4 This README contains high-level information about driver model, a unified
5 way of declaring and accessing drivers in U-Boot. The original work was done
8 Marek Vasut <marex@denx.de>
9 Pavel Herrmann <morpheus.ibis@gmail.com>
10 Viktor Křivák <viktor.krivak@gmail.com>
11 Tomas Hlavacek <tmshlvck@gmail.com>
13 This has been both simplified and extended into the current implementation
16 Simon Glass <sjg@chromium.org>
22 Uclass - a group of devices which operate in the same way. A uclass provides
23 a way of accessing individual devices within the group, but always
24 using the same interface. For example a GPIO uclass provides
25 operations for get/set value. An I2C uclass may have 10 I2C ports,
26 4 with one driver, and 6 with another.
28 Driver - some code which talks to a peripheral and presents a higher-level
31 Device - an instance of a driver, tied to a particular port or peripheral.
37 Build U-Boot sandbox and run it:
43 (type 'reset' to exit U-Boot)
46 There is a uclass called 'demo'. This uclass handles
47 saying hello, and reporting its status. There are two drivers in this
50 - simple: Just prints a message for hello, doesn't implement status
51 - shape: Prints shapes and reports number of characters printed as status
53 The demo class is pretty simple, but not trivial. The intention is that it
54 can be used for testing, so it will implement all driver model features and
55 provide good code coverage of them. It does have multiple drivers, it
56 handles parameter data and platdata (data which tells the driver how
57 to operate on a particular platform) and it uses private driver data.
59 To try it, see the example session below:
62 Hello '@' from 07981110: red 4
89 The intent with driver model is that the core portion has 100% test coverage
90 in sandbox, and every uclass has its own test. As a move towards this, tests
91 are provided in test/dm. To run them, try:
95 You should see something like this:
98 Running 14 driver model tests
99 Test: dm_test_autobind
100 Test: dm_test_autoprobe
101 Test: dm_test_children
103 Test: dm_test_fdt_pre_reloc
105 sandbox_gpio: sb_gpio_get_value: error: offset 4 not reserved
107 Test: dm_test_lifecycle
108 Test: dm_test_operations
109 Test: dm_test_ordering
110 Test: dm_test_platdata
111 Test: dm_test_pre_reloc
120 Let's start at the top. The demo command is in common/cmd_demo.c. It does
121 the usual command processing and then:
123 struct udevice *demo_dev;
125 ret = uclass_get_device(UCLASS_DEMO, devnum, &demo_dev);
127 UCLASS_DEMO means the class of devices which implement 'demo'. Other
128 classes might be MMC, or GPIO, hashing or serial. The idea is that the
129 devices in the class all share a particular way of working. The class
130 presents a unified view of all these devices to U-Boot.
132 This function looks up a device for the demo uclass. Given a device
133 number we can find the device because all devices have registered with
134 the UCLASS_DEMO uclass.
136 The device is automatically activated ready for use by uclass_get_device().
138 Now that we have the device we can do things like:
140 return demo_hello(demo_dev, ch);
142 This function is in the demo uclass. It takes care of calling the 'hello'
143 method of the relevant driver. Bearing in mind that there are two drivers,
144 this particular device may use one or other of them.
146 The code for demo_hello() is in drivers/demo/demo-uclass.c:
148 int demo_hello(struct udevice *dev, int ch)
150 const struct demo_ops *ops = device_get_ops(dev);
155 return ops->hello(dev, ch);
158 As you can see it just calls the relevant driver method. One of these is
159 in drivers/demo/demo-simple.c:
161 static int simple_hello(struct udevice *dev, int ch)
163 const struct dm_demo_pdata *pdata = dev_get_platdata(dev);
165 printf("Hello from %08x: %s %d\n", map_to_sysmem(dev),
166 pdata->colour, pdata->sides);
172 So that is a trip from top (command execution) to bottom (driver action)
173 but it leaves a lot of topics to address.
179 A driver declaration looks something like this (see
180 drivers/demo/demo-shape.c):
182 static const struct demo_ops shape_ops = {
183 .hello = shape_hello,
184 .status = shape_status,
187 U_BOOT_DRIVER(demo_shape_drv) = {
188 .name = "demo_shape_drv",
191 .priv_data_size = sizeof(struct shape_data),
195 This driver has two methods (hello and status) and requires a bit of
196 private data (accessible through dev_get_priv(dev) once the driver has
197 been probed). It is a member of UCLASS_DEMO so will register itself
200 In U_BOOT_DRIVER it is also possible to specify special methods for bind
201 and unbind, and these are called at appropriate times. For many drivers
202 it is hoped that only 'probe' and 'remove' will be needed.
204 The U_BOOT_DRIVER macro creates a data structure accessible from C,
205 so driver model can find the drivers that are available.
207 The methods a device can provide are documented in the device.h header.
210 bind - make the driver model aware of a device (bind it to its driver)
211 unbind - make the driver model forget the device
212 ofdata_to_platdata - convert device tree data to platdata - see later
213 probe - make a device ready for use
214 remove - remove a device so it cannot be used until probed again
216 The sequence to get a device to work is bind, ofdata_to_platdata (if using
217 device tree) and probe.
223 Platform data is like Linux platform data, if you are familiar with that.
224 It provides the board-specific information to start up a device.
226 Why is this information not just stored in the device driver itself? The
227 idea is that the device driver is generic, and can in principle operate on
228 any board that has that type of device. For example, with modern
229 highly-complex SoCs it is common for the IP to come from an IP vendor, and
230 therefore (for example) the MMC controller may be the same on chips from
231 different vendors. It makes no sense to write independent drivers for the
232 MMC controller on each vendor's SoC, when they are all almost the same.
233 Similarly, we may have 6 UARTs in an SoC, all of which are mostly the same,
234 but lie at different addresses in the address space.
236 Using the UART example, we have a single driver and it is instantiated 6
237 times by supplying 6 lots of platform data. Each lot of platform data
238 gives the driver name and a pointer to a structure containing information
239 about this instance - e.g. the address of the register space. It may be that
240 one of the UARTS supports RS-485 operation - this can be added as a flag in
241 the platform data, which is set for this one port and clear for the rest.
243 Think of your driver as a generic piece of code which knows how to talk to
244 a device, but needs to know where it is, any variant/option information and
245 so on. Platform data provides this link between the generic piece of code
246 and the specific way it is bound on a particular board.
248 Examples of platform data include:
250 - The base address of the IP block's register space
251 - Configuration options, like:
252 - the SPI polarity and maximum speed for a SPI controller
253 - the I2C speed to use for an I2C device
254 - the number of GPIOs available in a GPIO device
256 Where does the platform data come from? It is either held in a structure
257 which is compiled into U-Boot, or it can be parsed from the Device Tree
258 (see 'Device Tree' below).
260 For an example of how it can be compiled in, see demo-pdata.c which
261 sets up a table of driver names and their associated platform data.
262 The data can be interpreted by the drivers however they like - it is
263 basically a communication scheme between the board-specific code and
264 the generic drivers, which are intended to work on any board.
266 Drivers can access their data via dev->info->platdata. Here is
267 the declaration for the platform data, which would normally appear
270 static const struct dm_demo_cdata red_square = {
274 static const struct driver_info info[] = {
276 .name = "demo_shape_drv",
277 .platdata = &red_square,
281 demo1 = driver_bind(root, &info[0]);
287 While platdata is useful, a more flexible way of providing device data is
288 by using device tree. With device tree we replace the above code with the
289 following device tree fragment:
292 compatible = "demo-shape";
297 This means that instead of having lots of U_BOOT_DEVICE() declarations in
298 the board file, we put these in the device tree. This approach allows a lot
299 more generality, since the same board file can support many types of boards
300 (e,g. with the same SoC) just by using different device trees. An added
301 benefit is that the Linux device tree can be used, thus further simplifying
302 the task of board-bring up either for U-Boot or Linux devs (whoever gets to
305 The easiest way to make this work it to add a few members to the driver:
307 .platdata_auto_alloc_size = sizeof(struct dm_test_pdata),
308 .ofdata_to_platdata = testfdt_ofdata_to_platdata,
310 The 'auto_alloc' feature allowed space for the platdata to be allocated
311 and zeroed before the driver's ofdata_to_platdata() method is called. The
312 ofdata_to_platdata() method, which the driver write supplies, should parse
313 the device tree node for this device and place it in dev->platdata. Thus
314 when the probe method is called later (to set up the device ready for use)
315 the platform data will be present.
317 Note that both methods are optional. If you provide an ofdata_to_platdata
318 method then it will be called first (during activation). If you provide a
319 probe method it will be called next. See Driver Lifecycle below for more
322 If you don't want to have the platdata automatically allocated then you
323 can leave out platdata_auto_alloc_size. In this case you can use malloc
324 in your ofdata_to_platdata (or probe) method to allocate the required memory,
325 and you should free it in the remove method.
331 The demo uclass is declared like this:
333 U_BOOT_CLASS(demo) = {
337 It is also possible to specify special methods for probe, etc. The uclass
338 numbering comes from include/dm/uclass.h. To add a new uclass, add to the
339 end of the enum there, then declare your uclass as above.
345 Here are the stages that a device goes through in driver model. Note that all
346 methods mentioned here are optional - e.g. if there is no probe() method for
347 a device then it will not be called. A simple device may have very few
348 methods actually defined.
352 A device and its driver are bound using one of these two methods:
354 - Scan the U_BOOT_DEVICE() definitions. U-Boot It looks up the
355 name specified by each, to find the appropriate driver. It then calls
356 device_bind() to create a new device and bind' it to its driver. This will
357 call the device's bind() method.
359 - Scan through the device tree definitions. U-Boot looks at top-level
360 nodes in the the device tree. It looks at the compatible string in each node
361 and uses the of_match part of the U_BOOT_DRIVER() structure to find the
362 right driver for each node. It then calls device_bind() to bind the
363 newly-created device to its driver (thereby creating a device structure).
364 This will also call the device's bind() method.
366 At this point all the devices are known, and bound to their drivers. There
367 is a 'struct udevice' allocated for all devices. However, nothing has been
368 activated (except for the root device). Each bound device that was created
369 from a U_BOOT_DEVICE() declaration will hold the platdata pointer specified
370 in that declaration. For a bound device created from the device tree,
371 platdata will be NULL, but of_offset will be the offset of the device tree
372 node that caused the device to be created. The uclass is set correctly for
375 The device's bind() method is permitted to perform simple actions, but
376 should not scan the device tree node, not initialise hardware, nor set up
377 structures or allocate memory. All of these tasks should be left for
380 Note that compared to Linux, U-Boot's driver model has a separate step of
381 probe/remove which is independent of bind/unbind. This is partly because in
382 U-Boot it may be expensive to probe devices and we don't want to do it until
383 they are needed, or perhaps until after relocation.
387 When a device needs to be used, U-Boot activates it, by following these
388 steps (see device_probe()):
390 a. If priv_auto_alloc_size is non-zero, then the device-private space
391 is allocated for the device and zeroed. It will be accessible as
392 dev->priv. The driver can put anything it likes in there, but should use
393 it for run-time information, not platform data (which should be static
394 and known before the device is probed).
396 b. If platdata_auto_alloc_size is non-zero, then the platform data space
397 is allocated. This is only useful for device tree operation, since
398 otherwise you would have to specific the platform data in the
399 U_BOOT_DEVICE() declaration. The space is allocated for the device and
400 zeroed. It will be accessible as dev->platdata.
402 c. If the device's uclass specifies a non-zero per_device_auto_alloc_size,
403 then this space is allocated and zeroed also. It is allocated for and
404 stored in the device, but it is uclass data. owned by the uclass driver.
405 It is possible for the device to access it.
407 d. All parent devices are probed. It is not possible to activate a device
408 unless its predecessors (all the way up to the root device) are activated.
409 This means (for example) that an I2C driver will require that its bus
412 e. If the driver provides an ofdata_to_platdata() method, then this is
413 called to convert the device tree data into platform data. This should
414 do various calls like fdtdec_get_int(gd->fdt_blob, dev->of_offset, ...)
415 to access the node and store the resulting information into dev->platdata.
416 After this point, the device works the same way whether it was bound
417 using a device tree node or U_BOOT_DEVICE() structure. In either case,
418 the platform data is now stored in the platdata structure. Typically you
419 will use the platdata_auto_alloc_size feature to specify the size of the
420 platform data structure, and U-Boot will automatically allocate and zero
421 it for you before entry to ofdata_to_platdata(). But if not, you can
422 allocate it yourself in ofdata_to_platdata(). Note that it is preferable
423 to do all the device tree decoding in ofdata_to_platdata() rather than
424 in probe(). (Apart from the ugliness of mixing configuration and run-time
425 data, one day it is possible that U-Boot will cache platformat data for
426 devices which are regularly de/activated).
428 f. The device's probe() method is called. This should do anything that
429 is required by the device to get it going. This could include checking
430 that the hardware is actually present, setting up clocks for the
431 hardware and setting up hardware registers to initial values. The code
432 in probe() can access:
434 - platform data in dev->platdata (for configuration)
435 - private data in dev->priv (for run-time state)
436 - uclass data in dev->uclass_priv (for things the uclass stores
439 Note: If you don't use priv_auto_alloc_size then you will need to
440 allocate the priv space here yourself. The same applies also to
441 platdata_auto_alloc_size. Remember to free them in the remove() method.
443 g. The device is marked 'activated'
445 h. The uclass's post_probe() method is called, if one exists. This may
446 cause the uclass to do some housekeeping to record the device as
447 activated and 'known' by the uclass.
451 The device is now activated and can be used. From now until it is removed
452 all of the above structures are accessible. The device appears in the
453 uclass's list of devices (so if the device is in UCLASS_GPIO it will appear
454 as a device in the GPIO uclass). This is the 'running' state of the device.
458 When the device is no-longer required, you can call device_remove() to
459 remove it. This performs the probe steps in reverse:
461 a. The uclass's pre_remove() method is called, if one exists. This may
462 cause the uclass to do some housekeeping to record the device as
463 deactivated and no-longer 'known' by the uclass.
465 b. All the device's children are removed. It is not permitted to have
466 an active child device with a non-active parent. This means that
467 device_remove() is called for all the children recursively at this point.
469 c. The device's remove() method is called. At this stage nothing has been
470 deallocated so platform data, private data and the uclass data will all
471 still be present. This is where the hardware can be shut down. It is
472 intended that the device be completely inactive at this point, For U-Boot
473 to be sure that no hardware is running, it should be enough to remove
476 d. The device memory is freed (platform data, private data, uclass data).
478 Note: Because the platform data for a U_BOOT_DEVICE() is defined with a
479 static pointer, it is not de-allocated during the remove() method. For
480 a device instantiated using the device tree data, the platform data will
481 be dynamically allocated, and thus needs to be deallocated during the
482 remove() method, either:
484 1. if the platdata_auto_alloc_size is non-zero, the deallocation
485 happens automatically within the driver model core; or
487 2. when platdata_auto_alloc_size is 0, both the allocation (in probe()
488 or preferably ofdata_to_platdata()) and the deallocation in remove()
489 are the responsibility of the driver author.
491 e. The device is marked inactive. Note that it is still bound, so the
492 device structure itself is not freed at this point. Should the device be
493 activated again, then the cycle starts again at step 2 above.
497 The device is unbound. This is the step that actually destroys the device.
498 If a parent has children these will be destroyed first. After this point
499 the device does not exist and its memory has be deallocated.
505 Driver model uses a doubly-linked list as the basic data structure. Some
506 nodes have several lists running through them. Creating a more efficient
507 data structure might be worthwhile in some rare cases, once we understand
508 what the bottlenecks are.
514 For the record, this implementation uses a very similar approach to the
515 original patches, but makes at least the following changes:
517 - Tried to aggressively remove boilerplate, so that for most drivers there
518 is little or no 'driver model' code to write.
519 - Moved some data from code into data structure - e.g. store a pointer to
520 the driver operations structure in the driver, rather than passing it
521 to the driver bind function.
522 - Rename some structures to make them more similar to Linux (struct udevice
523 instead of struct instance, struct platdata, etc.)
524 - Change the name 'core' to 'uclass', meaning U-Boot class. It seems that
525 this concept relates to a class of drivers (or a subsystem). We shouldn't
526 use 'class' since it is a C++ reserved word, so U-Boot class (uclass) seems
528 - Remove 'struct driver_instance' and just use a single 'struct udevice'.
529 This removes a level of indirection that doesn't seem necessary.
530 - Built in device tree support, to avoid the need for platdata
531 - Removed the concept of driver relocation, and just make it possible for
532 the new driver (created after relocation) to access the old driver data.
533 I feel that relocation is a very special case and will only apply to a few
534 drivers, many of which can/will just re-init anyway. So the overhead of
535 dealing with this might not be worth it.
536 - Implemented a GPIO system, trying to keep it simple
539 Pre-Relocation Support
540 ----------------------
542 For pre-relocation we simply call the driver model init function. Only
543 drivers marked with DM_FLAG_PRE_RELOC or the device tree
544 'u-boot,dm-pre-reloc' flag are initialised prior to relocation. This helps
545 to reduce the driver model overhead.
547 Then post relocation we throw that away and re-init driver model again.
548 For drivers which require some sort of continuity between pre- and
549 post-relocation devices, we can provide access to the pre-relocation
550 device pointers, but this is not currently implemented (the root device
551 pointer is saved but not made available through the driver model API).
554 Things to punt for later
555 ------------------------
557 - SPL support - this will have to be present before many drivers can be
558 converted, but it seems like we can add it once we are happy with the
561 That is not to say that no thinking has gone into this - in fact there
562 is quite a lot there. However, getting these right is non-trivial and
563 there is a high cost associated with going down the wrong path.
565 For SPL, it may be possible to fit in a simplified driver model with only
566 bind and probe methods, to reduce size.
568 Uclasses are statically numbered at compile time. It would be possible to
569 change this to dynamic numbering, but then we would require some sort of
570 lookup service, perhaps searching by name. This is slightly less efficient
571 so has been left out for now. One small advantage of dynamic numbering might
572 be fewer merge conflicts in uclass-id.h.