iio: adis: Support different burst sizes

Add burst_max_len to `adis_burst`. This is useful for devices which
support different burst modes with different sizes. The buffer to be
used in the spi transfer is allocated with this variable making sure
that has space for all burst modes. The spi transfer length should hold
the "real" burst length depending on the current burst mode configured
in the device.

Moreover, `extra_len` in `adis_burst` is made const and it should
contain the smallest extra length necessary for a burst transfer. In
`struct adis` was added a new `burst_extra_len` that should hold the
extra bytes needed depending on the device instance being used.

Signed-off-by: Nuno Sá <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/imu/adis16400.c

···
 		indio_dev->available_scan_masks = st->avail_scan_mask;
 		st->adis.burst = &adis16400_burst;
 		if (st->variant->flags & ADIS16400_BURST_DIAG_STAT)
-			st->adis.burst->extra_len = sizeof(u16);
+			st->adis.burst_extra_len = sizeof(u16);
 	}
 
 	adis16400_data = &st->variant->adis_data;

drivers/iio/imu/adis_buffer.c

···
 	const unsigned long *scan_mask)
 {
 	struct adis *adis = iio_device_get_drvdata(indio_dev);
-	unsigned int burst_length;
+	unsigned int burst_length, burst_max_length;
 	u8 *tx;
 
 	/* All but the timestamp channel */
 	burst_length = (indio_dev->num_channels - 1) * sizeof(u16);
-	burst_length += adis->burst->extra_len;
+	burst_length += adis->burst->extra_len + adis->burst_extra_len;
+
+	if (adis->burst->burst_max_len)
+		burst_max_length = adis->burst->burst_max_len;
+	else
+		burst_max_length = burst_length;
 
 	adis->xfer = kcalloc(2, sizeof(*adis->xfer), GFP_KERNEL);
 	if (!adis->xfer)
 		return -ENOMEM;
 
-	adis->buffer = kzalloc(burst_length + sizeof(u16), GFP_KERNEL);
+	adis->buffer = kzalloc(burst_max_length + sizeof(u16), GFP_KERNEL);
 	if (!adis->buffer) {
 		kfree(adis->xfer);
 		adis->xfer = NULL;
 		return -ENOMEM;
 	}
 
-	tx = adis->buffer + burst_length;
+	tx = adis->buffer + burst_max_length;
 	tx[0] = ADIS_READ_REG(adis->burst->reg_cmd);
 	tx[1] = 0;
 

include/linux/iio/imu/adis.h

···
  * @trig: IIO trigger object data
  * @data: ADIS chip variant specific data
  * @burst: ADIS burst transfer information
+ * @burst_extra_len: Burst extra length. Should only be used by devices that can
+ *		     dynamically change their burst mode length.
  * @state_lock: Lock used by the device to protect state
  * @msg: SPI message object
  * @xfer: SPI transfer objects to be used for a @msg
···
 
 	const struct adis_data	*data;
 	struct adis_burst	*burst;
-
+	unsigned int		burst_extra_len;
 	/**
 	 * The state_lock is meant to be used during operations that require
 	 * a sequence of SPI R/W in order to protect the SPI transfer
···
  * @en			burst mode enabled
  * @reg_cmd		register command that triggers burst
  * @extra_len		extra length to account in the SPI RX buffer
+ * @burst_max_len	holds the maximum burst size when the device supports
+ *			more than one burst mode with different sizes
  */
 struct adis_burst {
 	bool		en;
 	unsigned int	reg_cmd;
-	unsigned int	extra_len;
+	const u32	extra_len;
+	const u32	burst_max_len;
 };
 
 int