sensors: Fix alignment issues

Add padding to the header and remove unnecessary memset in order to fix
alignment faults in cores such as M0 or ones that support
CONFIG_TRAP_UNALIGNED_ACCESS

Signed-off-by: Yuval Peress <peress@google.com>

drivers/sensor/default_rtio_sensor.c

@@ -12,6 +12,12 @@
 
 LOG_MODULE_REGISTER(sensor_compat, CONFIG_SENSOR_LOG_LEVEL);
 
+/*
+ * Ensure that the size of the generic header aligns with the sensor channel enum. If it doesn't,
+ * then cores that require aligned memory access will fail to read channel[0].
+ */
+BUILD_ASSERT((sizeof(struct sensor_data_generic_header) % sizeof(enum sensor_channel)) == 0);
+
 static void sensor_submit_fallback(const struct device *dev, struct rtio_iodev_sqe *iodev_sqe);
 
 static void sensor_iodev_submit(struct rtio_iodev_sqe *iodev_sqe)
@@ -49,6 +55,21 @@ static inline int compute_num_samples(const enum sensor_channel *channels, size_
 	return num_samples;
 }
 
+/**
+ * @brief Compute the required header size
+ *
+ * This function takes into account alignment of the q31 values that will follow the header.
+ *
+ * @param[in] num_output_samples The number of samples to represent
+ * @return The number of bytes needed for this sample frame's header
+ */
+static inline uint32_t compute_header_size(int num_output_samples)
+{
+	uint32_t size = sizeof(struct sensor_data_generic_header) +
+			(num_output_samples * sizeof(enum sensor_channel));
+	return (size + 3) & ~0x3;
+}
+
 /**
  * @brief Compute the minimum number of bytes needed
  *
@@ -57,8 +78,7 @@ static inline int compute_num_samples(const enum sensor_channel *channels, size_
  */
 static inline uint32_t compute_min_buf_len(int num_output_samples)
 {
-	return sizeof(struct sensor_data_generic_header) + (num_output_samples * sizeof(q31_t)) +
-	       (num_output_samples * sizeof(enum sensor_channel));
+	return compute_header_size(num_output_samples) + (num_output_samples * sizeof(q31_t));
 }
 
 /**
@@ -121,8 +141,7 @@ static void sensor_submit_fallback(const struct device *dev, struct rtio_iodev_s
 	header->num_channels = num_output_samples;
 	header->shift = 0;
 
-	q31_t *q = (q31_t *)(buf + sizeof(struct sensor_data_generic_header) +
-			     num_output_samples * sizeof(enum sensor_channel));
+	q31_t *q = (q31_t *)(buf + compute_header_size(num_output_samples));
 
 	/* Populate values, update shift, and set channels */
 	for (size_t i = 0, sample_idx = 0; i < cfg->count; ++i) {

drivers/sensor/icm42688/icm42688.c

@@ -251,12 +251,11 @@ int icm42688_init(const struct device *dev)
 	}
 #endif
 
-	memset(&data->cfg, 0, sizeof(struct icm42688_cfg));
 	data->cfg.accel_mode = ICM42688_ACCEL_LN;
-	data->cfg.gyro_mode = ICM42688_GYRO_LN;
 	data->cfg.accel_fs = ICM42688_ACCEL_FS_2G;
-	data->cfg.gyro_fs = ICM42688_GYRO_FS_125;
 	data->cfg.accel_odr = ICM42688_ACCEL_ODR_1000;
+	data->cfg.gyro_mode = ICM42688_GYRO_LN;
+	data->cfg.gyro_fs = ICM42688_GYRO_FS_125;
 	data->cfg.gyro_odr = ICM42688_GYRO_ODR_1000;
 	data->cfg.fifo_en = false;
 

include/zephyr/drivers/sensor.h

@@ -801,11 +801,14 @@ struct __attribute__((__packed__)) sensor_data_generic_header {
 	 * The number of channels present in the frame. This will be the true number of elements in
 	 * channel_info and in the q31 values that follow the header.
 	 */
-	size_t num_channels;
+	uint32_t num_channels;
 
 	/* Shift value for all samples in the frame */
 	int8_t shift;
 
+	/* This padding is needed to make sure that the 'channels' field is aligned */
+	int8_t _padding[sizeof(enum sensor_channel) - 1];
+
 	/* Channels present in the frame */
 	enum sensor_channel channels[0];
 };