[PATCH v9 3/3] iio: pressure: dps310: implement .hwfifo_flush_to_buffer()

[Rupesh Majhi <zoone.rupert@gmail.com>]

Let userspace drain the FIFO on demand rather than only from the
periodic drain.

With a trigger attached the FIFO is not running, so there is nothing to
flush and the hook returns 0.

Hold the mode across the drain, so a concurrent buffer disable cannot
free active_scan_mask while it runs.

Assisted-by: LLM
Signed-off-by: Rupesh Majhi <zoone.rupert@gmail.com>
---
 drivers/iio/pressure/dps310.c | 36 ++++++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/pressure/dps310.c b/drivers/iio/pressure/dps310.c
index 66f24adc8df7..2af2971de970 100644
--- a/drivers/iio/pressure/dps310.c
+++ b/drivers/iio/pressure/dps310.c
@@ -1088,22 +1088,30 @@ static unsigned int dps310_fifo_push_held(struct dps310_data *data)
 
 /*
  * Read the batch out before compensating it, so a pressure entry pairs with
- * the temperature preceding it rather than the last one in the batch.
+ * the temperature preceding it rather than the last one in the batch. Stop
+ * reading at max_scans rather than after it, since entries leave the hardware
+ * as they are read and any extra would have to be thrown away. max_scans of
+ * zero drains everything.
  *
  * Returns scans pushed.
  */
-static int dps310_fifo_drain(struct dps310_data *data)
+static int dps310_fifo_drain(struct dps310_data *data, unsigned int max_scans)
 	__must_hold(&data->lock)
 {
 	bool pressure_enabled = test_bit(DPS310_SCAN_PRESSURE,
 					 data->iio->active_scan_mask);
 	u8 kind[DPS310_FIFO_DEPTH];
 	s32 raw[DPS310_FIFO_DEPTH];
+	unsigned int scans = 0;
 	unsigned int pushed = 0;
 	unsigned int cnt, i;
+	bool is_pressure;
 	int rc;
 
 	for (cnt = 0; cnt < DPS310_FIFO_DEPTH; cnt++) {
+		if (max_scans && scans >= max_scans)
+			break;
+
 		rc = dps310_fifo_read_entry(data, &raw[cnt]);
 		if (rc < 0)
 			return rc;
@@ -1111,7 +1119,12 @@ static int dps310_fifo_drain(struct dps310_data *data)
 		if (rc == DPS310_FIFO_EMPTY)
 			break;
 
+		is_pressure = rc == DPS310_FIFO_PRESSURE;
 		kind[cnt] = rc;
+
+		/* Only the measurement that drives the scans counts */
+		if (is_pressure == pressure_enabled)
+			scans++;
 	}
 
 	for (i = 0; i < cnt; i++) {
@@ -1151,7 +1164,7 @@ static void dps310_fifo_work(struct work_struct *work)
 	int rc;
 
 	mutex_lock(&data->lock);
-	rc = dps310_fifo_drain(data);
+	rc = dps310_fifo_drain(data, 0);
 	mutex_unlock(&data->lock);
 
 	if (rc < 0)
@@ -1186,6 +1199,22 @@ static int dps310_fifo_hold_alloc(struct dps310_data *data, int prs_rate,
 	return 0;
 }
 
+static int dps310_hwfifo_flush(struct iio_dev *iio, unsigned int count)
+{
+	struct dps310_data *data = iio_priv(iio);
+
+	/* Holding the mode keeps active_scan_mask alive across the drain */
+	IIO_DEV_GUARD_CURRENT_MODE(iio);
+
+	/* A trigger drives the capture instead and leaves the FIFO empty */
+	if (iio_device_get_current_mode(iio) != INDIO_BUFFER_SOFTWARE)
+		return 0;
+
+	guard(mutex)(&data->lock);
+
+	return dps310_fifo_drain(data, count);
+}
+
 static int dps310_hwfifo_set_watermark(struct iio_dev *iio, unsigned int val)
 {
 	struct dps310_data *data = iio_priv(iio);
@@ -1327,6 +1356,7 @@ static const struct iio_info dps310_info = {
 	.read_raw = dps310_read_raw,
 	.write_raw = dps310_write_raw,
 	.hwfifo_set_watermark = dps310_hwfifo_set_watermark,
+	.hwfifo_flush_to_buffer = dps310_hwfifo_flush,
 };
 
 static int dps310_probe(struct i2c_client *client)
-- 
2.43.0