Documentation/driver-model/platform.txt update/rewrite

This is almost a rewrite of the driver-model/platform.txt documentation;
the previous text was obsolete (for several years), evidently it never
got updated to match the change from being a PC "legacy_bus" to the more
widely used core bus for most embedded systems.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

Documentation/driver-model/platform.txt

 Platform Devices and Drivers
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+See <linux/platform_device.h> for the driver model interface to the
+platform bus:  platform_device, and platform_driver.  This pseudo-bus
+is used to connect devices on busses with minimal infrastructure,
+like those used to integrate peripherals on many system-on-chip
+processors, or some "legacy" PC interconnects; as opposed to large
+formally specified ones like PCI or USB.
+
 
 Platform devices
 ~~~~~~~~~~~~~~~~
 Platform devices are devices that typically appear as autonomous
 entities in the system. This includes legacy port-based devices and
-host bridges to peripheral buses. 
-
-
-Platform drivers
-~~~~~~~~~~~~~~~~
-Drivers for platform devices are typically very simple and
-unstructured. Either the device was present at a particular I/O port
-and the driver was loaded, or it was not. There was no possibility
-of hotplugging or alternative discovery besides probing at a specific
-I/O address and expecting a specific response.
+host bridges to peripheral buses, and most controllers integrated
+into system-on-chip platforms.  What they usually have in common
+is direct addressing from a CPU bus.  Rarely, a platform_device will
+be connected through a segment of some other kind of bus; but its
+registers will still be directly addressible.
 
+Platform devices are given a name, used in driver binding, and a
+list of resources such as addresses and IRQs.
 
-Other Architectures, Modern Firmware, and new Platforms
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-These devices are not always at the legacy I/O ports. This is true on
-other architectures and on some modern architectures. In most cases,
-the drivers are modified to discover the devices at other well-known
-ports for the given platform. However, the firmware in these systems
-does usually know where exactly these devices reside, and in some
-cases, it's the only way of discovering them. 
+struct platform_device {
+	const char	*name;
+	u32		id;
+	struct device	dev;
+	u32		num_resources;
+	struct resource	*resource;
+};
 
 
-The Platform Bus
-~~~~~~~~~~~~~~~~
-A platform bus has been created to deal with these issues. First and
-foremost, it groups all the legacy devices under a common bus, and
-gives them a common parent if they don't already have one. 
-
-But, besides the organizational benefits, the platform bus can also
-accommodate firmware-based enumeration. 
-
-
-Device Discovery
+Platform drivers
 ~~~~~~~~~~~~~~~~
-The platform bus has no concept of probing for devices. Devices
-discovery is left up to either the legacy drivers or the
-firmware. These entities are expected to notify the platform of
-devices that it discovers via the bus's add() callback:
-
-	platform_bus.add(parent,bus_id).
-
-
-Bus IDs
-~~~~~~~
-Bus IDs are the canonical names for the devices. There is no globally
-standard addressing mechanism for legacy devices. In the IA-32 world,
-we have Pnp IDs to use, as well as the legacy I/O ports. However,
-neither tell what the device really is or have any meaning on other
-platforms. 
-
-Since both PnP IDs and the legacy I/O ports (and other standard I/O
-ports for specific devices) have a 1:1 mapping, we map the
-platform-specific name or identifier to a generic name (at least
-within the scope of the kernel).
-
-For example, a serial driver might find a device at I/O 0x3f8. The
-ACPI firmware might also discover a device with PnP ID (_HID)
-PNP0501. Both correspond to the same device and should be mapped to the
-canonical name 'serial'. 
-
-The bus_id field should be a concatenation of the canonical name and
-the instance of that type of device. For example, the device at I/O
-port 0x3f8 should have a bus_id of "serial0". This places the
-responsibility of enumerating devices of a particular type up to the
-discovery mechanism. But, they are the entity that should know best
-(as opposed to the platform bus driver).
-
-
-Drivers 
-~~~~~~~
-Drivers for platform devices should have a name that is the same as
-the canonical name of the devices they support. This allows the
-platform bus driver to do simple matching with the basic data
-structures to determine if a driver supports a certain device. 
-
-For example, a legacy serial driver should have a name of 'serial' and
-register itself with the platform bus. 
-
-
-Driver Binding
-~~~~~~~~~~~~~~
-Legacy drivers assume they are bound to the device once they start up
-and probe an I/O port. Divorcing them from this will be a difficult
-process. However, that shouldn't prevent us from implementing
-firmware-based enumeration. 
-
-The firmware should notify the platform bus about devices before the
-legacy drivers have had a chance to load. Once the drivers are loaded,
-they driver model core will attempt to bind the driver to any
-previously-discovered devices. Once that has happened, it will be free
-to discover any other devices it pleases.
+Platform drivers follow the standard driver model convention, where
+discovery/enumeration is handled outside the drivers, and drivers
+provide probe() and remove() methods.  They support power management
+and shutdown notifications using the standard conventions.
+
+struct platform_driver {
+	int (*probe)(struct platform_device *);
+	int (*remove)(struct platform_device *);
+	void (*shutdown)(struct platform_device *);
+	int (*suspend)(struct platform_device *, pm_message_t state);
+	int (*suspend_late)(struct platform_device *, pm_message_t state);
+	int (*resume_early)(struct platform_device *);
+	int (*resume)(struct platform_device *);
+	struct device_driver driver;
+};
+
+Note that probe() should general verify that the specified device hardware
+actually exists; sometimes platform setup code can't be sure.  The probing
+can use device resources, including clocks, and device platform_data.
+
+Platform drivers register themselves the normal way:
+
+	int platform_driver_register(struct platform_driver *drv);
+
+Or, in common situations where the device is known not to be hot-pluggable,
+the probe() routine can live in an init section to reduce the driver's
+runtime memory footprint:
+
+	int platform_driver_probe(struct platform_driver *drv,
+			  int (*probe)(struct platform_device *))
+
+
+Device Enumeration
+~~~~~~~~~~~~~~~~~~
+As a rule, platform specific (and often board-specific) setup code wil
+register platform devices:
+
+	int platform_device_register(struct platform_device *pdev);
+
+	int platform_add_devices(struct platform_device **pdevs, int ndev);
+
+The general rule is to register only those devices that actually exist,
+but in some cases extra devices might be registered.  For example, a kernel
+might be configured to work with an external network adapter that might not
+be populated on all boards, or likewise to work with an integrated controller
+that some boards might not hook up to any peripherals.
+
+In some cases, boot firmware will export tables describing the devices
+that are populated on a given board.   Without such tables, often the
+only way for system setup code to set up the correct devices is to build
+a kernel for a specific target board.  Such board-specific kernels are
+common with embedded and custom systems development.
+
+In many cases, the memory and IRQ resources associated with the platform
+device are not enough to let the device's driver work.  Board setup code
+will often provide additional information using the device's platform_data
+field to hold additional information.
+
+Embedded systems frequently need one or more clocks for platform devices,
+which are normally kept off until they're actively needed (to save power).
+System setup also associates those clocks with the device, so that that
+calls to clk_get(&pdev->dev, clock_name) return them as needed.
+
+
+Device Naming and Driver Binding
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The platform_device.dev.bus_id is the canonical name for the devices.
+It's built from two components:
+
+    * platform_device.name ... which is also used to for driver matching.
+
+    * platform_device.id ... the device instance number, or else "-1"
+      to indicate there's only one.
+
+These are catenated, so name/id "serial"/0 indicates bus_id "serial.0", and
+"serial/3" indicates bus_id "serial.3"; both would use the platform_driver
+named "serial".  While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id)
+and use the platform_driver called "my_rtc".
+
+Driver binding is performed automatically by the driver core, invoking
+driver probe() after finding a match between device and driver.  If the
+probe() succeeds, the driver and device are bound as usual.  There are
+three different ways to find such a match:
+
+    - Whenever a device is registered, the drivers for that bus are
+      checked for matches.  Platform devices should be registered very
+      early during system boot.
+
+    - When a driver is registered using platform_driver_register(), all
+      unbound devices on that bus are checked for matches.  Drivers
+      usually register later during booting, or by module loading.
+
+    - Registering a driver using platform_driver_probe() works just like
+      using platform_driver_register(), except that the the driver won't
+      be probed later if another device registers.  (Which is OK, since
+      this interface is only for use with non-hotpluggable devices.)