Re: Could the DTS experts look at this?

David Gibson wrote:

You don't.  If your agent takes a dtb, dtb layout and agent must
match.

So what I would like to see is a way for the agent to validate the dtb.  U-Boot 
could currently validate the SOC's compatible field.  However, if we add a 
special node that contains rules for modifying the rest of the tree, the only 
possible way to block older, incompatible U-Boots from accepting the tree is to 
bump the version number.  Since that is not the right thing to do, the best 
approach is to define a new node type that has conditional expression attached 
to it.  Then we can bump the version.

In fact, in one way of looking at it that's always what happens: the
dtb format is defined for passing hardware information from the
bootloader to the kernel; nothing else.  Passing a dtb *into* the
bootloader is just a bootloader implementation convenience, because
the possible variations on an output tree are small, so it's useful to
have a skeleton tree built-in.  But in order for the bootloader to
process those variations correctly, the skeleton *must* be in the
right format.  dtb input to a bootloader must match the bootloaders
expectations.  This has always been true, and will continue to be
true.

The problem with this approach is that you're replacing data with code, and that 
always makes things more difficult.

-- 
Timur Tabi
Linux kernel developer at Freescale