/Documentation/driver-model/platform.txt
【部分翻譯可能不準確，希望高人不吝賜教！】
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.
Platform drivers
~~~~~~~~~~~~~~~~
Platform drivers follow the standard driver model convention, where
discovery/enumeration(枚舉) is handled outside the drivers, and drivers
provide probe() and remove() methods. They support power management
and shutdown notifications using the standard conventions（慣例）.【使用標
準的規範，驅動部分支持電源管理以及關機通知】
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.
【probe()通常應確認指定的設備是否真正存在；有時，platform的setup代碼是靠不住
的。probe()可以使用設備的資源，包括時鐘和設備的platform_data】
Platform drivers register themselves the normal way:
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:
Device Enumeration
~~~~~~~~~~~~~~~~~~
As a rule, platform specific (and often board-specific) setup code will
register platform devices:
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(板載) on all boards, or likewise to work with an integrated controller
that some boards might not hook up(銜接) to any peripherals.
In some cases, boot firmware will export tables(輸出表格) 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.
【在很多情況下，由platform device提供的相關的memory和IRQ資源信息不足以讓device
driver工作。板子的setup代碼通常還使用設備的platform_data域來提供額外的信息。】
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.
【嵌入式系統中的platform devices常常需要1個或多個時鐘，爲了節省功耗，在不需要它們時，
它們通常會被隔離。系統setup也向device提供這些時鐘，設備可以通過clk_get(&pdev->dev,clock_name)
獲得他們想要的。】
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.
【一些(老式)驅動並不能完整的轉換爲現在的驅動模型，因爲它們扮演着一個“非驅動”的角色：
這種驅動註冊它的platform device，而不是留給系統底層結構去做。這種驅動不能支持熱拔插
或者冷拔插，因爲這種機制需要非驅動的不同系統組件來創建設備。】
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（有根據的） 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(更合適的位置),
outside the driver. This will usually be cleanup, since such drivers
tend to(往往) already have "normal" modes, such as ones using device nodes that
were created by PNP or by platform device setup.
None the less（依然）, there are some APIs to support such legacy drivers. Avoid
using these calls except with such hotplug-deficient drivers.
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:
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（注意格式） is the canonical(規範的) 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(實例) number, or else "-1"
to indicate there's only one.
These are concatenated(連接起來的), 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:
【driver core將會自動的執行驅動綁定工作，即在找到相匹配的device和driver後，調用驅動的probe()；
如果probe()執行成功，則driver和device像往常一樣被綁定在一起。有三種不同的方法可以找到一組匹配】
- Whenever a device is registered, the drivers for that bus are
checked for matches. Platform devices should be registered very
early during system boot.
【當一個device被註冊時，該總線上的驅動被檢測是否與該設備匹配。因此，在系統
啓動較早的時候，platform devices就應該被註冊。】
- 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.
【當一個driver被platform_driver_register()註冊時，該總線上所有沒有被綁定的devices被檢測是否
與該驅動匹配。通常，該驅動在啓動過程的後期就被註冊，或者被module加載。】
- 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.)
【使用platform_driver_probe()和使用platform_driver_register()註冊一個驅動是一樣的，不同的
是，如果當其他的devices註冊的時候，驅動不再被probed。不過，這樣也無所謂，因爲這種接口僅僅
在非熱拔插設備上使用。】
