[Linux driver]揭秘platform_driver， platform_device

Platform的出现，如同PCI总线一样，把没有“领导层”的设备全部纳入麾下，是个收养所，

专门收养如SPI I2C等连接多slave设备的微型总线，这样一来，linux的sysfs以及kobj内核管理数据结构

就可以很好的管理这些微总线，把他们出现在fs下面供用户查看其状态，也便于统一的进行电源管理工作；

下面是platform的原文介绍：

 

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 tobuild

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.)

 

 

Early Platform Devices and Drivers

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

The early platform interfaces provide platform data to platform device

drivers early on during the system boot. The code is built on top of the

early_param() command line parsing and can be executed very early on.

 

Example: "earlyprintk" class early serial console in 6 steps

 

1. Registering early platform device data

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

The architecture code registers platform device data using the function

early_platform_add_devices(). In the case of early serial console this

should be hardware configuration for the serial port. Devices registered

at this point will later on be matched against early platform drivers.

 

2. Parsing kernel command line

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

The architecture code calls parse_early_param() to parse the kernel

command line. This will execute all matching early_param() callbacks.

User specified early platform devices will be registered at this point.

For the early serial console case the user can specify port on the

kernel command line as "earlyprintk=serial.0" where "earlyprintk" is

the class string, "serial" is the name of the platform driver and

0 is the platform device id. If the id is -1 then the dot and the

id can be omitted.

 

3. Installing early platform drivers belonging to a certain class

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

The architecture code may optionally force registration of all early

platform drivers belonging to a certain class using the function

early_platform_driver_register_all(). User specified devices from

step 2 have priority over these. This step is omitted by the serial

driver example since the early serial driver code should be disabled

unless the user has specified port on the kernel command line.

 

4. Early platform driver registration

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

Compiled-in platform drivers making use of early_platform_init() are

automatically registered during step 2 or 3. The serial driver example

should use early_platform_init("earlyprintk", &platform_driver).

 

5. Probing of early platform drivers belonging to a certain class

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

The architecture code calls early_platform_driver_probe() to match

registered early platform devices associated with a certain class with

registered early platform drivers. Matched devices will get probed().

This step can be executed at any point during the early boot. As soon

as possible may be good for the serial port case.

 

6. Inside the early platform driver probe()

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

The driver code needs to take special care during early boot, especially

when it comes to memory allocation and interrupt registration. The code

in the probe() function can use is_early_platform_device() to check if

it is called at early platform device or at the regular platform device

time. The early serial driver performs register_console() at this point.

 

For further information, see <linux/platform_device.h>.