linux-kernel - Re: [PATCH v6 08/11] iio: adc: ad7768-1: add support for Synchronization over SPI

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Message-ID: <4fd62562-cd94-4019-952d-38624656ea81@baylibre.com>
Date: Wed, 30 Apr 2025 14:07:22 -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 v6 08/11] iio: adc: ad7768-1: add support for
 Synchronization over SPI

On 4/27/25 7:13 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
> and multi-device synchronization, as an alternative to adi,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 = NULL;
> +	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 (!device_property_present(dev, "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(dev_fwnode(dev),
> +						 "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)) {

Normally, having error be the indented path like this is preferred, but I think
this case is an exception since the following is "normal" code path, not error
return path.

I would understand this better as:

	st->gpio_sync_in = ad7768_trigger_source_get_gpio(dev, fwnode);
	if (!IS_ERR(st->gpio_sync_in))
		/* The trigger is a GPIO, our job is done here. */
		goto out_put_node;

	/* Second, ... */

> +		/*
> +		 * For this case, it requires one argument, which indicates the
> +		 * output pin referenced.
> +		 */
> +		if (args.nargs < 1)
> +			goto err_not_supp;
> +
> +		if (args.args[0] != AD7768_TRIGGER_SOURCE_SYNC_OUT)
> +			goto err_not_supp;
> +
> +		/*
> +		 * Only self trigger is supported for now, i.e.,
> +		 * external SYNC_OUT is not allowed.
> +		 */
> +		if (fwnode->dev == dev) {

As Andy pointed out, this is a bit odd. Technically, we should be allowing any
trigger provider with the right capabilities. But we don't have a trigger
subsystem yet to make that easy. Since we can already handle the SYNC_IN is
wired to SYNC_OUT of the same chip above by omitting the trigger-sources
property, I would just make a TODO here and always return the not supported
error for now.

> +			st->en_spi_sync = true;
> +			goto out_put_node;
> +		}
> +
> +		goto err_not_supp;
> +	}
> +
> +	goto out_put_node;
> +
> +err_not_supp:
> +	ret = dev_err_probe(dev, -EOPNOTSUPP,
> +			    "Invalid synchronization trigger source");
> +out_put_node:
> +	fwnode_handle_put(args.fwnode);
> +	return ret;
> +}
> +