gpio: swnode: Add ability to specify native chip selects for SPI

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

Reviewed-by: Linus Walleij <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Charles Keepax <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/gpio/Kconfig

···
 	select REGMAP
 	tristate
 
+config GPIO_SWNODE_UNDEFINED
+	bool
+	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.
+
 # put drivers in the right section, in alphabetical order
 
 # This symbol is selected by both I2C and SPI expanders

drivers/gpio/gpiolib-swnode.c

···
  *
  * Copyright 2022 Google LLC
  */
+
+#define pr_fmt(fmt) "gpiolib: swnode: " fmt
+
 #include <linux/err.h>
 #include <linux/errno.h>
+#include <linux/export.h>
+#include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/printk.h>
 #include <linux/property.h>
···
 
 #include "gpiolib.h"
 #include "gpiolib-swnode.h"
+
+#define GPIOLIB_SWNODE_UNDEFINED_NAME "swnode-gpio-undefined"
 
 static void swnode_format_propname(const char *con_id, char *propname,
 				   size_t max_size)
···
 	gdev_node = to_software_node(fwnode);
 	if (!gdev_node || !gdev_node->name)
 		return ERR_PTR(-EINVAL);
+
+	/*
+	 * Check for a special node that identifies undefined GPIOs, this is
+	 * primarily used as a key for internal chip selects in SPI bindings.
+	 */
+	if (IS_ENABLED(CONFIG_GPIO_SWNODE_UNDEFINED) &&
+	    !strcmp(gdev_node->name, GPIOLIB_SWNODE_UNDEFINED_NAME))
+		return ERR_PTR(-ENOENT);
 
 	gdev = gpio_device_find_by_label(gdev_node->name);
 	return gdev ?: ERR_PTR(-EPROBE_DEFER);
···
 
 	return count ?: -ENOENT;
 }
+
+#if IS_ENABLED(CONFIG_GPIO_SWNODE_UNDEFINED)
+/*
+ * A special node that identifies undefined GPIOs, this is primarily used as
+ * a key for internal chip selects in SPI bindings.
+ */
+const struct software_node swnode_gpio_undefined = {
+	.name = GPIOLIB_SWNODE_UNDEFINED_NAME,
+};
+EXPORT_SYMBOL_NS_GPL(swnode_gpio_undefined, GPIO_SWNODE);
+
+static int __init swnode_gpio_init(void)
+{
+	int ret;
+
+	ret = software_node_register(&swnode_gpio_undefined);
+	if (ret < 0)
+		pr_err("failed to register swnode: %d\n", ret);
+
+	return ret;
+}
+subsys_initcall(swnode_gpio_init);
+
+static void __exit swnode_gpio_cleanup(void)
+{
+	software_node_unregister(&swnode_gpio_undefined);
+}
+__exitcall(swnode_gpio_cleanup);
+#endif

include/linux/gpio/property.h

···
 #include <dt-bindings/gpio/gpio.h> /* for GPIO_* flags */
 #include <linux/property.h>
 
+struct software_node;
+
 #define PROPERTY_ENTRY_GPIO(_name_, _chip_node_, _idx_, _flags_) \
 	PROPERTY_ENTRY_REF(_name_, _chip_node_, _idx_, _flags_)
+
+extern const struct software_node swnode_gpio_undefined;
 
 #endif /* __LINUX_GPIO_PROPERTY_H */