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 20 driver model tests
99 Test: dm_test_autobind
100 Test: dm_test_autoprobe
101 Test: dm_test_bus_children
102 Device 'd-test': seq 3 is in use by 'b-test'
103 Device 'c-test@0': seq 0 is in use by 'a-test'
104 Device 'c-test@1': seq 1 is in use by 'd-test'
105 Test: dm_test_bus_children_funcs
106 Test: dm_test_bus_parent_data
107 Test: dm_test_children
109 Device 'd-test': seq 3 is in use by 'b-test'
110 Test: dm_test_fdt_offset
111 Test: dm_test_fdt_pre_reloc
112 Test: dm_test_fdt_uclass_seq
113 Device 'd-test': seq 3 is in use by 'b-test'
114 Device 'a-test': seq 0 is in use by 'd-test'
116 sandbox_gpio: sb_gpio_get_value: error: offset 4 not reserved
118 Test: dm_test_lifecycle
119 Test: dm_test_operations
120 Test: dm_test_ordering
121 Test: dm_test_platdata
122 Test: dm_test_pre_reloc
125 Test: dm_test_uclass_before_ready
132 Let's start at the top. The demo command is in common/cmd_demo.c. It does
133 the usual command processing and then:
135 struct udevice *demo_dev;
137 ret = uclass_get_device(UCLASS_DEMO, devnum, &demo_dev);
139 UCLASS_DEMO means the class of devices which implement 'demo'. Other
140 classes might be MMC, or GPIO, hashing or serial. The idea is that the
141 devices in the class all share a particular way of working. The class
142 presents a unified view of all these devices to U-Boot.
144 This function looks up a device for the demo uclass. Given a device
145 number we can find the device because all devices have registered with
146 the UCLASS_DEMO uclass.
148 The device is automatically activated ready for use by uclass_get_device().
150 Now that we have the device we can do things like:
152 return demo_hello(demo_dev, ch);
154 This function is in the demo uclass. It takes care of calling the 'hello'
155 method of the relevant driver. Bearing in mind that there are two drivers,
156 this particular device may use one or other of them.
158 The code for demo_hello() is in drivers/demo/demo-uclass.c:
160 int demo_hello(struct udevice *dev, int ch)
162 const struct demo_ops *ops = device_get_ops(dev);
167 return ops->hello(dev, ch);
170 As you can see it just calls the relevant driver method. One of these is
171 in drivers/demo/demo-simple.c:
173 static int simple_hello(struct udevice *dev, int ch)
175 const struct dm_demo_pdata *pdata = dev_get_platdata(dev);
177 printf("Hello from %08x: %s %d\n", map_to_sysmem(dev),
178 pdata->colour, pdata->sides);
184 So that is a trip from top (command execution) to bottom (driver action)
185 but it leaves a lot of topics to address.
191 A driver declaration looks something like this (see
192 drivers/demo/demo-shape.c):
194 static const struct demo_ops shape_ops = {
195 .hello = shape_hello,
196 .status = shape_status,
199 U_BOOT_DRIVER(demo_shape_drv) = {
200 .name = "demo_shape_drv",
203 .priv_data_size = sizeof(struct shape_data),
207 This driver has two methods (hello and status) and requires a bit of
208 private data (accessible through dev_get_priv(dev) once the driver has
209 been probed). It is a member of UCLASS_DEMO so will register itself
212 In U_BOOT_DRIVER it is also possible to specify special methods for bind
213 and unbind, and these are called at appropriate times. For many drivers
214 it is hoped that only 'probe' and 'remove' will be needed.
216 The U_BOOT_DRIVER macro creates a data structure accessible from C,
217 so driver model can find the drivers that are available.
219 The methods a device can provide are documented in the device.h header.
222 bind - make the driver model aware of a device (bind it to its driver)
223 unbind - make the driver model forget the device
224 ofdata_to_platdata - convert device tree data to platdata - see later
225 probe - make a device ready for use
226 remove - remove a device so it cannot be used until probed again
228 The sequence to get a device to work is bind, ofdata_to_platdata (if using
229 device tree) and probe.
235 Platform data is like Linux platform data, if you are familiar with that.
236 It provides the board-specific information to start up a device.
238 Why is this information not just stored in the device driver itself? The
239 idea is that the device driver is generic, and can in principle operate on
240 any board that has that type of device. For example, with modern
241 highly-complex SoCs it is common for the IP to come from an IP vendor, and
242 therefore (for example) the MMC controller may be the same on chips from
243 different vendors. It makes no sense to write independent drivers for the
244 MMC controller on each vendor's SoC, when they are all almost the same.
245 Similarly, we may have 6 UARTs in an SoC, all of which are mostly the same,
246 but lie at different addresses in the address space.
248 Using the UART example, we have a single driver and it is instantiated 6
249 times by supplying 6 lots of platform data. Each lot of platform data
250 gives the driver name and a pointer to a structure containing information
251 about this instance - e.g. the address of the register space. It may be that
252 one of the UARTS supports RS-485 operation - this can be added as a flag in
253 the platform data, which is set for this one port and clear for the rest.
255 Think of your driver as a generic piece of code which knows how to talk to
256 a device, but needs to know where it is, any variant/option information and
257 so on. Platform data provides this link between the generic piece of code
258 and the specific way it is bound on a particular board.
260 Examples of platform data include:
262 - The base address of the IP block's register space
263 - Configuration options, like:
264 - the SPI polarity and maximum speed for a SPI controller
265 - the I2C speed to use for an I2C device
266 - the number of GPIOs available in a GPIO device
268 Where does the platform data come from? It is either held in a structure
269 which is compiled into U-Boot, or it can be parsed from the Device Tree
270 (see 'Device Tree' below).
272 For an example of how it can be compiled in, see demo-pdata.c which
273 sets up a table of driver names and their associated platform data.
274 The data can be interpreted by the drivers however they like - it is
275 basically a communication scheme between the board-specific code and
276 the generic drivers, which are intended to work on any board.
278 Drivers can access their data via dev->info->platdata. Here is
279 the declaration for the platform data, which would normally appear
282 static const struct dm_demo_cdata red_square = {
286 static const struct driver_info info[] = {
288 .name = "demo_shape_drv",
289 .platdata = &red_square,
293 demo1 = driver_bind(root, &info[0]);
299 While platdata is useful, a more flexible way of providing device data is
300 by using device tree. With device tree we replace the above code with the
301 following device tree fragment:
304 compatible = "demo-shape";
309 This means that instead of having lots of U_BOOT_DEVICE() declarations in
310 the board file, we put these in the device tree. This approach allows a lot
311 more generality, since the same board file can support many types of boards
312 (e,g. with the same SoC) just by using different device trees. An added
313 benefit is that the Linux device tree can be used, thus further simplifying
314 the task of board-bring up either for U-Boot or Linux devs (whoever gets to
317 The easiest way to make this work it to add a few members to the driver:
319 .platdata_auto_alloc_size = sizeof(struct dm_test_pdata),
320 .ofdata_to_platdata = testfdt_ofdata_to_platdata,
322 The 'auto_alloc' feature allowed space for the platdata to be allocated
323 and zeroed before the driver's ofdata_to_platdata() method is called. The
324 ofdata_to_platdata() method, which the driver write supplies, should parse
325 the device tree node for this device and place it in dev->platdata. Thus
326 when the probe method is called later (to set up the device ready for use)
327 the platform data will be present.
329 Note that both methods are optional. If you provide an ofdata_to_platdata
330 method then it will be called first (during activation). If you provide a
331 probe method it will be called next. See Driver Lifecycle below for more
334 If you don't want to have the platdata automatically allocated then you
335 can leave out platdata_auto_alloc_size. In this case you can use malloc
336 in your ofdata_to_platdata (or probe) method to allocate the required memory,
337 and you should free it in the remove method.
343 The demo uclass is declared like this:
345 U_BOOT_CLASS(demo) = {
349 It is also possible to specify special methods for probe, etc. The uclass
350 numbering comes from include/dm/uclass.h. To add a new uclass, add to the
351 end of the enum there, then declare your uclass as above.
354 Device Sequence Numbers
355 -----------------------
357 U-Boot numbers devices from 0 in many situations, such as in the command
358 line for I2C and SPI buses, and the device names for serial ports (serial0,
359 serial1, ...). Driver model supports this numbering and permits devices
360 to be locating by their 'sequence'.
362 Sequence numbers start from 0 but gaps are permitted. For example, a board
363 may have I2C buses 0, 1, 4, 5 but no 2 or 3. The choice of how devices are
364 numbered is up to a particular board, and may be set by the SoC in some
365 cases. While it might be tempting to automatically renumber the devices
366 where there are gaps in the sequence, this can lead to confusion and is
367 not the way that U-Boot works.
369 Each device can request a sequence number. If none is required then the
370 device will be automatically allocated the next available sequence number.
372 To specify the sequence number in the device tree an alias is typically
376 serial2 = "/serial@22230000";
379 This indicates that in the uclass called "serial", the named node
380 ("/serial@22230000") will be given sequence number 2. Any command or driver
381 which requests serial device 2 will obtain this device.
383 Some devices represent buses where the devices on the bus are numbered or
384 addressed. For example, SPI typically numbers its slaves from 0, and I2C
385 uses a 7-bit address. In these cases the 'reg' property of the subnode is
390 spi2 = "/spi@22300000";
394 #address-cells = <1>;
405 In this case we have a SPI bus with two slaves at 0 and 1. The SPI bus
406 itself is numbered 2. So we might access the SPI flash with:
414 These commands simply need to look up the 2nd device in the SPI uclass to
415 find the right SPI bus. Then, they look at the children of that bus for the
416 right sequence number (0 or 1 in this case).
418 Typically the alias method is used for top-level nodes and the 'reg' method
419 is used only for buses.
421 Device sequence numbers are resolved when a device is probed. Before then
422 the sequence number is only a request which may or may not be honoured,
423 depending on what other devices have been probed. However the numbering is
424 entirely under the control of the board author so a conflict is generally
431 Here are the stages that a device goes through in driver model. Note that all
432 methods mentioned here are optional - e.g. if there is no probe() method for
433 a device then it will not be called. A simple device may have very few
434 methods actually defined.
438 A device and its driver are bound using one of these two methods:
440 - Scan the U_BOOT_DEVICE() definitions. U-Boot It looks up the
441 name specified by each, to find the appropriate driver. It then calls
442 device_bind() to create a new device and bind' it to its driver. This will
443 call the device's bind() method.
445 - Scan through the device tree definitions. U-Boot looks at top-level
446 nodes in the the device tree. It looks at the compatible string in each node
447 and uses the of_match part of the U_BOOT_DRIVER() structure to find the
448 right driver for each node. It then calls device_bind() to bind the
449 newly-created device to its driver (thereby creating a device structure).
450 This will also call the device's bind() method.
452 At this point all the devices are known, and bound to their drivers. There
453 is a 'struct udevice' allocated for all devices. However, nothing has been
454 activated (except for the root device). Each bound device that was created
455 from a U_BOOT_DEVICE() declaration will hold the platdata pointer specified
456 in that declaration. For a bound device created from the device tree,
457 platdata will be NULL, but of_offset will be the offset of the device tree
458 node that caused the device to be created. The uclass is set correctly for
461 The device's bind() method is permitted to perform simple actions, but
462 should not scan the device tree node, not initialise hardware, nor set up
463 structures or allocate memory. All of these tasks should be left for
466 Note that compared to Linux, U-Boot's driver model has a separate step of
467 probe/remove which is independent of bind/unbind. This is partly because in
468 U-Boot it may be expensive to probe devices and we don't want to do it until
469 they are needed, or perhaps until after relocation.
473 When a device needs to be used, U-Boot activates it, by following these
474 steps (see device_probe()):
476 a. If priv_auto_alloc_size is non-zero, then the device-private space
477 is allocated for the device and zeroed. It will be accessible as
478 dev->priv. The driver can put anything it likes in there, but should use
479 it for run-time information, not platform data (which should be static
480 and known before the device is probed).
482 b. If platdata_auto_alloc_size is non-zero, then the platform data space
483 is allocated. This is only useful for device tree operation, since
484 otherwise you would have to specific the platform data in the
485 U_BOOT_DEVICE() declaration. The space is allocated for the device and
486 zeroed. It will be accessible as dev->platdata.
488 c. If the device's uclass specifies a non-zero per_device_auto_alloc_size,
489 then this space is allocated and zeroed also. It is allocated for and
490 stored in the device, but it is uclass data. owned by the uclass driver.
491 It is possible for the device to access it.
493 d. If the device's immediate parent specifies a per_child_auto_alloc_size
494 then this space is allocated. This is intended for use by the parent
495 device to keep track of things related to the child. For example a USB
496 flash stick attached to a USB host controller would likely use this
497 space. The controller can hold information about the USB state of each
500 e. All parent devices are probed. It is not possible to activate a device
501 unless its predecessors (all the way up to the root device) are activated.
502 This means (for example) that an I2C driver will require that its bus
505 f. The device's sequence number is assigned, either the requested one
506 (assuming no conflicts) or the next available one if there is a conflict
507 or nothing particular is requested.
509 g. If the driver provides an ofdata_to_platdata() method, then this is
510 called to convert the device tree data into platform data. This should
511 do various calls like fdtdec_get_int(gd->fdt_blob, dev->of_offset, ...)
512 to access the node and store the resulting information into dev->platdata.
513 After this point, the device works the same way whether it was bound
514 using a device tree node or U_BOOT_DEVICE() structure. In either case,
515 the platform data is now stored in the platdata structure. Typically you
516 will use the platdata_auto_alloc_size feature to specify the size of the
517 platform data structure, and U-Boot will automatically allocate and zero
518 it for you before entry to ofdata_to_platdata(). But if not, you can
519 allocate it yourself in ofdata_to_platdata(). Note that it is preferable
520 to do all the device tree decoding in ofdata_to_platdata() rather than
521 in probe(). (Apart from the ugliness of mixing configuration and run-time
522 data, one day it is possible that U-Boot will cache platformat data for
523 devices which are regularly de/activated).
525 h. The device's probe() method is called. This should do anything that
526 is required by the device to get it going. This could include checking
527 that the hardware is actually present, setting up clocks for the
528 hardware and setting up hardware registers to initial values. The code
529 in probe() can access:
531 - platform data in dev->platdata (for configuration)
532 - private data in dev->priv (for run-time state)
533 - uclass data in dev->uclass_priv (for things the uclass stores
536 Note: If you don't use priv_auto_alloc_size then you will need to
537 allocate the priv space here yourself. The same applies also to
538 platdata_auto_alloc_size. Remember to free them in the remove() method.
540 i. The device is marked 'activated'
542 j. The uclass's post_probe() method is called, if one exists. This may
543 cause the uclass to do some housekeeping to record the device as
544 activated and 'known' by the uclass.
548 The device is now activated and can be used. From now until it is removed
549 all of the above structures are accessible. The device appears in the
550 uclass's list of devices (so if the device is in UCLASS_GPIO it will appear
551 as a device in the GPIO uclass). This is the 'running' state of the device.
555 When the device is no-longer required, you can call device_remove() to
556 remove it. This performs the probe steps in reverse:
558 a. The uclass's pre_remove() method is called, if one exists. This may
559 cause the uclass to do some housekeeping to record the device as
560 deactivated and no-longer 'known' by the uclass.
562 b. All the device's children are removed. It is not permitted to have
563 an active child device with a non-active parent. This means that
564 device_remove() is called for all the children recursively at this point.
566 c. The device's remove() method is called. At this stage nothing has been
567 deallocated so platform data, private data and the uclass data will all
568 still be present. This is where the hardware can be shut down. It is
569 intended that the device be completely inactive at this point, For U-Boot
570 to be sure that no hardware is running, it should be enough to remove
573 d. The device memory is freed (platform data, private data, uclass data,
576 Note: Because the platform data for a U_BOOT_DEVICE() is defined with a
577 static pointer, it is not de-allocated during the remove() method. For
578 a device instantiated using the device tree data, the platform data will
579 be dynamically allocated, and thus needs to be deallocated during the
580 remove() method, either:
582 1. if the platdata_auto_alloc_size is non-zero, the deallocation
583 happens automatically within the driver model core; or
585 2. when platdata_auto_alloc_size is 0, both the allocation (in probe()
586 or preferably ofdata_to_platdata()) and the deallocation in remove()
587 are the responsibility of the driver author.
589 e. The device sequence number is set to -1, meaning that it no longer
590 has an allocated sequence. If the device is later reactivated and that
591 sequence number is still free, it may well receive the name sequence
592 number again. But from this point, the sequence number previously used
593 by this device will no longer exist (think of SPI bus 2 being removed
594 and bus 2 is no longer available for use).
596 f. The device is marked inactive. Note that it is still bound, so the
597 device structure itself is not freed at this point. Should the device be
598 activated again, then the cycle starts again at step 2 above.
602 The device is unbound. This is the step that actually destroys the device.
603 If a parent has children these will be destroyed first. After this point
604 the device does not exist and its memory has be deallocated.
610 Driver model uses a doubly-linked list as the basic data structure. Some
611 nodes have several lists running through them. Creating a more efficient
612 data structure might be worthwhile in some rare cases, once we understand
613 what the bottlenecks are.
619 For the record, this implementation uses a very similar approach to the
620 original patches, but makes at least the following changes:
622 - Tried to aggressively remove boilerplate, so that for most drivers there
623 is little or no 'driver model' code to write.
624 - Moved some data from code into data structure - e.g. store a pointer to
625 the driver operations structure in the driver, rather than passing it
626 to the driver bind function.
627 - Rename some structures to make them more similar to Linux (struct udevice
628 instead of struct instance, struct platdata, etc.)
629 - Change the name 'core' to 'uclass', meaning U-Boot class. It seems that
630 this concept relates to a class of drivers (or a subsystem). We shouldn't
631 use 'class' since it is a C++ reserved word, so U-Boot class (uclass) seems
633 - Remove 'struct driver_instance' and just use a single 'struct udevice'.
634 This removes a level of indirection that doesn't seem necessary.
635 - Built in device tree support, to avoid the need for platdata
636 - Removed the concept of driver relocation, and just make it possible for
637 the new driver (created after relocation) to access the old driver data.
638 I feel that relocation is a very special case and will only apply to a few
639 drivers, many of which can/will just re-init anyway. So the overhead of
640 dealing with this might not be worth it.
641 - Implemented a GPIO system, trying to keep it simple
644 Pre-Relocation Support
645 ----------------------
647 For pre-relocation we simply call the driver model init function. Only
648 drivers marked with DM_FLAG_PRE_RELOC or the device tree
649 'u-boot,dm-pre-reloc' flag are initialised prior to relocation. This helps
650 to reduce the driver model overhead.
652 Then post relocation we throw that away and re-init driver model again.
653 For drivers which require some sort of continuity between pre- and
654 post-relocation devices, we can provide access to the pre-relocation
655 device pointers, but this is not currently implemented (the root device
656 pointer is saved but not made available through the driver model API).
659 Things to punt for later
660 ------------------------
662 - SPL support - this will have to be present before many drivers can be
663 converted, but it seems like we can add it once we are happy with the
666 That is not to say that no thinking has gone into this - in fact there
667 is quite a lot there. However, getting these right is non-trivial and
668 there is a high cost associated with going down the wrong path.
670 For SPL, it may be possible to fit in a simplified driver model with only
671 bind and probe methods, to reduce size.
673 Uclasses are statically numbered at compile time. It would be possible to
674 change this to dynamic numbering, but then we would require some sort of
675 lookup service, perhaps searching by name. This is slightly less efficient
676 so has been left out for now. One small advantage of dynamic numbering might
677 be fewer merge conflicts in uclass-id.h.