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-rw-r--r--Documentation/driver-model/platform.txt59
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diff --git a/Documentation/driver-model/platform.txt b/Documentation/driver-model/platform.txt
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@@ -169,3 +169,62 @@ three different ways to find such a match:
169 be probed later if another device registers. (Which is OK, since 169 be probed later if another device registers. (Which is OK, since
170 this interface is only for use with non-hotpluggable devices.) 170 this interface is only for use with non-hotpluggable devices.)
171 171
172
173Early Platform Devices and Drivers
174~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
175The early platform interfaces provide platform data to platform device
176drivers early on during the system boot. The code is built on top of the
177early_param() command line parsing and can be executed very early on.
178
179Example: "earlyprintk" class early serial console in 6 steps
180
1811. Registering early platform device data
182~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
183The architecture code registers platform device data using the function
184early_platform_add_devices(). In the case of early serial console this
185should be hardware configuration for the serial port. Devices registered
186at this point will later on be matched against early platform drivers.
187
1882. Parsing kernel command line
189~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
190The architecture code calls parse_early_param() to parse the kernel
191command line. This will execute all matching early_param() callbacks.
192User specified early platform devices will be registered at this point.
193For the early serial console case the user can specify port on the
194kernel command line as "earlyprintk=serial.0" where "earlyprintk" is
195the class string, "serial" is the name of the platfrom driver and
1960 is the platform device id. If the id is -1 then the dot and the
197id can be omitted.
198
1993. Installing early platform drivers belonging to a certain class
200~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
201The architecture code may optionally force registration of all early
202platform drivers belonging to a certain class using the function
203early_platform_driver_register_all(). User specified devices from
204step 2 have priority over these. This step is omitted by the serial
205driver example since the early serial driver code should be disabled
206unless the user has specified port on the kernel command line.
207
2084. Early platform driver registration
209~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
210Compiled-in platform drivers making use of early_platform_init() are
211automatically registered during step 2 or 3. The serial driver example
212should use early_platform_init("earlyprintk", &platform_driver).
213
2145. Probing of early platform drivers belonging to a certain class
215~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
216The architecture code calls early_platform_driver_probe() to match
217registered early platform devices associated with a certain class with
218registered early platform drivers. Matched devices will get probed().
219This step can be executed at any point during the early boot. As soon
220as possible may be good for the serial port case.
221
2226. Inside the early platform driver probe()
223~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
224The driver code needs to take special care during early boot, especially
225when it comes to memory allocation and interrupt registration. The code
226in the probe() function can use is_early_platform_device() to check if
227it is called at early platform device or at the regular platform device
228time. The early serial driver performs register_console() at this point.
229
230For further information, see <linux/platform_device.h>.