linux-kernel - Re: [PATCH v7 09/12] iio: adc: ad7768-1: add support for Synchronization over SPI

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Message-ID: <662fa2f9-b28c-4831-9f76-7d2af8267466@baylibre.com>
Date: Thu, 8 May 2025 13:45:37 -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: andy@...nel.org, nuno.sa@...log.com, Michael.Hennerich@...log.com,
 marcelo.schmitt@...log.com, jic23@...nel.org, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, marcelo.schmitt1@...il.com,
 linus.walleij@...aro.org, brgl@...ev.pl, lgirdwood@...il.com,
 broonie@...nel.org, jonath4nns@...il.com
Subject: Re: [PATCH v7 09/12] iio: adc: ad7768-1: add support for
 Synchronization over SPI

On 5/8/25 12:04 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 the SYNC_IN pin.
> 
> Use trigger-sources property to enable device synchronization over SPI
> and multi-device synchronization while replacing sync-in-gpios property.
> 
> Signed-off-by: Jonathan Santos <Jonathan.Santos@...log.com>
> ---

...

> +static int ad7768_trigger_sources_get_sync(struct device *dev,
> +					   struct ad7768_state *st)
> +{
> +	struct fwnode_reference_args args;
> +	struct fwnode_handle *fwnode = dev_fwnode(dev);
> +	int ret;
> +
> +	/*
> +	 * The AD7768-1 allows two primary methods for driving the SYNC_IN pin
> +	 * to synchronize one or more devices:
> +	 * 1. Using an external GPIO.
> +	 * 2. Using a SPI command, where the SYNC_OUT pin generates a
> +	 *    synchronization pulse that drives the SYNC_IN pin.
> +	 */
> +	if (!fwnode_property_present(fwnode, "trigger-sources")) {
> +		/*
> +		 * In the absence of trigger-sources property, enable self
> +		 * synchronization over SPI (SYNC_OUT).
> +		 */
> +		st->en_spi_sync = true;
> +		return 0;
> +	}
> +
> +	ret = fwnode_property_get_reference_args(fwnode,
> +						 "trigger-sources",
> +						 "#trigger-source-cells",
> +						 0,
> +						 AD7768_TRIGGER_SOURCE_SYNC_IDX,
> +						 &args);
> +	if (ret)
> +		return ret;
> +
> +	fwnode = args.fwnode;
> +	/*
> +	 * First, try getting the GPIO trigger source and fallback to
> +	 * synchronization over SPI in case of failure.
> +	 */
> +	st->gpio_sync_in = ad7768_trigger_source_get_gpio(dev, fwnode);
> +	if (!IS_ERR(st->gpio_sync_in))
> +		goto out_put_node;

I think we want to return the error in some cases here, e.g. deferred probe
so rather:

	st->gpio_sync_in = ad7768_trigger_source_get_gpio(dev, fwnode);
	ret = PTR_ERR_OR_ZERO(st->gpio_sync_in);
	if (ret != -EINVAL)
		goto out_put_node;

	/*
	 * EINVAL means that the trigger was not a gpio trigger and we should
	 * try something else.
	 */

This is assuming devm_fwnode_gpiod_get_index() doesn't return EINVAL which
could be confused as the return value for ad7768_trigger_source_get_gpio().

Or just inline ad7768_trigger_source_get_gpio() here and avoid that possibility.

> +
> +	/*
> +	 * TODO: Support the other cases when we have a trigger subsystem to
> +	 * reliably handle other types of devices as trigger sources.
> +	 *
> +	 * For now, return an error message. For self triggering, omit the
> +	 * trigger-sources property.
> +	 */
> +	ret = dev_err_probe(dev, -EOPNOTSUPP, "Invalid synchronization trigger source\n");
> +
> +out_put_node:
> +	fwnode_handle_put(args.fwnode);
> +	return ret;
> +}