Platform Devices and Drivers（English）

Platform Devices and Drivers

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

See 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.)