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Diffstat (limited to 'Documentation')
-rw-r--r-- | Documentation/driver-model/platform.txt | 59 |
1 files changed, 59 insertions, 0 deletions
diff --git a/Documentation/driver-model/platform.txt b/Documentation/driver-model/platform.txt index 83009fdcbbc8..2e2c2ea90ceb 100644 --- a/Documentation/driver-model/platform.txt +++ b/Documentation/driver-model/platform.txt | |||
@@ -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 | |||
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 platfrom 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>. | ||