linux-kernel - Re: [PATCH] iio: ad7380: add support for SPI offload

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Message-ID: <7b64c6a9-0606-45ba-be45-7ae11c4fdf39@baylibre.com>
Date: Sat, 22 Feb 2025 11:31:01 -0600
From: David Lechner <dlechner@...libre.com>
To: Angelo Dureghello <adureghello@...libre.com>,
 Jonathan Cameron <jic23@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Nuno Sá <nuno.sa@...log.com>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: ad7380: add support for SPI offload

On 2/20/25 12:03 PM, Angelo Dureghello wrote:
> From: Angelo Dureghello <adureghello@...libre.com>
> 
> Add support for SPI offload to the ad7380 driver. SPI offload allows
> sampling data at the max sample rate (2MSPS with one SDO line).
> 
> This is developed and tested against the ADI example FPGA design for
> this family of ADCs [1].
> 
> [1]: http://analogdevicesinc.github.io/hdl/projects/ad738x_fmc/index.html
> 
> Signed-off-by: Angelo Dureghello <adureghello@...libre.com>
> ---

We forgot to also update Documentation/iio/ad7380.rst. We can follow up
with a separate patch later though.

>  drivers/iio/adc/Kconfig  |   2 +
>  drivers/iio/adc/ad7380.c | 509 +++++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 475 insertions(+), 36 deletions(-)
> 

...

>  #define _AD7380_CHANNEL(index, bits, diff, sign, gain) {			\
>  	.type = IIO_VOLTAGE,							\
>  	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |				\
> @@ -237,48 +335,123 @@ static const struct iio_scan_type ad7380_scan_type_16_u[] = {
>  	.num_event_specs = ARRAY_SIZE(ad7380_events),				\
>  }
>  
> +#define _AD7380_OFFLOAD_CHANNEL(index, bits, diff, sign, gain) {		\
> +	.type = IIO_VOLTAGE,							\
> +	.info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |                          \
> +		((gain) ? BIT(IIO_CHAN_INFO_SCALE) : 0) |			\
> +		((diff) ? 0 : BIT(IIO_CHAN_INFO_OFFSET)),			\
> +	.info_mask_shared_by_type = ((gain) ? 0 : BIT(IIO_CHAN_INFO_SCALE)) |   \
> +		BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO) |				\
> +		BIT(IIO_CHAN_INFO_SAMP_FREQ),					\

Not sure if this is worth troubling with, but it might make more sense to make
IIO_CHAN_INFO_SAMP_FREQ info_mask_separate instead of info_mask_shared_by_type,
at least in the case of the single-ended chips.

This family of chips does simultaneous conversions so shared_by_type (or shared_by_all)
would typically be the right thing to do here. However, the single-ended versions
of these chips also have a multiplexer, so there are 2 banks of simultaneously
sampled inputs. So the effective sample rate as far as IIO is concerned would
actually be 1/2 of the sampling_frequency attribute value.

Since we have a channel mask restriction where we force all channels in a bank
to be enabled at once, I think it would work to make IIO_CHAN_INFO_SAMP_FREQ
info_mask_separate where the reported sampling frequency is the conversion rate
divided by the number of channels in a bank.

> +	.info_mask_shared_by_type_available =					\
> +		BIT(IIO_CHAN_INFO_OVERSAMPLING_RATIO) |				\
> +		BIT(IIO_CHAN_INFO_SAMP_FREQ),					\
> +	.indexed = 1,                                                           \
> +	.differential = (diff),                                                 \
> +	.channel = (diff) ? (2 * (index)) : (index),                            \
> +	.channel2 = (diff) ? (2 * (index) + 1) : 0,                             \
> +	.scan_index = (index),                                                  \
> +	.has_ext_scan_type = 1,                                                 \
> +	.ext_scan_type = ad7380_scan_type_##bits##_##sign##_offload,            \
> +	.num_ext_scan_type =                                                    \
> +		ARRAY_SIZE(ad7380_scan_type_##bits##_##sign##_offload),		\
> +	.event_spec = ad7380_events,                                            \
> +	.num_event_specs = ARRAY_SIZE(ad7380_events),                           \
> +}
> +