Introduce documentation regarding use of software nodes to describe
GPIOs on legacy boards that have not been converted to device tree.
Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Tested-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
---
v3: Addressed more Randy's comments, added his reviewed-by and tested-by
tags.
v2: Addressed Randy's comments.
Documentation/driver-api/gpio/board.rst | 65 ++++
Documentation/driver-api/gpio/index.rst | 1 +
.../driver-api/gpio/legacy-boards.rst | 298 ++++++++++++++++++
3 files changed, 364 insertions(+)
diff --git a/Documentation/driver-api/gpio/board.rst b/Documentation/driver-api/gpio/board.rst
index 4fd1cbd8296e..069b54d8591b 100644
--- a/Documentation/driver-api/gpio/board.rst
+++ b/Documentation/driver-api/gpio/board.rst
@@ -94,6 +94,71 @@ with the help of _DSD (Device Specific Data), introduced in ACPI 5.1::
For more information about the ACPI GPIO bindings see
Documentation/firmware-guide/acpi/gpio-properties.rst.
+Software Nodes
+--------------
+
+Software nodes allow board-specific code to construct an in-memory,
+device-tree-like structure using struct software_node and struct
+property_entry. This structure can then be associated with a platform device,
+allowing drivers to use the standard device properties API to query
+configuration, just as they would on an ACPI or device tree system.
+
+Software-node-backed GPIOs are described using the ``PROPERTY_ENTRY_GPIO()``
+macro, which ties a software node representing the GPIO controller with
+consumer device. It allows consumers to use regular gpiolib APIs, such as
+gpiod_get(), gpiod_get_optional().
+
+The software node representing a GPIO controller need not be attached to the
+GPIO controller device. The only requirement is that the node must be
+registered and its name must match the GPIO controller's label.
+
+For example, here is how to describe a single GPIO-connected LED. This is an
+alternative to using platform_data on legacy systems.
+
+.. code-block:: c
+
+ #include <linux/property.h>
+ #include <linux/gpio/machine.h>
+ #include <linux/gpio/property.h>
+
+ /*
+ * 1. Define a node for the GPIO controller. Its .name must match the
+ * controller's label.
+ */
+ static const struct software_node gpio_controller_node = {
+ .name = "gpio-foo",
+ };
+
+ /* 2. Define the properties for the LED device. */
+ static const struct property_entry led_device_props[] = {
+ PROPERTY_ENTRY_STRING("label", "myboard:green:status"),
+ PROPERTY_ENTRY_STRING("linux,default-trigger", "heartbeat"),
+ PROPERTY_ENTRY_GPIO("gpios", &gpio_controller_node, 42, GPIO_ACTIVE_HIGH),
+ { }
+ };
+
+ /* 3. Define the software node for the LED device. */
+ static const struct software_node led_device_swnode = {
+ .name = "status-led",
+ .properties = led_device_props,
+ };
+
+ /*
+ * 4. Register the software nodes and the platform device.
+ */
+ const struct software_node *swnodes[] = {
+ &gpio_controller_node,
+ &led_device_swnode,
+ NULL
+ };
+ software_node_register_node_group(swnodes);
+
+ // Then register a platform_device for "leds-gpio" and associate
+ // it with &led_device_swnode via .fwnode.
+
+For a complete guide on converting board files to use software nodes, see
+Documentation/driver-api/gpio/legacy-boards.rst.
+
Platform Data
-------------
Finally, GPIOs can be bound to devices and functions using platform data. Board
diff --git a/Documentation/driver-api/gpio/index.rst b/Documentation/driver-api/gpio/index.rst
index 43f6a3afe10b..87929840e85a 100644
--- a/Documentation/driver-api/gpio/index.rst
+++ b/Documentation/driver-api/gpio/index.rst
@@ -12,6 +12,7 @@ Contents:
driver
consumer
board
+ legacy-boards
drivers-on-gpio
bt8xxgpio
diff --git a/Documentation/driver-api/gpio/legacy-boards.rst b/Documentation/driver-api/gpio/legacy-boards.rst
new file mode 100644
index 000000000000..46e3a26dba77
--- /dev/null
+++ b/Documentation/driver-api/gpio/legacy-boards.rst
@@ -0,0 +1,298 @@
+Supporting Legacy Boards
+========================
+
+Many drivers in the kernel, such as ``leds-gpio`` and ``gpio-keys``, are
+migrating away from using board-specific ``platform_data`` to a unified device
+properties interface. This interface allows drivers to be simpler and more
+generic, as they can query properties in a standardized way.
+
+On modern systems, these properties are provided via device tree. However, some
+older platforms have not been converted to device tree and instead rely on
+board files to describe their hardware configuration. To bridge this gap and
+allow these legacy boards to work with modern, generic drivers, the kernel
+provides a mechanism called **software nodes**.
+
+This document provides a guide on how to convert a legacy board file from using
+``platform_data`` and ``gpiod_lookup_table`` to the modern software node
+approach for describing GPIO-connected devices.
+
+The Core Idea: Software Nodes
+-----------------------------
+
+Software nodes allow board-specific code to construct an in-memory,
+device-tree-like structure using struct software_node and struct
+property_entry. This structure can then be associated with a platform device,
+allowing drivers to use the standard device properties API (e.g.,
+device_property_read_u32(), device_property_read_string()) to query
+configuration, just as they would on an ACPI or device tree system.
+
+The gpiolib code has support for handling software nodes, so that if GPIO is
+described properly, as detailed in the section below, then regular gpiolib APIs,
+such as gpiod_get(), gpiod_get_optional(), and others will work.
+
+Requirements for GPIO Properties
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When using software nodes to describe GPIO connections, the following
+requirements must be met for the GPIO core to correctly resolve the reference:
+
+1. **The GPIO controller's software node "name" must match the controller's
+ "label".** The gpiolib core uses this name to find the corresponding
+ struct gpio_chip at runtime.
+ This software node has to be registered, but need not be attached to the
+ device representing the GPIO controller that is providing the GPIO in
+ question. It may be left as a "free floating" node.
+
+2. **The GPIO property must be a reference.** The ``PROPERTY_ENTRY_GPIO()``
+ macro handles this as it is an alias for ``PROPERTY_ENTRY_REF()``.
+
+3. **The reference must have exactly two arguments:**
+
+ - The first argument is the GPIO offset within the controller.
+ - The second argument is the flags for the GPIO line (e.g.,
+ GPIO_ACTIVE_HIGH, GPIO_ACTIVE_LOW).
+
+The ``PROPERTY_ENTRY_GPIO()`` macro is the preferred way of defining GPIO
+properties in software nodes.
+
+Conversion Example
+------------------
+
+Let's walk through an example of converting a board file that defines a GPIO-
+connected LED and a button.
+
+Before: Using Platform Data
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A typical legacy board file might look like this:
+
+.. code-block:: c
+
+ #include <linux/platform_device.h>
+ #include <linux/leds.h>
+ #include <linux/gpio_keys.h>
+ #include <linux/gpio/machine.h>
+
+ #define MYBOARD_GPIO_CONTROLLER "gpio-foo"
+
+ /* LED setup */
+ static const struct gpio_led myboard_leds[] = {
+ {
+ .name = "myboard:green:status",
+ .default_trigger = "heartbeat",
+ },
+ };
+
+ static const struct gpio_led_platform_data myboard_leds_pdata = {
+ .num_leds = ARRAY_SIZE(myboard_leds),
+ .leds = myboard_leds,
+ };
+
+ static struct gpiod_lookup_table myboard_leds_gpios = {
+ .dev_id = "leds-gpio",
+ .table = {
+ GPIO_LOOKUP_IDX(MYBOARD_GPIO_CONTROLLER, 42, NULL, 0, GPIO_ACTIVE_HIGH),
+ { },
+ },
+ };
+
+ /* Button setup */
+ static struct gpio_keys_button myboard_buttons[] = {
+ {
+ .code = KEY_WPS_BUTTON,
+ .desc = "WPS Button",
+ .active_low = 1,
+ },
+ };
+
+ static const struct gpio_keys_platform_data myboard_buttons_pdata = {
+ .buttons = myboard_buttons,
+ .nbuttons = ARRAY_SIZE(myboard_buttons),
+ };
+
+ static struct gpiod_lookup_table myboard_buttons_gpios = {
+ .dev_id = "gpio-keys",
+ .table = {
+ GPIO_LOOKUP_IDX(MYBOARD_GPIO_CONTROLLER, 15, NULL, 0, GPIO_ACTIVE_LOW),
+ { },
+ },
+ };
+
+ /* Device registration */
+ static int __init myboard_init(void)
+ {
+ gpiod_add_lookup_table(&myboard_leds_gpios);
+ gpiod_add_lookup_table(&myboard_buttons_gpios);
+
+ platform_device_register_data(NULL, "leds-gpio", -1,
+ &myboard_leds_pdata, sizeof(myboard_leds_pdata));
+ platform_device_register_data(NULL, "gpio-keys", -1,
+ &myboard_buttons_pdata, sizeof(myboard_buttons_pdata));
+
+ return 0;
+ }
+
+After: Using Software Nodes
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Here is how the same configuration can be expressed using software nodes.
+
+Step 1: Define the GPIO Controller Node
+***************************************
+
+First, define a software node that represents the GPIO controller that the
+LEDs and buttons are connected to. The ``name`` of this node must match the
+name of the driver for the GPIO controller (e.g., "gpio-foo").
+
+.. code-block:: c
+
+ #include <linux/property.h>
+ #include <linux/gpio/property.h>
+
+ #define MYBOARD_GPIO_CONTROLLER "gpio-foo"
+
+ static const struct software_node myboard_gpio_controller_node = {
+ .name = MYBOARD_GPIO_CONTROLLER,
+ };
+
+Step 2: Define Consumer Device Nodes and Properties
+***************************************************
+
+Next, define the software nodes for the consumer devices (the LEDs and buttons).
+This involves creating a parent node for each device type and child nodes for
+each individual LED or button.
+
+.. code-block:: c
+
+ /* LED setup */
+ static const struct software_node myboard_leds_node = {
+ .name = "myboard-leds",
+ };
+
+ static const struct property_entry myboard_status_led_props[] = {
+ PROPERTY_ENTRY_STRING("label", "myboard:green:status"),
+ PROPERTY_ENTRY_STRING("linux,default-trigger", "heartbeat"),
+ PROPERTY_ENTRY_GPIO("gpios", &myboard_gpio_controller_node, 42, GPIO_ACTIVE_HIGH),
+ { }
+ };
+
+ static const struct software_node myboard_status_led_swnode = {
+ .name = "status-led",
+ .parent = &myboard_leds_node,
+ .properties = myboard_status_led_props,
+ };
+
+ /* Button setup */
+ static const struct software_node myboard_keys_node = {
+ .name = "myboard-keys",
+ };
+
+ static const struct property_entry myboard_wps_button_props[] = {
+ PROPERTY_ENTRY_STRING("label", "WPS Button"),
+ PROPERTY_ENTRY_U32("linux,code", KEY_WPS_BUTTON),
+ PROPERTY_ENTRY_GPIO("gpios", &myboard_gpio_controller_node, 15, GPIO_ACTIVE_LOW),
+ { }
+ };
+
+ static const struct software_node myboard_wps_button_swnode = {
+ .name = "wps-button",
+ .parent = &myboard_keys_node,
+ .properties = myboard_wps_button_props,
+ };
+
+
+
+Step 3: Group and Register the Nodes
+************************************
+
+For maintainability, it is often beneficial to group all software nodes into a
+single array and register them with one call.
+
+.. code-block:: c
+
+ static const struct software_node * const myboard_swnodes[] = {
+ &myboard_gpio_controller_node,
+ &myboard_leds_node,
+ &myboard_status_led_swnode,
+ &myboard_keys_node,
+ &myboard_wps_button_swnode,
+ NULL
+ };
+
+ static int __init myboard_init(void)
+ {
+ int error;
+
+ error = software_node_register_node_group(myboard_swnodes);
+ if (error) {
+ pr_err("Failed to register software nodes: %d\n", error);
+ return error;
+ }
+
+ // ... platform device registration follows
+ }
+
+.. note::
+ When splitting registration of nodes by devices that they represent, it is
+ essential that the software node representing the GPIO controller itself
+ is registered first, before any of the nodes that reference it.
+
+Step 4: Register Platform Devices with Software Nodes
+*****************************************************
+
+Finally, register the platform devices and associate them with their respective
+software nodes using the ``fwnode`` field in struct platform_device_info.
+
+.. code-block:: c
+
+ static struct platform_device *leds_pdev;
+ static struct platform_device *keys_pdev;
+
+ static int __init myboard_init(void)
+ {
+ struct platform_device_info pdev_info;
+ int error;
+
+ error = software_node_register_node_group(myboard_swnodes);
+ if (error)
+ return error;
+
+ memset(&pdev_info, 0, sizeof(pdev_info));
+ pdev_info.name = "leds-gpio";
+ pdev_info.id = PLATFORM_DEVID_NONE;
+ pdev_info.fwnode = software_node_fwnode(&myboard_leds_node);
+ leds_pdev = platform_device_register_full(&pdev_info);
+ if (IS_ERR(leds_pdev)) {
+ error = PTR_ERR(leds_pdev);
+ goto err_unregister_nodes;
+ }
+
+ memset(&pdev_info, 0, sizeof(pdev_info));
+ pdev_info.name = "gpio-keys";
+ pdev_info.id = PLATFORM_DEVID_NONE;
+ pdev_info.fwnode = software_node_fwnode(&myboard_keys_node);
+ keys_pdev = platform_device_register_full(&pdev_info);
+ if (IS_ERR(keys_pdev)) {
+ error = PTR_ERR(keys_pdev);
+ platform_device_unregister(leds_pdev);
+ goto err_unregister_nodes;
+ }
+
+ return 0;
+
+ err_unregister_nodes:
+ software_node_unregister_node_group(myboard_swnodes);
+ return error;
+ }
+
+ static void __exit myboard_exit(void)
+ {
+ platform_device_unregister(keys_pdev);
+ platform_device_unregister(leds_pdev);
+ software_node_unregister_node_group(myboard_swnodes);
+ }
+
+With these changes, the generic ``leds-gpio`` and ``gpio-keys`` drivers will
+be able to probe successfully and get their configuration from the properties
+defined in the software nodes, removing the need for board-specific platform
+data.
--
2.51.0.rc1.163.g2494970778-goog
--
Dmitry
