sensors/lsm6ds3trc: add FIFO drain functionality

Documentation/components/drivers/special/sensors/lsm6ds3trc.rst

@@ -132,6 +132,42 @@ sensor, read in the same burst transaction as whichever sub-sensor's
 sample triggered it -- it does not have an independent output data rate
 of its own in this driver.
 
+FIFO mode
+=========
+
+Setting ``CONFIG_SENSORS_LSM6DS3TRC_FIFO=y`` switches interrupt-driven
+delivery from one uORB event per data-ready interrupt to draining the
+chip's hardware FIFO on a watermark interrupt instead -- one interrupt and
+one I2C burst read per
+``CONFIG_SENSORS_LSM6DS3TRC_FIFO_WATERMARK`` samples, rather than per
+sample. This cuts interrupt and I2C-wakeup frequency roughly by the
+watermark size, at the cost of three limitations while it's on:
+
+- Both the accelerometer and gyroscope are forced to the same output
+  data rate. Whichever sub-sensor activates first sets it; a second
+  subscriber joins that rate instead of using its own.
+- Per-sample temperature isn't available -- the FIFO pattern doesn't
+  include it. One direct temperature read happens per drain and is
+  applied to every sample in that batch instead.
+- The chip's FIFO write trigger only fires while both the accelerometer
+  and the gyroscope are physically running, even if only one of them is
+  actually subscribed. This driver forces the unsubscribed sub-sensor
+  on at the shared rate to make the FIFO work at all -- it's still left
+  out of the FIFO pattern itself, so this doesn't cost extra I2C
+  bandwidth on drain, but it does mean FIFO mode cannot power down the
+  unused sub-sensor the way non-FIFO interrupt-driven mode can.
+
+.. warning::
+   ``CONFIG_SENSORS_LSM6DS3TRC_FIFO_WATERMARK`` is in samples, not raw
+   FIFO words, but it must not exceed
+   ``CONFIG_SENSORS_LSM6DS3TRC_ACCEL_ORB_BUFSIZE`` or
+   ``CONFIG_SENSORS_LSM6DS3TRC_GYRO_ORB_BUFSIZE`` -- a single drain can
+   push that many events into each topic's uORB ring buffer at once, and
+   a smaller buffer drops the oldest ones.
+
+FIFO mode only takes effect in interrupt-driven mode (a real ``attach()``
+in ``lsm6ds3trc_config_s``); it's silently unused in kthread polling mode.
+
 This sensor also has an additional command for gaining access to extra
 functionality.
 

drivers/sensors/Kconfig

@@ -1083,8 +1083,8 @@ config SENSORS_LSM6DS3TRC
 		Enable uORB driver support for the STM LSM6DS3TR-C 6-axis IMU
 		over I2C. Needs the high-priority work queue even in kthread
 		polling mode: the driver's interrupt handlers reference it
-		unconditionally (they're only dead code until a future phase wires
-		up INT1/INT2).
+		unconditionally, whether or not a board actually wires up INT1
+		to use them.
 
 if SENSORS_LSM6DS3TRC
 
@@ -1115,6 +1115,35 @@ config SENSORS_LSM6DS3TRC_GYRO_ORB_BUFSIZE
 	---help---
 		The circular buffer size for the gyroscope uORB measurements
 
+config SENSORS_LSM6DS3TRC_FIFO
+	bool "LSM6DS3TR-C FIFO drain (interrupt mode only)"
+	default n
+	---help---
+		Drain the chip's hardware FIFO on a watermark interrupt instead of
+		pushing one uORB event per physical sample on every data-ready
+		interrupt. Cuts the number of I2C transactions and interrupt
+		wakeups by roughly the watermark size, at the cost of both
+		sub-sensors being forced to the same output data rate while this
+		is on (whichever one is activated first sets it; the other joins
+		it) and per-sample temperature (the FIFO pattern doesn't include
+		it; one direct temperature read per drain is applied to the whole
+		batch instead). Only takes effect when the board registers this
+		driver with a real interrupt attach() -- it's silently unused in
+		kthread polling mode.
+
+config SENSORS_LSM6DS3TRC_FIFO_WATERMARK
+	int "LSM6DS3TR-C FIFO watermark (samples)"
+	default 8
+	range 1 300
+	depends on SENSORS_LSM6DS3TRC_FIFO
+	---help---
+		How many samples (not raw FIFO words) accumulate before the FIFO
+		threshold interrupt fires and a drain happens. Keep this at or
+		below CONFIG_SENSORS_LSM6DS3TRC_ACCEL_ORB_BUFSIZE and
+		CONFIG_SENSORS_LSM6DS3TRC_GYRO_ORB_BUFSIZE -- one drain can push
+		up to this many events into each topic's uORB ring buffer in a
+		single burst, and a smaller ring buffer will drop the oldest ones.
+
 endif # SENSORS_LSM6DS3TRC
 
 config SENSORS_LSM9DS1