[U-Boot] Merging device trees at runtime for module-based systems

[Stephen Warren]

On 10/24/2012 03:47 AM, Daniel Mack wrote:
> Hi,
> 
> a project I'm involved in uses a module/baseboard combo, and components
> on either board are described in DT. I'm currently using separate dts
> files which build upon each other with include statements, which works
> fine for development.
> 
> In production though, we will certainly have running changes (and hence
> different versions) over the lifetime of the product for both the
> baseboard and the module, and the hardware has support for identifying
> the versions of both sides at runtime.
> 
> So let's say we have n versions of the baseboard and m versions of the
> module, we would much like to only prepare n + m files, instead of n * m
> by pre-compiling every possible combination (some of which may actually
> never occur 'in the wild').
> 
> So my question is: is it possible to do that kind of assembly of a
> number of files at runtime in U-Boot? I guess all it takes is merging a
> number of trees together, right? I browsed through the APIs but couldn't
> yet find an clear approach to that kind of problem. If not, what would
> it take to add that functionality? I can probably help with the
> implementation if someone tells me what would be the right way.

Yes, solving this would be very useful; it's a wide-spread problem.

Some thoughts though:

Simply overlaying two DTBs on top of each-other (in the same fashion
that dtc's /include/ statement would do at compile-time) might not be
fully general enough, although perhaps it would be sufficient for your
immediate needs.

For example, lets say that a GPIO is routed from a device on the main
board to a device on a daughter board, or even from one daughter board
into the main board and back out to a different daughter board. Now,
consider that the different board(s) that are the source of the GPIO
might use completely different SoCs or versions of the SoC, which might
require using a different GPIO specifier to represent the signal. That
means you need to change the .dtb file for the "client" of the GPIO
depending on the HW or .dtb that provides the GPIO. That's certainly not
a simple matter of merging multiple .dtb blobs together.

The same issue could easily apply to I2C or SPI buses, chip selects, etc.

One solution would be to explicitly represent a connector or
connection-point in DT, such that the connector can implement the naming
of all signals that pass through it, and provide a translation point for
hooking the two DT fragments together. This seems within the spirit of DT.

Another solution might be some form of variables/macros/code in the DTB
that can be used to parameterize other DTBs that get merged with it.
This is probably an enormous can of worms.

I wonder if similar yet more subtle issues might arise, such as some
motherboards requiring an active-low IRQ signal yet others requiring an
active-high IRQ signal, thus requiring a daughter-board to program its
IRQ source differently. Similarly, what about different drive strength
requirements for a signal source, depending on what board version
receives the signal?