MAX_ID,
 };
 
+#define ST_LSM6DSX_BUFF_SIZE		256
 #define ST_LSM6DSX_CHAN_SIZE		2
 #define ST_LSM6DSX_SAMPLE_SIZE		6
 #define ST_LSM6DSX_MAX_WORD_LEN		((32 / ST_LSM6DSX_SAMPLE_SIZE) * \
@@ -122,6 +123,7 @@ struct st_lsm6dsx_sensor {
  * @fifo_mode: FIFO operating mode supported by the device.
  * @enable_mask: Enabled sensor bitmask.
  * @sip: Total number of samples (acc/gyro) in a given pattern.
+ * @buff: Device read buffer.
  * @iio_devs: Pointers to acc/gyro iio_dev instances.
  * @settings: Pointer to the specific sensor settings in use.
  */
@@ -137,6 +139,8 @@ struct st_lsm6dsx_hw {
 	u8 enable_mask;
 	u8 sip;
 
+	u8 *buff;
+
 	struct iio_dev *iio_devs[ST_LSM6DSX_ID_MAX];
 
 	const struct st_lsm6dsx_settings *settings;

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_buffer.c

@@ -247,7 +247,6 @@ static int st_lsm6dsx_read_fifo(struct st_lsm6dsx_hw *hw)
 	struct st_lsm6dsx_sensor *acc_sensor, *gyro_sensor;
 	s64 acc_ts, acc_delta_ts, gyro_ts, gyro_delta_ts;
 	u8 iio_buff[ALIGN(ST_LSM6DSX_SAMPLE_SIZE, sizeof(s64)) + sizeof(s64)];
-	u8 buff[pattern_len];
 	__le16 fifo_status;
 
 	err = regmap_bulk_read(hw->regmap,
@@ -280,7 +279,7 @@ static int st_lsm6dsx_read_fifo(struct st_lsm6dsx_hw *hw)
 				samples);
 
 	for (read_len = 0; read_len < fifo_len; read_len += pattern_len) {
-		err = st_lsm6dsx_read_block(hw, buff, sizeof(buff));
+		err = st_lsm6dsx_read_block(hw, hw->buff, pattern_len);
 		if (err < 0)
 			return err;
 
@@ -305,7 +304,7 @@ static int st_lsm6dsx_read_fifo(struct st_lsm6dsx_hw *hw)
 
 		while (acc_sip > 0 || gyro_sip > 0) {
 			if (gyro_sip-- > 0) {
-				memcpy(iio_buff, &buff[offset],
+				memcpy(iio_buff, &hw->buff[offset],
 				       ST_LSM6DSX_SAMPLE_SIZE);
 				iio_push_to_buffers_with_timestamp(
 					hw->iio_devs[ST_LSM6DSX_ID_GYRO],
@@ -315,7 +314,7 @@ static int st_lsm6dsx_read_fifo(struct st_lsm6dsx_hw *hw)
 			}
 
 			if (acc_sip-- > 0) {
-				memcpy(iio_buff, &buff[offset],
+				memcpy(iio_buff, &hw->buff[offset],
 				       ST_LSM6DSX_SAMPLE_SIZE);
 				iio_push_to_buffers_with_timestamp(
 					hw->iio_devs[ST_LSM6DSX_ID_ACC],

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c

@@ -722,6 +722,10 @@ int st_lsm6dsx_probe(struct device *dev, int irq, int hw_id, const char *name,
 	mutex_init(&hw->fifo_lock);
 	mutex_init(&hw->conf_lock);
 
+	hw->buff = devm_kzalloc(dev, ST_LSM6DSX_BUFF_SIZE, GFP_KERNEL);
+	if (!hw->buff)
+		return -ENOMEM;
+
 	hw->dev = dev;
 	hw->irq = irq;
 	hw->regmap = regmap;