iio: imu: inv_mpu6050: read the full fifo when processing data

When processing data read the full fifo data in 1 time. If there
are several samples in the FIFO, it means we are experiencing
system delay. In this case, it is better to read all data with 1
bus access than to add additional latency by doing several ones.

This requires to use a bigger buffer depending on chip FIFO size
and do an additional local data copy before sending. But the cost
is minimal and behavior is still better like this under system
heavy load.

Signed-off-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Link: https://lore.kernel.org/r/20230623082924.283967-1-inv.git-commit@tdk.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/imu/inv_mpu6050/inv_mpu_core.c

@@ -1345,6 +1345,9 @@ static int inv_check_and_setup_chip(struct inv_mpu6050_state *st)
 	st->reg = hw_info[st->chip_type].reg;
 	memcpy(&st->chip_config, hw_info[st->chip_type].config,
 	       sizeof(st->chip_config));
+	st->data = devm_kzalloc(regmap_get_device(st->map), st->hw->fifo_size, GFP_KERNEL);
+	if (st->data == NULL)
+		return -ENOMEM;
 
 	/* check chip self-identification */
 	result = regmap_read(st->map, INV_MPU6050_REG_WHOAMI, &regval);

drivers/iio/imu/inv_mpu6050/inv_mpu_iio.h

@@ -179,7 +179,7 @@ struct inv_mpu6050_hw {
  *  @magn_raw_to_gauss:	coefficient to convert mag raw value to Gauss.
  *  @magn_orient:       magnetometer sensor chip orientation if available.
  *  @suspended_sensors:	sensors mask of sensors turned off for suspend
- *  @data:		dma safe buffer used for bulk reads.
+ *  @data:		read buffer used for bulk reads.
  */
 struct inv_mpu6050_state {
 	struct mutex lock;
@@ -203,7 +203,7 @@ struct inv_mpu6050_state {
 	s32 magn_raw_to_gauss[3];
 	struct iio_mount_matrix magn_orient;
 	unsigned int suspended_sensors;
-	u8 data[INV_MPU6050_OUTPUT_DATA_SIZE] __aligned(IIO_DMA_MINALIGN);
+	u8 *data;
 };
 
 /*register and associated bit definition*/

drivers/iio/imu/inv_mpu6050/inv_mpu_ring.c

@@ -52,6 +52,7 @@ irqreturn_t inv_mpu6050_read_fifo(int irq, void *p)
 	u16 fifo_count;
 	u32 fifo_period;
 	s64 timestamp;
+	u8 data[INV_MPU6050_OUTPUT_DATA_SIZE];
 	int int_status;
 	size_t i, nb;
 
@@ -105,24 +106,30 @@ irqreturn_t inv_mpu6050_read_fifo(int irq, void *p)
 		goto flush_fifo;
 	}
 
-	/* compute and process all complete datum */
+	/* compute and process only all complete datum */
 	nb = fifo_count / bytes_per_datum;
+	fifo_count = nb * bytes_per_datum;
 	/* Each FIFO data contains all sensors, so same number for FIFO and sensor data */
 	fifo_period = NSEC_PER_SEC / INV_MPU6050_DIVIDER_TO_FIFO_RATE(st->chip_config.divider);
 	inv_sensors_timestamp_interrupt(&st->timestamp, fifo_period, nb, nb, pf->timestamp);
 	inv_sensors_timestamp_apply_odr(&st->timestamp, fifo_period, nb, 0);
+
+	/* clear internal data buffer for avoiding kernel data leak */
+	memset(data, 0, sizeof(data));
+
+	/* read all data once and process every samples */
+	result = regmap_noinc_read(st->map, st->reg->fifo_r_w, st->data, fifo_count);
+	if (result)
+		goto flush_fifo;
 	for (i = 0; i < nb; ++i) {
-		result = regmap_noinc_read(st->map, st->reg->fifo_r_w,
-					   st->data, bytes_per_datum);
-		if (result)
-			goto flush_fifo;
 		/* skip first samples if needed */
 		if (st->skip_samples) {
 			st->skip_samples--;
 			continue;
 		}
+		memcpy(data, &st->data[i * bytes_per_datum], bytes_per_datum);
 		timestamp = inv_sensors_timestamp_pop(&st->timestamp);
-		iio_push_to_buffers_with_timestamp(indio_dev, st->data, timestamp);
+		iio_push_to_buffers_with_timestamp(indio_dev, data, timestamp);
 	}
 
 end_session: