133 lines
5.8 KiB
ReStructuredText
133 lines
5.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0+
|
|
.. 2017-01-06, Mario Six <mario.six@gdsys.cc>
|
|
|
|
Pre-relocation device tree manipulation
|
|
=======================================
|
|
|
|
Purpose
|
|
-------
|
|
|
|
In certain markets, it is beneficial for manufacturers of embedded devices to
|
|
offer certain ranges of products, where the functionality of the devices within
|
|
one series either don't differ greatly from another, or can be thought of as
|
|
"extensions" of each other, where one device only differs from another in the
|
|
addition of a small number of features (e.g. an additional output connector).
|
|
|
|
To realize this in hardware, one method is to have a motherboard, and several
|
|
possible daughter boards that can be attached to this mother board. Different
|
|
daughter boards then either offer the slightly different functionality, or the
|
|
addition of the daughter board to the device realizes the "extension" of
|
|
functionality to the device described previously.
|
|
|
|
For the software, we obviously want to reuse components for all these
|
|
variations of the device. This means that the software somehow needs to cope
|
|
with the situation that certain ICs may or may not be present on any given
|
|
system, depending on which daughter boards are connected to the motherboard.
|
|
|
|
In the Linux kernel, one possible solution to this problem is to employ the
|
|
device tree overlay mechanism: There exists one "base" device tree, which
|
|
features only the components guaranteed to exist in all varieties of the
|
|
device. At the start of the kernel, the presence and type of the daughter
|
|
boards is then detected, and the corresponding device tree overlays are applied
|
|
to support the components on the daughter boards.
|
|
|
|
Note that the components present on every variety of the board must, of course,
|
|
provide a way to find out if and which daughter boards are installed for this
|
|
mechanism to work.
|
|
|
|
In the U-Boot boot loader, support for device tree overlays has recently been
|
|
integrated, and is used on some boards to alter the device tree that is later
|
|
passed to Linux. But since U-Boot's driver model, which is device tree-based as
|
|
well, is being used in more and more drivers, the same problem of altering the
|
|
device tree starts cropping up in U-Boot itself as well.
|
|
|
|
An additional problem with the device tree in U-Boot is that it is read-only,
|
|
and the current mechanisms don't allow easy manipulation of the device tree
|
|
after the driver model has been initialized. While migrating to a live device
|
|
tree (at least after the relocation) would greatly simplify the solution of
|
|
this problem, it is a non-negligible task to implement it, an a interim
|
|
solution is needed to address the problem at least in the medium-term.
|
|
|
|
Hence, we propose a solution to this problem by offering a board-specific
|
|
call-back function, which is passed a writeable pointer to the device tree.
|
|
This function is called before the device tree is relocated, and specifically
|
|
before the main U-Boot's driver model is instantiated, hence the main U-Boot
|
|
"sees" all modifications to the device tree made in this function. Furthermore,
|
|
we have the pre-relocation driver model at our disposal at this stage, which
|
|
means that we can query the hardware for the existence and variety of the
|
|
components easily.
|
|
|
|
Implementation
|
|
--------------
|
|
|
|
To take advantage of the pre-relocation device tree manipulation mechanism,
|
|
boards have to implement the function board_fix_fdt, which has the following
|
|
signature:
|
|
|
|
.. code-block:: c
|
|
|
|
int board_fix_fdt (void *rw_fdt_blob)
|
|
|
|
The passed-in void pointer is a writeable pointer to the device tree, which can
|
|
be used to manipulate the device tree using e.g. functions from
|
|
include/fdt_support.h. The return value should either be 0 in case of
|
|
successful execution of the device tree manipulation or something else for a
|
|
failure. Note that returning a non-null value from the function will
|
|
unrecoverably halt the boot process, as with any function from init_sequence_f
|
|
(in common/board_f.c).
|
|
|
|
Furthermore, the Kconfig option OF_BOARD_FIXUP has to be set for the function
|
|
to be called::
|
|
|
|
Device Tree Control
|
|
-> [*] Board-specific manipulation of Device Tree
|
|
|
|
+----------------------------------------------------------+
|
|
| WARNING: The actual manipulation of the device tree has |
|
|
| to be the _last_ set of operations in board_fix_fdt! |
|
|
| Since the pre-relocation driver model does not adapt to |
|
|
| changes made to the device tree either, its references |
|
|
| into the device tree will be invalid after manipulating |
|
|
| it, and unpredictable behavior might occur when |
|
|
| functions that rely on them are executed! |
|
|
+----------------------------------------------------------+
|
|
|
|
Hence, the recommended layout of the board_fixup_fdt call-back function is the
|
|
following:
|
|
|
|
.. code-block:: c
|
|
|
|
int board_fix_fdt(void *rw_fdt_blob)
|
|
{
|
|
/*
|
|
* Collect information about device's hardware and store
|
|
* them in e.g. local variables
|
|
*/
|
|
|
|
/* Do device tree manipulation using the values previously collected */
|
|
|
|
/* Return 0 on successful manipulation and non-zero otherwise */
|
|
}
|
|
|
|
If this convention is kept, both an "additive" approach, meaning that nodes for
|
|
detected components are added to the device tree, as well as a "subtractive"
|
|
approach, meaning that nodes for absent components are removed from the tree,
|
|
as well as a combination of both approaches should work.
|
|
|
|
Example
|
|
-------
|
|
|
|
The controlcenterdc board (board/gdsys/a38x/controlcenterdc.c) features a
|
|
board_fix_fdt function, in which six GPIO expanders (which might be present or
|
|
not, since they are on daughter boards) on a I2C bus are queried for, and
|
|
subsequently deactivated in the device tree if they are not present.
|
|
|
|
Note that the dm_i2c_simple_probe function does not use the device tree, hence
|
|
it is safe to call it after the tree has already been manipulated.
|
|
|
|
Work to be done
|
|
---------------
|
|
|
|
* The application of device tree overlay should be possible in board_fixup_fdt,
|
|
but has not been tested at this stage.
|