[RFC] Device Tree Overlays Proposal (Was Re: capebus moving omap_devices to mach-omap2)

[Grant Likely]

On Wed, Nov 7, 2012 at 12:54 AM, Mitch Bradley <wmb at firmworks.com> wrote:
> On 11/6/2012 12:37 PM, Stephen Warren wrote:
>> This proposal is very oriented at an overlay-based approach. I'm not
>> totally convinced that a pure overlay approach (as in how dtc does
>> overlayed DT nodes) will be flexible enough, but would love to be
>> persuaded. Again see below.
>
>
> An overlay approach will not be powerful enough to solve the sorts of
> problems that occur when a product goes into full production, becomes a
> family, and starts to evolve.  Issues like second-source parts that
> aren't quite compatible and need to be detected and reported,
> board-stuff options for different customer profiles, speed grades of
> parts that aren't properly probeable but instead need to be identified
> by some subterfuge, the list of tedious issues goes on and on.
>
> It's nice to pretend that the world fits into a few coherent simple
> use cases, but 30 years of experience shipping computer product families
> proves otherwise.  You need a programming language to solve the full
> set of problems - which I why the device tree is designed so it can
> be generated and modified by a program.

I'm not going to argue with that. There is nothing saying that the
overlay wouldn't be generated or applied by a script. However, I do
strongly think that the data model needs to be independent of any tool
or execution environment used to manipulate it. I certainly am not
interested in encoding scripts or bytecode into the tree - the
opposite of the approach used by ACPI which must execute AML to even
retrieve the device tree. I like the overlay approach because it is
conceptually straightforward and provides a working model of how
changes could be made from userspace while still being usable by
firmware.

An alternate approach from userspace would be to use a live virtual
filesystem to manipulate the device tree, though that approach only
applies to Linux userspace. Firmware would still need its own
approach.

g.