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 22 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_children_iterators
107 Test: dm_test_bus_parent_data
108 Test: dm_test_bus_parent_ops
109 Test: dm_test_children
111 Device 'd-test': seq 3 is in use by 'b-test'
112 Test: dm_test_fdt_offset
113 Test: dm_test_fdt_pre_reloc
114 Test: dm_test_fdt_uclass_seq
115 Device 'd-test': seq 3 is in use by 'b-test'
116 Device 'a-test': seq 0 is in use by 'd-test'
118 sandbox_gpio: sb_gpio_get_value: error: offset 4 not reserved
120 Test: dm_test_lifecycle
121 Test: dm_test_operations
122 Test: dm_test_ordering
123 Test: dm_test_platdata
124 Test: dm_test_pre_reloc
127 Test: dm_test_uclass_before_ready
134 Let's start at the top. The demo command is in common/cmd_demo.c. It does
135 the usual command processing and then:
137 struct udevice *demo_dev;
139 ret = uclass_get_device(UCLASS_DEMO, devnum, &demo_dev);
141 UCLASS_DEMO means the class of devices which implement 'demo'. Other
142 classes might be MMC, or GPIO, hashing or serial. The idea is that the
143 devices in the class all share a particular way of working. The class
144 presents a unified view of all these devices to U-Boot.
146 This function looks up a device for the demo uclass. Given a device
147 number we can find the device because all devices have registered with
148 the UCLASS_DEMO uclass.
150 The device is automatically activated ready for use by uclass_get_device().
152 Now that we have the device we can do things like:
154 return demo_hello(demo_dev, ch);
156 This function is in the demo uclass. It takes care of calling the 'hello'
157 method of the relevant driver. Bearing in mind that there are two drivers,
158 this particular device may use one or other of them.
160 The code for demo_hello() is in drivers/demo/demo-uclass.c:
162 int demo_hello(struct udevice *dev, int ch)
164 const struct demo_ops *ops = device_get_ops(dev);
169 return ops->hello(dev, ch);
172 As you can see it just calls the relevant driver method. One of these is
173 in drivers/demo/demo-simple.c:
175 static int simple_hello(struct udevice *dev, int ch)
177 const struct dm_demo_pdata *pdata = dev_get_platdata(dev);
179 printf("Hello from %08x: %s %d\n", map_to_sysmem(dev),
180 pdata->colour, pdata->sides);
186 So that is a trip from top (command execution) to bottom (driver action)
187 but it leaves a lot of topics to address.
193 A driver declaration looks something like this (see
194 drivers/demo/demo-shape.c):
196 static const struct demo_ops shape_ops = {
197 .hello = shape_hello,
198 .status = shape_status,
201 U_BOOT_DRIVER(demo_shape_drv) = {
202 .name = "demo_shape_drv",
205 .priv_data_size = sizeof(struct shape_data),
209 This driver has two methods (hello and status) and requires a bit of
210 private data (accessible through dev_get_priv(dev) once the driver has
211 been probed). It is a member of UCLASS_DEMO so will register itself
214 In U_BOOT_DRIVER it is also possible to specify special methods for bind
215 and unbind, and these are called at appropriate times. For many drivers
216 it is hoped that only 'probe' and 'remove' will be needed.
218 The U_BOOT_DRIVER macro creates a data structure accessible from C,
219 so driver model can find the drivers that are available.
221 The methods a device can provide are documented in the device.h header.
224 bind - make the driver model aware of a device (bind it to its driver)
225 unbind - make the driver model forget the device
226 ofdata_to_platdata - convert device tree data to platdata - see later
227 probe - make a device ready for use
228 remove - remove a device so it cannot be used until probed again
230 The sequence to get a device to work is bind, ofdata_to_platdata (if using
231 device tree) and probe.
237 Platform data is like Linux platform data, if you are familiar with that.
238 It provides the board-specific information to start up a device.
240 Why is this information not just stored in the device driver itself? The
241 idea is that the device driver is generic, and can in principle operate on
242 any board that has that type of device. For example, with modern
243 highly-complex SoCs it is common for the IP to come from an IP vendor, and
244 therefore (for example) the MMC controller may be the same on chips from
245 different vendors. It makes no sense to write independent drivers for the
246 MMC controller on each vendor's SoC, when they are all almost the same.
247 Similarly, we may have 6 UARTs in an SoC, all of which are mostly the same,
248 but lie at different addresses in the address space.
250 Using the UART example, we have a single driver and it is instantiated 6
251 times by supplying 6 lots of platform data. Each lot of platform data
252 gives the driver name and a pointer to a structure containing information
253 about this instance - e.g. the address of the register space. It may be that
254 one of the UARTS supports RS-485 operation - this can be added as a flag in
255 the platform data, which is set for this one port and clear for the rest.
257 Think of your driver as a generic piece of code which knows how to talk to
258 a device, but needs to know where it is, any variant/option information and
259 so on. Platform data provides this link between the generic piece of code
260 and the specific way it is bound on a particular board.
262 Examples of platform data include:
264 - The base address of the IP block's register space
265 - Configuration options, like:
266 - the SPI polarity and maximum speed for a SPI controller
267 - the I2C speed to use for an I2C device
268 - the number of GPIOs available in a GPIO device
270 Where does the platform data come from? It is either held in a structure
271 which is compiled into U-Boot, or it can be parsed from the Device Tree
272 (see 'Device Tree' below).
274 For an example of how it can be compiled in, see demo-pdata.c which
275 sets up a table of driver names and their associated platform data.
276 The data can be interpreted by the drivers however they like - it is
277 basically a communication scheme between the board-specific code and
278 the generic drivers, which are intended to work on any board.
280 Drivers can access their data via dev->info->platdata. Here is
281 the declaration for the platform data, which would normally appear
284 static const struct dm_demo_cdata red_square = {
288 static const struct driver_info info[] = {
290 .name = "demo_shape_drv",
291 .platdata = &red_square,
295 demo1 = driver_bind(root, &info[0]);
301 While platdata is useful, a more flexible way of providing device data is
302 by using device tree. With device tree we replace the above code with the
303 following device tree fragment:
306 compatible = "demo-shape";
311 This means that instead of having lots of U_BOOT_DEVICE() declarations in
312 the board file, we put these in the device tree. This approach allows a lot
313 more generality, since the same board file can support many types of boards
314 (e,g. with the same SoC) just by using different device trees. An added
315 benefit is that the Linux device tree can be used, thus further simplifying
316 the task of board-bring up either for U-Boot or Linux devs (whoever gets to
319 The easiest way to make this work it to add a few members to the driver:
321 .platdata_auto_alloc_size = sizeof(struct dm_test_pdata),
322 .ofdata_to_platdata = testfdt_ofdata_to_platdata,
324 The 'auto_alloc' feature allowed space for the platdata to be allocated
325 and zeroed before the driver's ofdata_to_platdata() method is called. The
326 ofdata_to_platdata() method, which the driver write supplies, should parse
327 the device tree node for this device and place it in dev->platdata. Thus
328 when the probe method is called later (to set up the device ready for use)
329 the platform data will be present.
331 Note that both methods are optional. If you provide an ofdata_to_platdata
332 method then it will be called first (during activation). If you provide a
333 probe method it will be called next. See Driver Lifecycle below for more
336 If you don't want to have the platdata automatically allocated then you
337 can leave out platdata_auto_alloc_size. In this case you can use malloc
338 in your ofdata_to_platdata (or probe) method to allocate the required memory,
339 and you should free it in the remove method.
345 The demo uclass is declared like this:
347 U_BOOT_CLASS(demo) = {
351 It is also possible to specify special methods for probe, etc. The uclass
352 numbering comes from include/dm/uclass.h. To add a new uclass, add to the
353 end of the enum there, then declare your uclass as above.
356 Device Sequence Numbers
357 -----------------------
359 U-Boot numbers devices from 0 in many situations, such as in the command
360 line for I2C and SPI buses, and the device names for serial ports (serial0,
361 serial1, ...). Driver model supports this numbering and permits devices
362 to be locating by their 'sequence'.
364 Sequence numbers start from 0 but gaps are permitted. For example, a board
365 may have I2C buses 0, 1, 4, 5 but no 2 or 3. The choice of how devices are
366 numbered is up to a particular board, and may be set by the SoC in some
367 cases. While it might be tempting to automatically renumber the devices
368 where there are gaps in the sequence, this can lead to confusion and is
369 not the way that U-Boot works.
371 Each device can request a sequence number. If none is required then the
372 device will be automatically allocated the next available sequence number.
374 To specify the sequence number in the device tree an alias is typically
378 serial2 = "/serial@22230000";
381 This indicates that in the uclass called "serial", the named node
382 ("/serial@22230000") will be given sequence number 2. Any command or driver
383 which requests serial device 2 will obtain this device.
385 Some devices represent buses where the devices on the bus are numbered or
386 addressed. For example, SPI typically numbers its slaves from 0, and I2C
387 uses a 7-bit address. In these cases the 'reg' property of the subnode is
392 spi2 = "/spi@22300000";
396 #address-cells = <1>;
407 In this case we have a SPI bus with two slaves at 0 and 1. The SPI bus
408 itself is numbered 2. So we might access the SPI flash with:
416 These commands simply need to look up the 2nd device in the SPI uclass to
417 find the right SPI bus. Then, they look at the children of that bus for the
418 right sequence number (0 or 1 in this case).
420 Typically the alias method is used for top-level nodes and the 'reg' method
421 is used only for buses.
423 Device sequence numbers are resolved when a device is probed. Before then
424 the sequence number is only a request which may or may not be honoured,
425 depending on what other devices have been probed. However the numbering is
426 entirely under the control of the board author so a conflict is generally
433 A common use of driver model is to implement a bus, a device which provides
434 access to other devices. Example of buses include SPI and I2C. Typically
435 the bus provides some sort of transport or translation that makes it
436 possible to talk to the devices on the bus.
438 Driver model provides a few useful features to help with implementing
439 buses. Firstly, a bus can request that its children store some 'parent
440 data' which can be used to keep track of child state. Secondly, the bus can
441 define methods which are called when a child is probed or removed. This is
442 similar to the methods the uclass driver provides.
444 Here an explanation of how a bus fits with a uclass may be useful. Consider
445 a USB bus with several devices attached to it, each from a different (made
448 xhci_usb (UCLASS_USB)
449 eth (UCLASS_ETHERNET)
450 camera (UCLASS_CAMERA)
451 flash (UCLASS_FLASH_STORAGE)
453 Each of the devices is connected to a different address on the USB bus.
454 The bus device wants to store this address and some other information such
455 as the bus speed for each device.
457 To achieve this, the bus device can use dev->parent_priv in each of its
458 three children. This can be auto-allocated if the bus driver has a non-zero
459 value for per_child_auto_alloc_size. If not, then the bus device can
460 allocate the space itself before the child device is probed.
462 Also the bus driver can define the child_pre_probe() and child_post_remove()
463 methods to allow it to do some processing before the child is activated or
464 after it is deactivated.
466 Note that the information that controls this behaviour is in the bus's
467 driver, not the child's. In fact it is possible that child has no knowledge
468 that it is connected to a bus. The same child device may even be used on two
469 different bus types. As an example. the 'flash' device shown above may also
470 be connected on a SATA bus or standalone with no bus:
472 xhci_usb (UCLASS_USB)
473 flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by USB bus
476 flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by SATA bus
478 flash (UCLASS_FLASH_STORAGE) - no parent data/methods (not on a bus)
480 Above you can see that the driver for xhci_usb/sata controls the child's
481 bus methods. In the third example the device is not on a bus, and therefore
482 will not have these methods at all. Consider the case where the flash
483 device defines child methods. These would be used for *its* children, and
484 would be quite separate from the methods defined by the driver for the bus
485 that the flash device is connetced to. The act of attaching a device to a
486 parent device which is a bus, causes the device to start behaving like a
487 bus device, regardless of its own views on the matter.
489 The uclass for the device can also contain data private to that uclass.
490 But note that each device on the bus may be a memeber of a different
491 uclass, and this data has nothing to do with the child data for each child
498 Here are the stages that a device goes through in driver model. Note that all
499 methods mentioned here are optional - e.g. if there is no probe() method for
500 a device then it will not be called. A simple device may have very few
501 methods actually defined.
505 A device and its driver are bound using one of these two methods:
507 - Scan the U_BOOT_DEVICE() definitions. U-Boot It looks up the
508 name specified by each, to find the appropriate driver. It then calls
509 device_bind() to create a new device and bind' it to its driver. This will
510 call the device's bind() method.
512 - Scan through the device tree definitions. U-Boot looks at top-level
513 nodes in the the device tree. It looks at the compatible string in each node
514 and uses the of_match part of the U_BOOT_DRIVER() structure to find the
515 right driver for each node. It then calls device_bind() to bind the
516 newly-created device to its driver (thereby creating a device structure).
517 This will also call the device's bind() method.
519 At this point all the devices are known, and bound to their drivers. There
520 is a 'struct udevice' allocated for all devices. However, nothing has been
521 activated (except for the root device). Each bound device that was created
522 from a U_BOOT_DEVICE() declaration will hold the platdata pointer specified
523 in that declaration. For a bound device created from the device tree,
524 platdata will be NULL, but of_offset will be the offset of the device tree
525 node that caused the device to be created. The uclass is set correctly for
528 The device's bind() method is permitted to perform simple actions, but
529 should not scan the device tree node, not initialise hardware, nor set up
530 structures or allocate memory. All of these tasks should be left for
533 Note that compared to Linux, U-Boot's driver model has a separate step of
534 probe/remove which is independent of bind/unbind. This is partly because in
535 U-Boot it may be expensive to probe devices and we don't want to do it until
536 they are needed, or perhaps until after relocation.
540 When a device needs to be used, U-Boot activates it, by following these
541 steps (see device_probe()):
543 a. If priv_auto_alloc_size is non-zero, then the device-private space
544 is allocated for the device and zeroed. It will be accessible as
545 dev->priv. The driver can put anything it likes in there, but should use
546 it for run-time information, not platform data (which should be static
547 and known before the device is probed).
549 b. If platdata_auto_alloc_size is non-zero, then the platform data space
550 is allocated. This is only useful for device tree operation, since
551 otherwise you would have to specific the platform data in the
552 U_BOOT_DEVICE() declaration. The space is allocated for the device and
553 zeroed. It will be accessible as dev->platdata.
555 c. If the device's uclass specifies a non-zero per_device_auto_alloc_size,
556 then this space is allocated and zeroed also. It is allocated for and
557 stored in the device, but it is uclass data. owned by the uclass driver.
558 It is possible for the device to access it.
560 d. If the device's immediate parent specifies a per_child_auto_alloc_size
561 then this space is allocated. This is intended for use by the parent
562 device to keep track of things related to the child. For example a USB
563 flash stick attached to a USB host controller would likely use this
564 space. The controller can hold information about the USB state of each
567 e. All parent devices are probed. It is not possible to activate a device
568 unless its predecessors (all the way up to the root device) are activated.
569 This means (for example) that an I2C driver will require that its bus
572 f. The device's sequence number is assigned, either the requested one
573 (assuming no conflicts) or the next available one if there is a conflict
574 or nothing particular is requested.
576 g. If the driver provides an ofdata_to_platdata() method, then this is
577 called to convert the device tree data into platform data. This should
578 do various calls like fdtdec_get_int(gd->fdt_blob, dev->of_offset, ...)
579 to access the node and store the resulting information into dev->platdata.
580 After this point, the device works the same way whether it was bound
581 using a device tree node or U_BOOT_DEVICE() structure. In either case,
582 the platform data is now stored in the platdata structure. Typically you
583 will use the platdata_auto_alloc_size feature to specify the size of the
584 platform data structure, and U-Boot will automatically allocate and zero
585 it for you before entry to ofdata_to_platdata(). But if not, you can
586 allocate it yourself in ofdata_to_platdata(). Note that it is preferable
587 to do all the device tree decoding in ofdata_to_platdata() rather than
588 in probe(). (Apart from the ugliness of mixing configuration and run-time
589 data, one day it is possible that U-Boot will cache platformat data for
590 devices which are regularly de/activated).
592 h. The device's probe() method is called. This should do anything that
593 is required by the device to get it going. This could include checking
594 that the hardware is actually present, setting up clocks for the
595 hardware and setting up hardware registers to initial values. The code
596 in probe() can access:
598 - platform data in dev->platdata (for configuration)
599 - private data in dev->priv (for run-time state)
600 - uclass data in dev->uclass_priv (for things the uclass stores
603 Note: If you don't use priv_auto_alloc_size then you will need to
604 allocate the priv space here yourself. The same applies also to
605 platdata_auto_alloc_size. Remember to free them in the remove() method.
607 i. The device is marked 'activated'
609 j. The uclass's post_probe() method is called, if one exists. This may
610 cause the uclass to do some housekeeping to record the device as
611 activated and 'known' by the uclass.
615 The device is now activated and can be used. From now until it is removed
616 all of the above structures are accessible. The device appears in the
617 uclass's list of devices (so if the device is in UCLASS_GPIO it will appear
618 as a device in the GPIO uclass). This is the 'running' state of the device.
622 When the device is no-longer required, you can call device_remove() to
623 remove it. This performs the probe steps in reverse:
625 a. The uclass's pre_remove() method is called, if one exists. This may
626 cause the uclass to do some housekeeping to record the device as
627 deactivated and no-longer 'known' by the uclass.
629 b. All the device's children are removed. It is not permitted to have
630 an active child device with a non-active parent. This means that
631 device_remove() is called for all the children recursively at this point.
633 c. The device's remove() method is called. At this stage nothing has been
634 deallocated so platform data, private data and the uclass data will all
635 still be present. This is where the hardware can be shut down. It is
636 intended that the device be completely inactive at this point, For U-Boot
637 to be sure that no hardware is running, it should be enough to remove
640 d. The device memory is freed (platform data, private data, uclass data,
643 Note: Because the platform data for a U_BOOT_DEVICE() is defined with a
644 static pointer, it is not de-allocated during the remove() method. For
645 a device instantiated using the device tree data, the platform data will
646 be dynamically allocated, and thus needs to be deallocated during the
647 remove() method, either:
649 1. if the platdata_auto_alloc_size is non-zero, the deallocation
650 happens automatically within the driver model core; or
652 2. when platdata_auto_alloc_size is 0, both the allocation (in probe()
653 or preferably ofdata_to_platdata()) and the deallocation in remove()
654 are the responsibility of the driver author.
656 e. The device sequence number is set to -1, meaning that it no longer
657 has an allocated sequence. If the device is later reactivated and that
658 sequence number is still free, it may well receive the name sequence
659 number again. But from this point, the sequence number previously used
660 by this device will no longer exist (think of SPI bus 2 being removed
661 and bus 2 is no longer available for use).
663 f. The device is marked inactive. Note that it is still bound, so the
664 device structure itself is not freed at this point. Should the device be
665 activated again, then the cycle starts again at step 2 above.
669 The device is unbound. This is the step that actually destroys the device.
670 If a parent has children these will be destroyed first. After this point
671 the device does not exist and its memory has be deallocated.
677 Driver model uses a doubly-linked list as the basic data structure. Some
678 nodes have several lists running through them. Creating a more efficient
679 data structure might be worthwhile in some rare cases, once we understand
680 what the bottlenecks are.
686 For the record, this implementation uses a very similar approach to the
687 original patches, but makes at least the following changes:
689 - Tried to aggressively remove boilerplate, so that for most drivers there
690 is little or no 'driver model' code to write.
691 - Moved some data from code into data structure - e.g. store a pointer to
692 the driver operations structure in the driver, rather than passing it
693 to the driver bind function.
694 - Rename some structures to make them more similar to Linux (struct udevice
695 instead of struct instance, struct platdata, etc.)
696 - Change the name 'core' to 'uclass', meaning U-Boot class. It seems that
697 this concept relates to a class of drivers (or a subsystem). We shouldn't
698 use 'class' since it is a C++ reserved word, so U-Boot class (uclass) seems
700 - Remove 'struct driver_instance' and just use a single 'struct udevice'.
701 This removes a level of indirection that doesn't seem necessary.
702 - Built in device tree support, to avoid the need for platdata
703 - Removed the concept of driver relocation, and just make it possible for
704 the new driver (created after relocation) to access the old driver data.
705 I feel that relocation is a very special case and will only apply to a few
706 drivers, many of which can/will just re-init anyway. So the overhead of
707 dealing with this might not be worth it.
708 - Implemented a GPIO system, trying to keep it simple
711 Pre-Relocation Support
712 ----------------------
714 For pre-relocation we simply call the driver model init function. Only
715 drivers marked with DM_FLAG_PRE_RELOC or the device tree
716 'u-boot,dm-pre-reloc' flag are initialised prior to relocation. This helps
717 to reduce the driver model overhead.
719 Then post relocation we throw that away and re-init driver model again.
720 For drivers which require some sort of continuity between pre- and
721 post-relocation devices, we can provide access to the pre-relocation
722 device pointers, but this is not currently implemented (the root device
723 pointer is saved but not made available through the driver model API).
726 Things to punt for later
727 ------------------------
729 - SPL support - this will have to be present before many drivers can be
730 converted, but it seems like we can add it once we are happy with the
733 That is not to say that no thinking has gone into this - in fact there
734 is quite a lot there. However, getting these right is non-trivial and
735 there is a high cost associated with going down the wrong path.
737 For SPL, it may be possible to fit in a simplified driver model with only
738 bind and probe methods, to reduce size.
740 Uclasses are statically numbered at compile time. It would be possible to
741 change this to dynamic numbering, but then we would require some sort of
742 lookup service, perhaps searching by name. This is slightly less efficient
743 so has been left out for now. One small advantage of dynamic numbering might
744 be fewer merge conflicts in uclass-id.h.