doc/driver-model/fdt-fixup.txt

9 4. Work to be done
14 In certain markets, it is beneficial for manufacturers of embedded devices to
20 To realize this in hardware, one method is to have a motherboard, and several
21 possible daughter boards that can be attached to this mother board. Different
23 addition of the daughter board to the device realizes the "extension" of
24 functionality to the device described previously.
26 For the software, we obviously want to reuse components for all these
27 variations of the device. This means that the software somehow needs to cope
29 system, depending on which daughter boards are connected to the motherboard.
31 In the Linux kernel, one possible solution to this problem is to employ the
33 features only the components guaranteed to exist in all varieties of the
36 to support the components on the daughter boards.
39 provide a way to find out if and which daughter boards are installed for this
40 mechanism to work.
43 integrated, and is used on some boards to alter the device tree that is later
44 passed to Linux. But since U-Boot's driver model, which is device tree-based as
50 after the driver model has been initialized. While migrating to a live device
52 this problem, it is a non-negligible task to implement it, an a interim
53 solution is needed to address the problem at least in the medium-term.
55 Hence, we propose a solution to this problem by offering a board-specific
56 call-back function, which is passed a writeable pointer to the device tree.
59 "sees" all modifications to the device tree made in this function. Furthermore,
68 boards have to implement the function board_fix_fdt, which has the following
73 The passed-in void pointer is a writeable pointer to the device tree, which can
74 be used to manipulate the device tree using e.g. functions from
81 Furthermore, the Kconfig option OF_BOARD_FIXUP has to be set for the function
82 to be called:
89 | to be the _last_ set of operations in board_fix_fdt!     |
90 | Since the pre-relocation driver model does not adapt to  |
91 | changes made to the device tree either, its references   |
111 detected components are added to the device tree, as well as a "subtractive"
124 it is safe to call it after the tree has already been manipulated.
126 4. Work to be done