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@baylibre.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@baylibre.com>

On Fri, 19 Sep 2025 15:50:34 -0500
David Lechner <dlechner@baylibre.com> wrote:

> 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@baylibre.com>
Applied to my temporary fixes branch (always fun at this point in cycle).
It'll become the normal fixes branch after rc1.

J
> ---
> 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,