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