[PATCH] iio: imu: inv_mpu6050: Use as standalone trigger

[Linus Walleij <linus.walleij@linaro.org>]

It may happen that the MPU6050 is the only hardware
trigger available on your system such as this:

> lsiio
Device 003: hscdtd008a
Device 001: mpu6050
Device 002: gp2ap002
Device 000: ab8500-gpadc
Trigger 000: mpu6050-dev1

And when you want to use it to read periodically from
your magnetometer like this:

> iio_generic_buffer -a -c 100 -n hscdtd008a -t mpu6050-dev1

Then the following happens:

[  209.951334] Internal error: Oops: 5 [#1] SMP ARM
(...)
[  209.981969] Hardware name: ST-Ericsson Ux5x0 platform (Device Tree Support)
[  209.988925] PC is at inv_scan_query_mpu6050+0x8/0xb8
[  209.993914] LR is at inv_mpu6050_set_enable+0x40/0x194

This is because since we are not using any channels from the
same device, the indio_dev->active_scan_mask is NULL.

Just checking for that and bailing out is however not enough:
we have to enable some kind of FIFO for the readout to work.
So enable the temperature as a dummy FIFO and all works
fine.

Cc: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c b/drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c
index f7b5a70be30f..6946d50b874a 100644
--- a/drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c
+++ b/drivers/iio/imu/inv_mpu6050/inv_mpu_trigger.c
@@ -11,6 +11,17 @@ static unsigned int inv_scan_query_mpu6050(struct iio_dev *indio_dev)
 	struct inv_mpu6050_state  *st = iio_priv(indio_dev);
 	unsigned int mask;
 
+	/*
+	 * If the MPU6050 is just used as a trigger, then the scan mask
+	 * is not allocated so we simply enable the temperature channel
+	 * as a dummy and bail out.
+	 */
+	if (!indio_dev->active_scan_mask) {
+		st->chip_config.temp_fifo_enable = true;
+		mask = INV_MPU6050_SENSOR_TEMP;
+		return mask;
+	}
+
 	st->chip_config.gyro_fifo_enable =
 		test_bit(INV_MPU6050_SCAN_GYRO_X,
 			 indio_dev->active_scan_mask) ||
-- 
2.29.2