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

[Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxx>]

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.

Signed-off-by: Charles Keepax <ckeepax@xxxxxxxxxxxxxxxxxxxxx>
---

Changes since v3:
 - Add Kconfig to make swnode conditionally built
 - Add define for swnode name
 - Add custom init and exit calls
 - Use export namespaces

Thanks,
Charles

 drivers/gpio/Kconfig          |  9 +++++++++
 drivers/gpio/gpiolib-swnode.c | 38 +++++++++++++++++++++++++++++++++++
 include/linux/gpio/consumer.h |  4 ++++
 3 files changed, 51 insertions(+)

diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index b50d0b470849..e9b977139674 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -103,6 +103,15 @@ config GPIO_REGMAP
 	select REGMAP
 	tristate
 
+config GPIO_SWNODE_UNDEFINED
+	bool "Undefined swnode GPIOs"
+	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
diff --git a/drivers/gpio/gpiolib-swnode.c b/drivers/gpio/gpiolib-swnode.c
index fa52bdb1a29a..57722d829c61 100644
--- a/drivers/gpio/gpiolib-swnode.c
+++ b/drivers/gpio/gpiolib-swnode.c
@@ -17,6 +17,8 @@
 #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)
 {
@@ -40,6 +42,13 @@ static struct gpio_device *swnode_get_gpio_device(struct fwnode_handle *fwnode)
 	if (!gdev_node || !gdev_node->name)
 		return ERR_PTR(-EINVAL);
 
+	/*
+	 * Check for special node that identifies undefined GPIOs, this is
+	 * primarily used as a key for internal chip selects in SPI bindings.
+	 */
+	if (!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);
 }
@@ -121,3 +130,32 @@ int swnode_gpio_count(const struct fwnode_handle *fwnode, const char *con_id)
 
 	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("gpiolib: 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
diff --git a/include/linux/gpio/consumer.h b/include/linux/gpio/consumer.h
index db2dfbae8edb..e685fac43398 100644
--- a/include/linux/gpio/consumer.h
+++ b/include/linux/gpio/consumer.h
@@ -12,6 +12,8 @@ struct fwnode_handle;
 struct gpio_array;
 struct gpio_desc;
 
+struct software_node;
+
 /**
  * struct gpio_descs - Struct containing an array of descriptors that can be
  *                     obtained using gpiod_get_array()
@@ -54,6 +56,8 @@ enum gpiod_flags {
 	GPIOD_OUT_HIGH_OPEN_DRAIN = GPIOD_OUT_HIGH | GPIOD_FLAGS_BIT_OPEN_DRAIN,
 };
 
+extern const struct software_node swnode_gpio_undefined;
+
 #ifdef CONFIG_GPIOLIB
 
 /* Return the number of GPIOs associated with a device / function */
-- 
2.39.2