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, 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 addressable.

Platform devices are given a name, used in driver binding, and a
list of resources such as addresses and IRQs.

struct platform_device {
        const char      *name;
        u32             id;
        struct device   dev;
        u32             num_resources;
        struct resource *resource;
};


Platform drivers
~~~~~~~~~~~~~~~~
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 will
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.


Legacy Drivers:  Device Probing
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Some drivers are not fully converted to the driver model, because they take
on a non-driver role:  the driver registers its platform device, rather than
leaving that for system infrastructure.  Such drivers can't be hotplugged
or coldplugged, since those mechanisms require device creation to be in a
different system component than the driver.

The only "good" reason for this is to handle older system designs which, like
original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware
configuration.  Newer systems have largely abandoned that model, in favor of
bus-level support for dynamic configuration (PCI, USB), or device tables
provided by the boot firmware (e.g. PNPACPI on x86).  There are too many
conflicting options about what might be where, and even educated guesses by
an operating system will be wrong often enough to make trouble.

This style of driver is discouraged.  If you're updating such a driver,
please try to move the device enumeration to a more appropriate location,
outside the driver.  This will usually be cleanup, since such drivers
tend to already have "normal" modes, such as ones using device nodes that
were created by PNP or by platform device setup.

None the less, there are some APIs to support such legacy drivers.  Avoid
using these calls except with such hotplug-deficient drivers.

        struct platform_device *platform_device_alloc(
                        const char *name, int id);

You can use platform_device_alloc() to dynamically allocate a device, which
you will then initialize with resources and platform_device_register().
A better solution is usually:

        struct platform_device *platform_device_register_simple(
                        const char *name, int id,
                        struct resource *res, unsigned int nres);

You can use platform_device_register_simple() as a one-step call to allocate
and register a device.


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 concatenated, 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 driver won't
      be probed later if another device registers.  (Which is OK, since
      this interface is only for use with non-hotpluggable devices.)