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

[Pantelis Antoniou]

Hi Grant,

On Nov 9, 2012, at 7:02 PM, Grant Likely wrote:

> 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.

I completely agree here. 

I certainly wouldn't want to introduce any kind of bytecode or a
programming model for manipulating DT. I know OF has a full blown
forth interpreter, but that's not what modern DT should be (IMHO).

Having DT provide such a programming model will preclude a number
of users of accessing it, and on top of that, complexity will 
increase considerably. 

When faced with a new problem, vendors will just code a workaround, 
never bothering fixing it properly.

In a nutshell, let's not turn DT into ACPI, please.

Regards

-- Pantelis