bmi160: Keep timestamp and FIFO synchronized

The BMI160 will clear the FIFO watermark/full interrupt when the FIFO
crosses the threshold condition - which happens upon reading from it
in load_fifo(). As a result of this, and the fact that the soft IRQ
handler for the BMI160 tries to loop until it has finished serving all
interrupt causes, there are possibilities for a race condition such as
the following to occur:

read timestamp, value is t0
read interrupt cause: it's a watermark interrupt
load fifo(t0): this clears the interrupt, allowing a new one to happen
read timestamp, value is still t0
a watermark interrupt happens at t1
read interrupt cause: watermark
load fifo(t0)

At this point, the fifo will process events at t1 as-if they had happened
at t0, which is bad and causes incorrect timestamps on the AP side.

This changes moves reading the timestamp value as close as possible
to load_fifo, and in a relative order where a new watermark interrupt
cannot happen, which ensures the timestamp and the FIFO data for it
are synchronized

BUG=b:120508077
TEST=on Bobba and Nocturne, observe that no events happen
     where an identical "a" time matches two distinct (b,c) times
BRANCH=nocturne,bobba

Change-Id: I454b257366bccf2b9e4d78df5dc005a8ad7313a0
Signed-off-by: Enrico Granata <egranata@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1367013
Reviewed-by: Alexandru M Stan <amstan@chromium.org>
Reviewed-by: Diana Z <dzigterman@chromium.org>

1 files changed, 2 insertions, 8 deletions

diff --git a/driver/accelgyro_bmi160.c b/driver/accelgyro_bmi160.c
index 82ef7b0117..3c56626b98 100644
--- a/driver/accelgyro_bmi160.c
+++ b/driver/accelgyro_bmi160.c
@@ -1103,7 +1103,7 @@ static void irq_set_orientation(struct motion_sensor_t *s,
  */
 static int irq_handler(struct motion_sensor_t *s, uint32_t *event)
 {
-	uint32_t interrupt, last_ts;
+	uint32_t interrupt;
 	int rv;
 
 	if ((s->type != MOTIONSENSE_TYPE_ACCEL) ||
@@ -1111,12 +1111,6 @@ static int irq_handler(struct motion_sensor_t *s, uint32_t *event)
 		return EC_ERROR_NOT_HANDLED;
 
 	do {
-		/*
-		 * Collect timestamp before resetting the interrupt line:
-		 * After reading, it is possible for the timestamp to change,
-		 * before we can process the FIFO.
-		 */
-		last_ts = last_interrupt_timestamp;
 		rv = raw_read32(s->port, s->addr, BMI160_INT_STATUS_0,
 				&interrupt);
 		/* Bail out of this loop if the sensor isn't powered. */
@@ -1133,7 +1127,7 @@ static int irq_handler(struct motion_sensor_t *s, uint32_t *event)
 #endif
 #ifdef CONFIG_ACCEL_FIFO
 		if (interrupt & (BMI160_FWM_INT | BMI160_FFULL_INT))
-			load_fifo(s, last_ts);
+			load_fifo(s, last_interrupt_timestamp);
 #endif
 #ifdef CONFIG_BMI160_ORIENTATION_SENSOR
 		irq_set_orientation(s, interrupt);