--- /dev/null
+Pre-relocation device tree manipulation
+=======================================
+
+Contents:
+
+1. Purpose
+2. Implementation
+3. Example
+4. Work to be done
+
+1. 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.
+
+2. 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:
+
+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:
+
+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.
+
+3. 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.
+
+4. 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.
+
+2017-01-06, Mario Six <mario.six@gdsys.cc>
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