Re: [PATCH v4 1/3] gpio: swnode: Add ability to specify native chip selects for SPI

[Andy Shevchenko <andy@black.fi.intel.com>]

On Tue, Apr 09, 2024 at 02:21:24PM +0100, Charles Keepax wrote:
> SPI devices can specify a cs-gpios property to enumerate their
> chip selects. Under device tree, a zero entry in this property can
> be used to specify that a particular chip select is using the SPI
> controllers native chip select, for example:
> 
>         cs-gpios = <&gpio1 0 0>, <0>;
> 
> Here the second chip select is native. However, when using swnodes
> there is currently no way to specify a native chip select. The
> proposal here is to register a swnode_gpio_undefined software node,
> that can be specified to allow the indication of a native chip
> select. For example:
> 
> static const struct software_node_ref_args device_cs_refs[] = {
> 	{
> 		.node  = &device_gpiochip_swnode,
> 		.nargs = 2,
> 		.args  = { 0, GPIO_ACTIVE_LOW },
> 	},
> 	{
> 		.node  = &swnode_gpio_undefined,
> 		.nargs = 0,
> 	},
> };
> 
> Register the swnode as the gpiolib is initialised and
> check in swnode_get_gpio_device if the returned node matches

swnode_get_gpio_device()

> swnode_gpio_undefined and return -ENOENT, which matches the behaviour
> of the device tree system when it encounters a 0 phandle.

...

> +config GPIO_SWNODE_UNDEFINED
> +	bool "Undefined swnode GPIOs"

Why is this user visible?

> +	help
> +	  This adds a special place holder for software nodes to contain an
> +	  undefined GPIO reference, this is primarily used by SPI to allow a
> +	  list of GPIO chip selects to mark a certain chip select as being
> +	  controlled the SPI device's internal chip select mechanism and not
> +	  a GPIO.

How are drivers supposed to work in case this is not selected?

...

> +EXPORT_SYMBOL_NS_GPL(swnode_gpio_undefined, GPIO_SWNODE);

+ export.h

> +static int __init swnode_gpio_init(void)

+ init.h

> +		pr_err("gpiolib: failed to register swnode: %d\n", ret);

+ printk.h

-- 
With Best Regards,
Andy Shevchenko