linux-kernel - Re: [PATCH v4 14/17] iio: adc: ad7768-1: add support for Synchronization over SPI

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Message-ID: <c5858d5d-687a-46f1-8447-e9c9d802f87f@baylibre.com>
Date: Tue, 1 Apr 2025 12:02:40 -0500
From: David Lechner <dlechner@...libre.com>
To: Jonathan Santos <Jonathan.Santos@...log.com>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-gpio@...r.kernel.org
Cc: lars@...afoo.de, Michael.Hennerich@...log.com,
 marcelo.schmitt@...log.com, jic23@...nel.org, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, linus.walleij@...aro.org,
 brgl@...ev.pl, lgirdwood@...il.com, broonie@...nel.org,
 marcelo.schmitt1@...il.com, jonath4nns@...il.com
Subject: Re: [PATCH v4 14/17] iio: adc: ad7768-1: add support for
 Synchronization over SPI

On 3/6/25 3:03 PM, Jonathan Santos wrote:
> The synchronization method using GPIO requires the generated pulse to be
> truly synchronous with the base MCLK signal. When it is not possible to
> do that in hardware, the datasheet recommends using synchronization over
> SPI, where the generated pulse is already synchronous with MCLK. This
> requires the SYNC_OUT pin to be connected to SYNC_IN pin.
> 
> Use trigger-sources property to enable device synchronization over SPI.
> 
> Signed-off-by: Jonathan Santos <Jonathan.Santos@...log.com>
> ---
...

> +static int ad7768_setup_spi_sync(struct device *dev, struct ad7768_state *st)
> +{
> +	struct fwnode_reference_args args;
> +	int ret;
> +
> +	ret = fwnode_property_get_reference_args(dev_fwnode(dev),
> +						 "trigger-sources",
> +						 "#trigger-source-cells",
> +						 0, 0, &args);

As in the DT binding patch, we may need to allow an arg here to be
able to tell the difference between /SYNC_OUT and /DRDY triggers.

Also, in reviews on previous versions of this series, Jonathan and
Conor both mentioned that it would be sensible to allow omitting
the trigger-sources property when /SYNC_OUT is connected to /SYNC_IN
if you want to consider that.

> +	if (ret)
> +		return dev_err_probe(dev, ret, "Failed to get trigger-sources reference\n");
> +
> +	/*
> +	 * Currently, the driver supports SPI-based synchronization only for
> +	 * single-device setups, where the device's own SYNC_OUT is looped back
> +	 * to its SYNC_IN. Only enable this feature if the trigger-sources
> +	 * references the current device.
> +	 */
> +	st->en_spi_sync = args.fwnode->dev == dev;
> +	fwnode_handle_put(args.fwnode);
> +
> +	return st->en_spi_sync ? 0 : -EOPNOTSUPP;
> +}