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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:02 EST.

1	Platform Devices and Drivers
2	~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3	See <linux/platform_device.h> for the driver model interface to the
4	platform bus:  platform_device, and platform_driver.  This pseudo-bus
5	is used to connect devices on busses with minimal infrastructure,
6	like those used to integrate peripherals on many system-on-chip
7	processors, or some "legacy" PC interconnects; as opposed to large
8	formally specified ones like PCI or USB.
9	
10	
11	Platform devices
12	~~~~~~~~~~~~~~~~
13	Platform devices are devices that typically appear as autonomous
14	entities in the system. This includes legacy port-based devices and
15	host bridges to peripheral buses, and most controllers integrated
16	into system-on-chip platforms.  What they usually have in common
17	is direct addressing from a CPU bus.  Rarely, a platform_device will
18	be connected through a segment of some other kind of bus; but its
19	registers will still be directly addressable.
20	
21	Platform devices are given a name, used in driver binding, and a
22	list of resources such as addresses and IRQs.
23	
24	struct platform_device {
25		const char	*name;
26		u32		id;
27		struct device	dev;
28		u32		num_resources;
29		struct resource	*resource;
30	};
31	
32	
33	Platform drivers
34	~~~~~~~~~~~~~~~~
35	Platform drivers follow the standard driver model convention, where
36	discovery/enumeration is handled outside the drivers, and drivers
37	provide probe() and remove() methods.  They support power management
38	and shutdown notifications using the standard conventions.
39	
40	struct platform_driver {
41		int (*probe)(struct platform_device *);
42		int (*remove)(struct platform_device *);
43		void (*shutdown)(struct platform_device *);
44		int (*suspend)(struct platform_device *, pm_message_t state);
45		int (*suspend_late)(struct platform_device *, pm_message_t state);
46		int (*resume_early)(struct platform_device *);
47		int (*resume)(struct platform_device *);
48		struct device_driver driver;
49	};
50	
51	Note that probe() should in general verify that the specified device hardware
52	actually exists; sometimes platform setup code can't be sure.  The probing
53	can use device resources, including clocks, and device platform_data.
54	
55	Platform drivers register themselves the normal way:
56	
57		int platform_driver_register(struct platform_driver *drv);
58	
59	Or, in common situations where the device is known not to be hot-pluggable,
60	the probe() routine can live in an init section to reduce the driver's
61	runtime memory footprint:
62	
63		int platform_driver_probe(struct platform_driver *drv,
64				  int (*probe)(struct platform_device *))
65	
66	
67	Device Enumeration
68	~~~~~~~~~~~~~~~~~~
69	As a rule, platform specific (and often board-specific) setup code will
70	register platform devices:
71	
72		int platform_device_register(struct platform_device *pdev);
73	
74		int platform_add_devices(struct platform_device **pdevs, int ndev);
75	
76	The general rule is to register only those devices that actually exist,
77	but in some cases extra devices might be registered.  For example, a kernel
78	might be configured to work with an external network adapter that might not
79	be populated on all boards, or likewise to work with an integrated controller
80	that some boards might not hook up to any peripherals.
81	
82	In some cases, boot firmware will export tables describing the devices
83	that are populated on a given board.   Without such tables, often the
84	only way for system setup code to set up the correct devices is to build
85	a kernel for a specific target board.  Such board-specific kernels are
86	common with embedded and custom systems development.
87	
88	In many cases, the memory and IRQ resources associated with the platform
89	device are not enough to let the device's driver work.  Board setup code
90	will often provide additional information using the device's platform_data
91	field to hold additional information.
92	
93	Embedded systems frequently need one or more clocks for platform devices,
94	which are normally kept off until they're actively needed (to save power).
95	System setup also associates those clocks with the device, so that that
96	calls to clk_get(&pdev->dev, clock_name) return them as needed.
97	
98	
99	Legacy Drivers:  Device Probing
100	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
101	Some drivers are not fully converted to the driver model, because they take
102	on a non-driver role:  the driver registers its platform device, rather than
103	leaving that for system infrastructure.  Such drivers can't be hotplugged
104	or coldplugged, since those mechanisms require device creation to be in a
105	different system component than the driver.
106	
107	The only "good" reason for this is to handle older system designs which, like
108	original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware
109	configuration.  Newer systems have largely abandoned that model, in favor of
110	bus-level support for dynamic configuration (PCI, USB), or device tables
111	provided by the boot firmware (e.g. PNPACPI on x86).  There are too many
112	conflicting options about what might be where, and even educated guesses by
113	an operating system will be wrong often enough to make trouble.
114	
115	This style of driver is discouraged.  If you're updating such a driver,
116	please try to move the device enumeration to a more appropriate location,
117	outside the driver.  This will usually be cleanup, since such drivers
118	tend to already have "normal" modes, such as ones using device nodes that
119	were created by PNP or by platform device setup.
120	
121	None the less, there are some APIs to support such legacy drivers.  Avoid
122	using these calls except with such hotplug-deficient drivers.
123	
124		struct platform_device *platform_device_alloc(
125				const char *name, int id);
126	
127	You can use platform_device_alloc() to dynamically allocate a device, which
128	you will then initialize with resources and platform_device_register().
129	A better solution is usually:
130	
131		struct platform_device *platform_device_register_simple(
132				const char *name, int id,
133				struct resource *res, unsigned int nres);
134	
135	You can use platform_device_register_simple() as a one-step call to allocate
136	and register a device.
137	
138	
139	Device Naming and Driver Binding
140	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
141	The platform_device.dev.bus_id is the canonical name for the devices.
142	It's built from two components:
143	
144	    * platform_device.name ... which is also used to for driver matching.
145	
146	    * platform_device.id ... the device instance number, or else "-1"
147	      to indicate there's only one.
148	
149	These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and
150	"serial/3" indicates bus_id "serial.3"; both would use the platform_driver
151	named "serial".  While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id)
152	and use the platform_driver called "my_rtc".
153	
154	Driver binding is performed automatically by the driver core, invoking
155	driver probe() after finding a match between device and driver.  If the
156	probe() succeeds, the driver and device are bound as usual.  There are
157	three different ways to find such a match:
158	
159	    - Whenever a device is registered, the drivers for that bus are
160	      checked for matches.  Platform devices should be registered very
161	      early during system boot.
162	
163	    - When a driver is registered using platform_driver_register(), all
164	      unbound devices on that bus are checked for matches.  Drivers
165	      usually register later during booting, or by module loading.
166	
167	    - Registering a driver using platform_driver_probe() works just like
168	      using platform_driver_register(), except that the driver won't
169	      be probed later if another device registers.  (Which is OK, since
170	      this interface is only for use with non-hotpluggable devices.)
171	
172	
173	Early Platform Devices and Drivers
174	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
175	The early platform interfaces provide platform data to platform device
176	drivers early on during the system boot. The code is built on top of the
177	early_param() command line parsing and can be executed very early on.
178	
179	Example: "earlyprintk" class early serial console in 6 steps
180	
181	1. Registering early platform device data
182	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
183	The architecture code registers platform device data using the function
184	early_platform_add_devices(). In the case of early serial console this
185	should be hardware configuration for the serial port. Devices registered
186	at this point will later on be matched against early platform drivers.
187	
188	2. Parsing kernel command line
189	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
190	The architecture code calls parse_early_param() to parse the kernel
191	command line. This will execute all matching early_param() callbacks.
192	User specified early platform devices will be registered at this point.
193	For the early serial console case the user can specify port on the
194	kernel command line as "earlyprintk=serial.0" where "earlyprintk" is
195	the class string, "serial" is the name of the platform driver and
196	0 is the platform device id. If the id is -1 then the dot and the
197	id can be omitted.
198	
199	3. Installing early platform drivers belonging to a certain class
200	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
201	The architecture code may optionally force registration of all early
202	platform drivers belonging to a certain class using the function
203	early_platform_driver_register_all(). User specified devices from
204	step 2 have priority over these. This step is omitted by the serial
205	driver example since the early serial driver code should be disabled
206	unless the user has specified port on the kernel command line.
207	
208	4. Early platform driver registration
209	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
210	Compiled-in platform drivers making use of early_platform_init() are
211	automatically registered during step 2 or 3. The serial driver example
212	should use early_platform_init("earlyprintk", &platform_driver).
213	
214	5. Probing of early platform drivers belonging to a certain class
215	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
216	The architecture code calls early_platform_driver_probe() to match
217	registered early platform devices associated with a certain class with
218	registered early platform drivers. Matched devices will get probed().
219	This step can be executed at any point during the early boot. As soon
220	as possible may be good for the serial port case.
221	
222	6. Inside the early platform driver probe()
223	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
224	The driver code needs to take special care during early boot, especially
225	when it comes to memory allocation and interrupt registration. The code
226	in the probe() function can use is_early_platform_device() to check if
227	it is called at early platform device or at the regular platform device
228	time. The early serial driver performs register_console() at this point.
229	
230	For further information, see <linux/platform_device.h>.
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