1 ============================
2 Platform Devices and Drivers
3 ============================
4
5 See <linux/platform_device.h> for the driver model interface to the
6 platform bus: platform_device, and platform_driver. This pseudo-bus
7 is used to connect devices on busses with minimal infrastructure,
8 like those used to integrate peripherals on many system-on-chip
9 processors, or some "legacy" PC interconnects; as opposed to large
10 formally specified ones like PCI or USB.
11
12
13 Platform devices
14 ~~~~~~~~~~~~~~~~
15 Platform devices are devices that typically appear as autonomous
16 entities in the system. This includes legacy port-based devices and
17 host bridges to peripheral buses, and most controllers integrated
18 into system-on-chip platforms. What they usually have in common
19 is direct addressing from a CPU bus. Rarely, a platform_device will
20 be connected through a segment of some other kind of bus; but its
21 registers will still be directly addressable.
22
23 Platform devices are given a name, used in driver binding, and a
24 list of resources such as addresses and IRQs::
25
26 struct platform_device {
27 const char *name;
28 u32 id;
29 struct device dev;
30 u32 num_resources;
31 struct resource *resource;
32 };
33
34
35 Platform drivers
36 ~~~~~~~~~~~~~~~~
37 Platform drivers follow the standard driver model convention, where
38 discovery/enumeration is handled outside the drivers, and drivers
39 provide probe() and remove() methods. They support power management
40 and shutdown notifications using the standard conventions::
41
42 struct platform_driver {
43 int (*probe)(struct platform_device *);
44 void (*remove)(struct platform_device *);
45 void (*shutdown)(struct platform_device *);
46 int (*suspend)(struct platform_device *, pm_message_t state);
47 int (*resume)(struct platform_device *);
48 struct device_driver driver;
49 const struct platform_device_id *id_table;
50 bool prevent_deferred_probe;
51 bool driver_managed_dma;
52 };
53
54 Note that probe() should in general verify that the specified device hardware
55 actually exists; sometimes platform setup code can't be sure. The probing
56 can use device resources, including clocks, and device platform_data.
57
58 Platform drivers register themselves the normal way::
59
60 int platform_driver_register(struct platform_driver *drv);
61
62 Or, in common situations where the device is known not to be hot-pluggable,
63 the probe() routine can live in an init section to reduce the driver's
64 runtime memory footprint::
65
66 int platform_driver_probe(struct platform_driver *drv,
67 int (*probe)(struct platform_device *))
68
69 Kernel modules can be composed of several platform drivers. The platform core
70 provides helpers to register and unregister an array of drivers::
71
72 int __platform_register_drivers(struct platform_driver * const *drivers,
73 unsigned int count, struct module *owner);
74 void platform_unregister_drivers(struct platform_driver * const *drivers,
75 unsigned int count);
76
77 If one of the drivers fails to register, all drivers registered up to that
78 point will be unregistered in reverse order. Note that there is a convenience
79 macro that passes THIS_MODULE as owner parameter::
80
81 #define platform_register_drivers(drivers, count)
82
83
84 Device Enumeration
85 ~~~~~~~~~~~~~~~~~~
86 As a rule, platform specific (and often board-specific) setup code will
87 register platform devices::
88
89 int platform_device_register(struct platform_device *pdev);
90
91 int platform_add_devices(struct platform_device **pdevs, int ndev);
92
93 The general rule is to register only those devices that actually exist,
94 but in some cases extra devices might be registered. For example, a kernel
95 might be configured to work with an external network adapter that might not
96 be populated on all boards, or likewise to work with an integrated controller
97 that some boards might not hook up to any peripherals.
98
99 In some cases, boot firmware will export tables describing the devices
100 that are populated on a given board. Without such tables, often the
101 only way for system setup code to set up the correct devices is to build
102 a kernel for a specific target board. Such board-specific kernels are
103 common with embedded and custom systems development.
104
105 In many cases, the memory and IRQ resources associated with the platform
106 device are not enough to let the device's driver work. Board setup code
107 will often provide additional information using the device's platform_data
108 field to hold additional information.
109
110 Embedded systems frequently need one or more clocks for platform devices,
111 which are normally kept off until they're actively needed (to save power).
112 System setup also associates those clocks with the device, so that
113 calls to clk_get(&pdev->dev, clock_name) return them as needed.
114
115
116 Legacy Drivers: Device Probing
117 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
118 Some drivers are not fully converted to the driver model, because they take
119 on a non-driver role: the driver registers its platform device, rather than
120 leaving that for system infrastructure. Such drivers can't be hotplugged
121 or coldplugged, since those mechanisms require device creation to be in a
122 different system component than the driver.
123
124 The only "good" reason for this is to handle older system designs which, like
125 original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware
126 configuration. Newer systems have largely abandoned that model, in favor of
127 bus-level support for dynamic configuration (PCI, USB), or device tables
128 provided by the boot firmware (e.g. PNPACPI on x86). There are too many
129 conflicting options about what might be where, and even educated guesses by
130 an operating system will be wrong often enough to make trouble.
131
132 This style of driver is discouraged. If you're updating such a driver,
133 please try to move the device enumeration to a more appropriate location,
134 outside the driver. This will usually be cleanup, since such drivers
135 tend to already have "normal" modes, such as ones using device nodes that
136 were created by PNP or by platform device setup.
137
138 None the less, there are some APIs to support such legacy drivers. Avoid
139 using these calls except with such hotplug-deficient drivers::
140
141 struct platform_device *platform_device_alloc(
142 const char *name, int id);
143
144 You can use platform_device_alloc() to dynamically allocate a device, which
145 you will then initialize with resources and platform_device_register().
146 A better solution is usually::
147
148 struct platform_device *platform_device_register_simple(
149 const char *name, int id,
150 struct resource *res, unsigned int nres);
151
152 You can use platform_device_register_simple() as a one-step call to allocate
153 and register a device.
154
155
156 Device Naming and Driver Binding
157 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
158 The platform_device.dev.bus_id is the canonical name for the devices.
159 It's built from two components:
160
161 * platform_device.name ... which is also used to for driver matching.
162
163 * platform_device.id ... the device instance number, or else "-1"
164 to indicate there's only one.
165
166 These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and
167 "serial/3" indicates bus_id "serial.3"; both would use the platform_driver
168 named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id)
169 and use the platform_driver called "my_rtc".
170
171 Driver binding is performed automatically by the driver core, invoking
172 driver probe() after finding a match between device and driver. If the
173 probe() succeeds, the driver and device are bound as usual. There are
174 three different ways to find such a match:
175
176 - Whenever a device is registered, the drivers for that bus are
177 checked for matches. Platform devices should be registered very
178 early during system boot.
179
180 - When a driver is registered using platform_driver_register(), all
181 unbound devices on that bus are checked for matches. Drivers
182 usually register later during booting, or by module loading.
183
184 - Registering a driver using platform_driver_probe() works just like
185 using platform_driver_register(), except that the driver won't
186 be probed later if another device registers. (Which is OK, since
187 this interface is only for use with non-hotpluggable devices.)
188
189
190 Early Platform Devices and Drivers
191 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
192 The early platform interfaces provide platform data to platform device
193 drivers early on during the system boot. The code is built on top of the
194 early_param() command line parsing and can be executed very early on.
195
196 Example: "earlyprintk" class early serial console in 6 steps
197
198 1. Registering early platform device data
199 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
200 The architecture code registers platform device data using the function
201 early_platform_add_devices(). In the case of early serial console this
202 should be hardware configuration for the serial port. Devices registered
203 at this point will later on be matched against early platform drivers.
204
205 2. Parsing kernel command line
206 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
207 The architecture code calls parse_early_param() to parse the kernel
208 command line. This will execute all matching early_param() callbacks.
209 User specified early platform devices will be registered at this point.
210 For the early serial console case the user can specify port on the
211 kernel command line as "earlyprintk=serial.0" where "earlyprintk" is
212 the class string, "serial" is the name of the platform driver and
213 0 is the platform device id. If the id is -1 then the dot and the
214 id can be omitted.
215
216 3. Installing early platform drivers belonging to a certain class
217 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
218 The architecture code may optionally force registration of all early
219 platform drivers belonging to a certain class using the function
220 early_platform_driver_register_all(). User specified devices from
221 step 2 have priority over these. This step is omitted by the serial
222 driver example since the early serial driver code should be disabled
223 unless the user has specified port on the kernel command line.
224
225 4. Early platform driver registration
226 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
227 Compiled-in platform drivers making use of early_platform_init() are
228 automatically registered during step 2 or 3. The serial driver example
229 should use early_platform_init("earlyprintk", &platform_driver).
230
231 5. Probing of early platform drivers belonging to a certain class
232 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
233 The architecture code calls early_platform_driver_probe() to match
234 registered early platform devices associated with a certain class with
235 registered early platform drivers. Matched devices will get probed().
236 This step can be executed at any point during the early boot. As soon
237 as possible may be good for the serial port case.
238
239 6. Inside the early platform driver probe()
240 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
241 The driver code needs to take special care during early boot, especially
242 when it comes to memory allocation and interrupt registration. The code
243 in the probe() function can use is_early_platform_device() to check if
244 it is called at early platform device or at the regular platform device
245 time. The early serial driver performs register_console() at this point.
246
247 For further information, see <linux/platform_device.h>.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.