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

[Charles Keepax <ckeepax@opensource.cirrus.com>]

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@opensource.cirrus.com>
---
 drivers/gpio/gpiolib-swnode.c | 8 ++++++++
 drivers/gpio/gpiolib.c        | 9 +++++++++
 include/linux/gpio/consumer.h | 4 ++++
 3 files changed, 21 insertions(+)

diff --git a/drivers/gpio/gpiolib-swnode.c b/drivers/gpio/gpiolib-swnode.c
index fa52bdb1a29a..801b5a660307 100644
--- a/drivers/gpio/gpiolib-swnode.c
+++ b/drivers/gpio/gpiolib-swnode.c
@@ -17,6 +17,11 @@
 #include "gpiolib.h"
 #include "gpiolib-swnode.h"
 
+const struct software_node swnode_gpio_undefined = {
+	.name = "gpio-internal-undefined",
+};
+EXPORT_SYMBOL_GPL(swnode_gpio_undefined);
+
 static void swnode_format_propname(const char *con_id, char *propname,
 				   size_t max_size)
 {
@@ -40,6 +45,9 @@ static struct gpio_device *swnode_get_gpio_device(struct fwnode_handle *fwnode)
 	if (!gdev_node || !gdev_node->name)
 		return ERR_PTR(-EINVAL);
 
+	if (!strcmp(gdev_node->name, "gpio-internal-undefined"))
+		return ERR_PTR(-ENOENT);
+
 	gdev = gpio_device_find_by_label(gdev_node->name);
 	return gdev ?: ERR_PTR(-EPROBE_DEFER);
 }
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index ce94e37bcbee..e3a7e2a3a323 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -4892,8 +4892,17 @@ DEFINE_SEQ_ATTRIBUTE(gpiolib);
 
 static int __init gpiolib_debugfs_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;
+	}
+
 	/* /sys/kernel/debug/gpio */
 	debugfs_create_file("gpio", 0444, NULL, NULL, &gpiolib_fops);
+
 	return 0;
 }
 subsys_initcall(gpiolib_debugfs_init);
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