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 26 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 extra-gpios: get_value: error: gpio b5 not reserved
119 Test: dm_test_gpio_anon
121 Test: dm_test_lifecycle
122 Test: dm_test_operations
123 Test: dm_test_ordering
124 Test: dm_test_platdata
125 Test: dm_test_pre_reloc
127 Test: dm_test_spi_find
128 Invalid chip select 0:0 (err=-19)
129 SF: Failed to get idcodes
130 Device 'name-emul': seq 0 is in use by 'name-emul'
131 SF: Detected M25P16 with page size 256 Bytes, erase size 64 KiB, total 2 MiB
132 Test: dm_test_spi_flash
133 2097152 bytes written in 0 ms
134 SF: Detected M25P16 with page size 256 Bytes, erase size 64 KiB, total 2 MiB
136 0 erase: 0 ticks, 65536000 KiB/s 524288.000 Mbps
137 1 check: 0 ticks, 65536000 KiB/s 524288.000 Mbps
138 2 write: 0 ticks, 65536000 KiB/s 524288.000 Mbps
139 3 read: 0 ticks, 65536000 KiB/s 524288.000 Mbps
141 0 erase: 0 ticks, 65536000 KiB/s 524288.000 Mbps
142 1 check: 0 ticks, 65536000 KiB/s 524288.000 Mbps
143 2 write: 0 ticks, 65536000 KiB/s 524288.000 Mbps
144 3 read: 0 ticks, 65536000 KiB/s 524288.000 Mbps
145 Test: dm_test_spi_xfer
146 SF: Detected M25P16 with page size 256 Bytes, erase size 64 KiB, total 2 MiB
148 Test: dm_test_uclass_before_ready
155 Let's start at the top. The demo command is in common/cmd_demo.c. It does
156 the usual command processing and then:
158 struct udevice *demo_dev;
160 ret = uclass_get_device(UCLASS_DEMO, devnum, &demo_dev);
162 UCLASS_DEMO means the class of devices which implement 'demo'. Other
163 classes might be MMC, or GPIO, hashing or serial. The idea is that the
164 devices in the class all share a particular way of working. The class
165 presents a unified view of all these devices to U-Boot.
167 This function looks up a device for the demo uclass. Given a device
168 number we can find the device because all devices have registered with
169 the UCLASS_DEMO uclass.
171 The device is automatically activated ready for use by uclass_get_device().
173 Now that we have the device we can do things like:
175 return demo_hello(demo_dev, ch);
177 This function is in the demo uclass. It takes care of calling the 'hello'
178 method of the relevant driver. Bearing in mind that there are two drivers,
179 this particular device may use one or other of them.
181 The code for demo_hello() is in drivers/demo/demo-uclass.c:
183 int demo_hello(struct udevice *dev, int ch)
185 const struct demo_ops *ops = device_get_ops(dev);
190 return ops->hello(dev, ch);
193 As you can see it just calls the relevant driver method. One of these is
194 in drivers/demo/demo-simple.c:
196 static int simple_hello(struct udevice *dev, int ch)
198 const struct dm_demo_pdata *pdata = dev_get_platdata(dev);
200 printf("Hello from %08x: %s %d\n", map_to_sysmem(dev),
201 pdata->colour, pdata->sides);
207 So that is a trip from top (command execution) to bottom (driver action)
208 but it leaves a lot of topics to address.
214 A driver declaration looks something like this (see
215 drivers/demo/demo-shape.c):
217 static const struct demo_ops shape_ops = {
218 .hello = shape_hello,
219 .status = shape_status,
222 U_BOOT_DRIVER(demo_shape_drv) = {
223 .name = "demo_shape_drv",
226 .priv_data_size = sizeof(struct shape_data),
230 This driver has two methods (hello and status) and requires a bit of
231 private data (accessible through dev_get_priv(dev) once the driver has
232 been probed). It is a member of UCLASS_DEMO so will register itself
235 In U_BOOT_DRIVER it is also possible to specify special methods for bind
236 and unbind, and these are called at appropriate times. For many drivers
237 it is hoped that only 'probe' and 'remove' will be needed.
239 The U_BOOT_DRIVER macro creates a data structure accessible from C,
240 so driver model can find the drivers that are available.
242 The methods a device can provide are documented in the device.h header.
245 bind - make the driver model aware of a device (bind it to its driver)
246 unbind - make the driver model forget the device
247 ofdata_to_platdata - convert device tree data to platdata - see later
248 probe - make a device ready for use
249 remove - remove a device so it cannot be used until probed again
251 The sequence to get a device to work is bind, ofdata_to_platdata (if using
252 device tree) and probe.
258 Platform data is like Linux platform data, if you are familiar with that.
259 It provides the board-specific information to start up a device.
261 Why is this information not just stored in the device driver itself? The
262 idea is that the device driver is generic, and can in principle operate on
263 any board that has that type of device. For example, with modern
264 highly-complex SoCs it is common for the IP to come from an IP vendor, and
265 therefore (for example) the MMC controller may be the same on chips from
266 different vendors. It makes no sense to write independent drivers for the
267 MMC controller on each vendor's SoC, when they are all almost the same.
268 Similarly, we may have 6 UARTs in an SoC, all of which are mostly the same,
269 but lie at different addresses in the address space.
271 Using the UART example, we have a single driver and it is instantiated 6
272 times by supplying 6 lots of platform data. Each lot of platform data
273 gives the driver name and a pointer to a structure containing information
274 about this instance - e.g. the address of the register space. It may be that
275 one of the UARTS supports RS-485 operation - this can be added as a flag in
276 the platform data, which is set for this one port and clear for the rest.
278 Think of your driver as a generic piece of code which knows how to talk to
279 a device, but needs to know where it is, any variant/option information and
280 so on. Platform data provides this link between the generic piece of code
281 and the specific way it is bound on a particular board.
283 Examples of platform data include:
285 - The base address of the IP block's register space
286 - Configuration options, like:
287 - the SPI polarity and maximum speed for a SPI controller
288 - the I2C speed to use for an I2C device
289 - the number of GPIOs available in a GPIO device
291 Where does the platform data come from? It is either held in a structure
292 which is compiled into U-Boot, or it can be parsed from the Device Tree
293 (see 'Device Tree' below).
295 For an example of how it can be compiled in, see demo-pdata.c which
296 sets up a table of driver names and their associated platform data.
297 The data can be interpreted by the drivers however they like - it is
298 basically a communication scheme between the board-specific code and
299 the generic drivers, which are intended to work on any board.
301 Drivers can access their data via dev->info->platdata. Here is
302 the declaration for the platform data, which would normally appear
305 static const struct dm_demo_cdata red_square = {
309 static const struct driver_info info[] = {
311 .name = "demo_shape_drv",
312 .platdata = &red_square,
316 demo1 = driver_bind(root, &info[0]);
322 While platdata is useful, a more flexible way of providing device data is
323 by using device tree. With device tree we replace the above code with the
324 following device tree fragment:
327 compatible = "demo-shape";
332 This means that instead of having lots of U_BOOT_DEVICE() declarations in
333 the board file, we put these in the device tree. This approach allows a lot
334 more generality, since the same board file can support many types of boards
335 (e,g. with the same SoC) just by using different device trees. An added
336 benefit is that the Linux device tree can be used, thus further simplifying
337 the task of board-bring up either for U-Boot or Linux devs (whoever gets to
340 The easiest way to make this work it to add a few members to the driver:
342 .platdata_auto_alloc_size = sizeof(struct dm_test_pdata),
343 .ofdata_to_platdata = testfdt_ofdata_to_platdata,
345 The 'auto_alloc' feature allowed space for the platdata to be allocated
346 and zeroed before the driver's ofdata_to_platdata() method is called. The
347 ofdata_to_platdata() method, which the driver write supplies, should parse
348 the device tree node for this device and place it in dev->platdata. Thus
349 when the probe method is called later (to set up the device ready for use)
350 the platform data will be present.
352 Note that both methods are optional. If you provide an ofdata_to_platdata
353 method then it will be called first (during activation). If you provide a
354 probe method it will be called next. See Driver Lifecycle below for more
357 If you don't want to have the platdata automatically allocated then you
358 can leave out platdata_auto_alloc_size. In this case you can use malloc
359 in your ofdata_to_platdata (or probe) method to allocate the required memory,
360 and you should free it in the remove method.
366 The demo uclass is declared like this:
368 U_BOOT_CLASS(demo) = {
372 It is also possible to specify special methods for probe, etc. The uclass
373 numbering comes from include/dm/uclass.h. To add a new uclass, add to the
374 end of the enum there, then declare your uclass as above.
377 Device Sequence Numbers
378 -----------------------
380 U-Boot numbers devices from 0 in many situations, such as in the command
381 line for I2C and SPI buses, and the device names for serial ports (serial0,
382 serial1, ...). Driver model supports this numbering and permits devices
383 to be locating by their 'sequence'. This numbering unique identifies a
384 device in its uclass, so no two devices within a particular uclass can have
385 the same sequence number.
387 Sequence numbers start from 0 but gaps are permitted. For example, a board
388 may have I2C buses 0, 1, 4, 5 but no 2 or 3. The choice of how devices are
389 numbered is up to a particular board, and may be set by the SoC in some
390 cases. While it might be tempting to automatically renumber the devices
391 where there are gaps in the sequence, this can lead to confusion and is
392 not the way that U-Boot works.
394 Each device can request a sequence number. If none is required then the
395 device will be automatically allocated the next available sequence number.
397 To specify the sequence number in the device tree an alias is typically
401 serial2 = "/serial@22230000";
404 This indicates that in the uclass called "serial", the named node
405 ("/serial@22230000") will be given sequence number 2. Any command or driver
406 which requests serial device 2 will obtain this device.
408 Some devices represent buses where the devices on the bus are numbered or
409 addressed. For example, SPI typically numbers its slaves from 0, and I2C
410 uses a 7-bit address. In these cases the 'reg' property of the subnode is
415 spi2 = "/spi@22300000";
419 #address-cells = <1>;
430 In this case we have a SPI bus with two slaves at 0 and 1. The SPI bus
431 itself is numbered 2. So we might access the SPI flash with:
439 These commands simply need to look up the 2nd device in the SPI uclass to
440 find the right SPI bus. Then, they look at the children of that bus for the
441 right sequence number (0 or 1 in this case).
443 Typically the alias method is used for top-level nodes and the 'reg' method
444 is used only for buses.
446 Device sequence numbers are resolved when a device is probed. Before then
447 the sequence number is only a request which may or may not be honoured,
448 depending on what other devices have been probed. However the numbering is
449 entirely under the control of the board author so a conflict is generally
456 A common use of driver model is to implement a bus, a device which provides
457 access to other devices. Example of buses include SPI and I2C. Typically
458 the bus provides some sort of transport or translation that makes it
459 possible to talk to the devices on the bus.
461 Driver model provides a few useful features to help with implementing
462 buses. Firstly, a bus can request that its children store some 'parent
463 data' which can be used to keep track of child state. Secondly, the bus can
464 define methods which are called when a child is probed or removed. This is
465 similar to the methods the uclass driver provides.
467 Here an explanation of how a bus fits with a uclass may be useful. Consider
468 a USB bus with several devices attached to it, each from a different (made
471 xhci_usb (UCLASS_USB)
472 eth (UCLASS_ETHERNET)
473 camera (UCLASS_CAMERA)
474 flash (UCLASS_FLASH_STORAGE)
476 Each of the devices is connected to a different address on the USB bus.
477 The bus device wants to store this address and some other information such
478 as the bus speed for each device.
480 To achieve this, the bus device can use dev->parent_priv in each of its
481 three children. This can be auto-allocated if the bus driver has a non-zero
482 value for per_child_auto_alloc_size. If not, then the bus device can
483 allocate the space itself before the child device is probed.
485 Also the bus driver can define the child_pre_probe() and child_post_remove()
486 methods to allow it to do some processing before the child is activated or
487 after it is deactivated.
489 Note that the information that controls this behaviour is in the bus's
490 driver, not the child's. In fact it is possible that child has no knowledge
491 that it is connected to a bus. The same child device may even be used on two
492 different bus types. As an example. the 'flash' device shown above may also
493 be connected on a SATA bus or standalone with no bus:
495 xhci_usb (UCLASS_USB)
496 flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by USB bus
499 flash (UCLASS_FLASH_STORAGE) - parent data/methods defined by SATA bus
501 flash (UCLASS_FLASH_STORAGE) - no parent data/methods (not on a bus)
503 Above you can see that the driver for xhci_usb/sata controls the child's
504 bus methods. In the third example the device is not on a bus, and therefore
505 will not have these methods at all. Consider the case where the flash
506 device defines child methods. These would be used for *its* children, and
507 would be quite separate from the methods defined by the driver for the bus
508 that the flash device is connetced to. The act of attaching a device to a
509 parent device which is a bus, causes the device to start behaving like a
510 bus device, regardless of its own views on the matter.
512 The uclass for the device can also contain data private to that uclass.
513 But note that each device on the bus may be a memeber of a different
514 uclass, and this data has nothing to do with the child data for each child
521 Here are the stages that a device goes through in driver model. Note that all
522 methods mentioned here are optional - e.g. if there is no probe() method for
523 a device then it will not be called. A simple device may have very few
524 methods actually defined.
528 A device and its driver are bound using one of these two methods:
530 - Scan the U_BOOT_DEVICE() definitions. U-Boot It looks up the
531 name specified by each, to find the appropriate driver. It then calls
532 device_bind() to create a new device and bind' it to its driver. This will
533 call the device's bind() method.
535 - Scan through the device tree definitions. U-Boot looks at top-level
536 nodes in the the device tree. It looks at the compatible string in each node
537 and uses the of_match part of the U_BOOT_DRIVER() structure to find the
538 right driver for each node. It then calls device_bind() to bind the
539 newly-created device to its driver (thereby creating a device structure).
540 This will also call the device's bind() method.
542 At this point all the devices are known, and bound to their drivers. There
543 is a 'struct udevice' allocated for all devices. However, nothing has been
544 activated (except for the root device). Each bound device that was created
545 from a U_BOOT_DEVICE() declaration will hold the platdata pointer specified
546 in that declaration. For a bound device created from the device tree,
547 platdata will be NULL, but of_offset will be the offset of the device tree
548 node that caused the device to be created. The uclass is set correctly for
551 The device's bind() method is permitted to perform simple actions, but
552 should not scan the device tree node, not initialise hardware, nor set up
553 structures or allocate memory. All of these tasks should be left for
556 Note that compared to Linux, U-Boot's driver model has a separate step of
557 probe/remove which is independent of bind/unbind. This is partly because in
558 U-Boot it may be expensive to probe devices and we don't want to do it until
559 they are needed, or perhaps until after relocation.
563 When a device needs to be used, U-Boot activates it, by following these
564 steps (see device_probe()):
566 a. If priv_auto_alloc_size is non-zero, then the device-private space
567 is allocated for the device and zeroed. It will be accessible as
568 dev->priv. The driver can put anything it likes in there, but should use
569 it for run-time information, not platform data (which should be static
570 and known before the device is probed).
572 b. If platdata_auto_alloc_size is non-zero, then the platform data space
573 is allocated. This is only useful for device tree operation, since
574 otherwise you would have to specific the platform data in the
575 U_BOOT_DEVICE() declaration. The space is allocated for the device and
576 zeroed. It will be accessible as dev->platdata.
578 c. If the device's uclass specifies a non-zero per_device_auto_alloc_size,
579 then this space is allocated and zeroed also. It is allocated for and
580 stored in the device, but it is uclass data. owned by the uclass driver.
581 It is possible for the device to access it.
583 d. If the device's immediate parent specifies a per_child_auto_alloc_size
584 then this space is allocated. This is intended for use by the parent
585 device to keep track of things related to the child. For example a USB
586 flash stick attached to a USB host controller would likely use this
587 space. The controller can hold information about the USB state of each
590 e. All parent devices are probed. It is not possible to activate a device
591 unless its predecessors (all the way up to the root device) are activated.
592 This means (for example) that an I2C driver will require that its bus
595 f. The device's sequence number is assigned, either the requested one
596 (assuming no conflicts) or the next available one if there is a conflict
597 or nothing particular is requested.
599 g. If the driver provides an ofdata_to_platdata() method, then this is
600 called to convert the device tree data into platform data. This should
601 do various calls like fdtdec_get_int(gd->fdt_blob, dev->of_offset, ...)
602 to access the node and store the resulting information into dev->platdata.
603 After this point, the device works the same way whether it was bound
604 using a device tree node or U_BOOT_DEVICE() structure. In either case,
605 the platform data is now stored in the platdata structure. Typically you
606 will use the platdata_auto_alloc_size feature to specify the size of the
607 platform data structure, and U-Boot will automatically allocate and zero
608 it for you before entry to ofdata_to_platdata(). But if not, you can
609 allocate it yourself in ofdata_to_platdata(). Note that it is preferable
610 to do all the device tree decoding in ofdata_to_platdata() rather than
611 in probe(). (Apart from the ugliness of mixing configuration and run-time
612 data, one day it is possible that U-Boot will cache platformat data for
613 devices which are regularly de/activated).
615 h. The device's probe() method is called. This should do anything that
616 is required by the device to get it going. This could include checking
617 that the hardware is actually present, setting up clocks for the
618 hardware and setting up hardware registers to initial values. The code
619 in probe() can access:
621 - platform data in dev->platdata (for configuration)
622 - private data in dev->priv (for run-time state)
623 - uclass data in dev->uclass_priv (for things the uclass stores
626 Note: If you don't use priv_auto_alloc_size then you will need to
627 allocate the priv space here yourself. The same applies also to
628 platdata_auto_alloc_size. Remember to free them in the remove() method.
630 i. The device is marked 'activated'
632 j. The uclass's post_probe() method is called, if one exists. This may
633 cause the uclass to do some housekeeping to record the device as
634 activated and 'known' by the uclass.
638 The device is now activated and can be used. From now until it is removed
639 all of the above structures are accessible. The device appears in the
640 uclass's list of devices (so if the device is in UCLASS_GPIO it will appear
641 as a device in the GPIO uclass). This is the 'running' state of the device.
645 When the device is no-longer required, you can call device_remove() to
646 remove it. This performs the probe steps in reverse:
648 a. The uclass's pre_remove() method is called, if one exists. This may
649 cause the uclass to do some housekeeping to record the device as
650 deactivated and no-longer 'known' by the uclass.
652 b. All the device's children are removed. It is not permitted to have
653 an active child device with a non-active parent. This means that
654 device_remove() is called for all the children recursively at this point.
656 c. The device's remove() method is called. At this stage nothing has been
657 deallocated so platform data, private data and the uclass data will all
658 still be present. This is where the hardware can be shut down. It is
659 intended that the device be completely inactive at this point, For U-Boot
660 to be sure that no hardware is running, it should be enough to remove
663 d. The device memory is freed (platform data, private data, uclass data,
666 Note: Because the platform data for a U_BOOT_DEVICE() is defined with a
667 static pointer, it is not de-allocated during the remove() method. For
668 a device instantiated using the device tree data, the platform data will
669 be dynamically allocated, and thus needs to be deallocated during the
670 remove() method, either:
672 1. if the platdata_auto_alloc_size is non-zero, the deallocation
673 happens automatically within the driver model core; or
675 2. when platdata_auto_alloc_size is 0, both the allocation (in probe()
676 or preferably ofdata_to_platdata()) and the deallocation in remove()
677 are the responsibility of the driver author.
679 e. The device sequence number is set to -1, meaning that it no longer
680 has an allocated sequence. If the device is later reactivated and that
681 sequence number is still free, it may well receive the name sequence
682 number again. But from this point, the sequence number previously used
683 by this device will no longer exist (think of SPI bus 2 being removed
684 and bus 2 is no longer available for use).
686 f. The device is marked inactive. Note that it is still bound, so the
687 device structure itself is not freed at this point. Should the device be
688 activated again, then the cycle starts again at step 2 above.
692 The device is unbound. This is the step that actually destroys the device.
693 If a parent has children these will be destroyed first. After this point
694 the device does not exist and its memory has be deallocated.
700 Driver model uses a doubly-linked list as the basic data structure. Some
701 nodes have several lists running through them. Creating a more efficient
702 data structure might be worthwhile in some rare cases, once we understand
703 what the bottlenecks are.
709 For the record, this implementation uses a very similar approach to the
710 original patches, but makes at least the following changes:
712 - Tried to aggressively remove boilerplate, so that for most drivers there
713 is little or no 'driver model' code to write.
714 - Moved some data from code into data structure - e.g. store a pointer to
715 the driver operations structure in the driver, rather than passing it
716 to the driver bind function.
717 - Rename some structures to make them more similar to Linux (struct udevice
718 instead of struct instance, struct platdata, etc.)
719 - Change the name 'core' to 'uclass', meaning U-Boot class. It seems that
720 this concept relates to a class of drivers (or a subsystem). We shouldn't
721 use 'class' since it is a C++ reserved word, so U-Boot class (uclass) seems
723 - Remove 'struct driver_instance' and just use a single 'struct udevice'.
724 This removes a level of indirection that doesn't seem necessary.
725 - Built in device tree support, to avoid the need for platdata
726 - Removed the concept of driver relocation, and just make it possible for
727 the new driver (created after relocation) to access the old driver data.
728 I feel that relocation is a very special case and will only apply to a few
729 drivers, many of which can/will just re-init anyway. So the overhead of
730 dealing with this might not be worth it.
731 - Implemented a GPIO system, trying to keep it simple
734 Pre-Relocation Support
735 ----------------------
737 For pre-relocation we simply call the driver model init function. Only
738 drivers marked with DM_FLAG_PRE_RELOC or the device tree
739 'u-boot,dm-pre-reloc' flag are initialised prior to relocation. This helps
740 to reduce the driver model overhead.
742 Then post relocation we throw that away and re-init driver model again.
743 For drivers which require some sort of continuity between pre- and
744 post-relocation devices, we can provide access to the pre-relocation
745 device pointers, but this is not currently implemented (the root device
746 pointer is saved but not made available through the driver model API).
749 Things to punt for later
750 ------------------------
752 - SPL support - this will have to be present before many drivers can be
753 converted, but it seems like we can add it once we are happy with the
756 That is not to say that no thinking has gone into this - in fact there
757 is quite a lot there. However, getting these right is non-trivial and
758 there is a high cost associated with going down the wrong path.
760 For SPL, it may be possible to fit in a simplified driver model with only
761 bind and probe methods, to reduce size.
763 Uclasses are statically numbered at compile time. It would be possible to
764 change this to dynamic numbering, but then we would require some sort of
765 lookup service, perhaps searching by name. This is slightly less efficient
766 so has been left out for now. One small advantage of dynamic numbering might
767 be fewer merge conflicts in uclass-id.h.