linux-kernel - [PATCH] iio: adc: ad7380: fix SPI offload trigger rate

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Message-Id: <20250919-iio-adc-ad7380-fix-sampling-frequency-for-single-ended-chips-v1-1-7b2d761766cf@baylibre.com>
Date: Fri, 19 Sep 2025 15:50:34 -0500
From: David Lechner <dlechner@...libre.com>
To: Michael Hennerich <michael.hennerich@...log.com>, 
 Nuno Sá <nuno.sa@...log.com>, 
 Jonathan Cameron <jic23@...nel.org>, Andy Shevchenko <andy@...nel.org>, 
 Angelo Dureghello <adureghello@...libre.com>
Cc: Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
 David Lechner <dlechner@...libre.com>
Subject: [PATCH] iio: adc: ad7380: fix SPI offload trigger rate

Add a special case to double the SPI offload trigger rate when all
channels of a single-ended chip are enabled in a buffered read.

The single-ended chips in the AD738x family can only do simultaneous
sampling of half their channels and have a multiplexer to allow reading
the other half. To comply with the IIO definition of sampling_frequency,
we need to trigger twice as often when the sequencer is enabled to so
that both banks can be read in a single sample period.

Fixes: bbeaec81a03e ("iio: ad7380: add support for SPI offload")
Signed-off-by: David Lechner <dlechner@...libre.com>
---
There was some unresolved discussion about this when we were adding
SPI offload support to the ad7380 driver, but it turns out that handling
the sampling frequency correctly for single-ended chips is actually
quite simple.
---
 drivers/iio/adc/ad7380.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/iio/adc/ad7380.c b/drivers/iio/adc/ad7380.c
index fa251dc1aae6ab0a0d36792fa37b2cc22b99dfe6..bfd908deefc0f40b42bd8a44bfce7a2510b2fdf1 100644
--- a/drivers/iio/adc/ad7380.c
+++ b/drivers/iio/adc/ad7380.c
@@ -1227,6 +1227,14 @@ static int ad7380_offload_buffer_postenable(struct iio_dev *indio_dev)
 	if (ret)
 		return ret;
 
+	/*
+	 * When the sequencer is required to read all channels, we need to
+	 * trigger twice per sample period in order to read one complete set
+	 * of samples.
+	 */
+	if (st->seq)
+		config.periodic.frequency_hz *= 2;
+
 	ret = spi_offload_trigger_enable(st->offload, st->offload_trigger, &config);
 	if (ret)
 		spi_unoptimize_message(&st->offload_msg);

---
base-commit: 561285d048053fec8a3d6d1e3ddc60df11c393a0
change-id: 20250919-iio-adc-ad7380-fix-sampling-frequency-for-single-ended-chips-05bbfea8891c

Best regards,
-- 
David Lechner <dlechner@...libre.com>