Re: [PATCH 2/4] of: DT quirks infrastructure

[Rob Herring <robherring2@gmail.com>]

On Wed, Feb 18, 2015 at 8:08 PM, Frank Rowand <frowand.list@gmail.com> wrote:
> On 2/18/2015 6:59 AM, Pantelis Antoniou wrote:
>> Implement a method of applying DT quirks early in the boot sequence.
>>
>> A DT quirk is a subtree of the boot DT that can be applied to
>> a target in the base DT resulting in a modification of the live
>> tree. The format of the quirk nodes is that of a device tree overlay.
>
> The use of the word "quirk" is a different mental model for me than what
> this patch series appears to be addressing.  I would suggest totally
> removing the word "quirk" from this proposal to avoid confusing the
> mental models of future generations of kernel folks.

This comes from me as quirks are a different usecase I had in mind,
but one that could use a similar mechanism. Although, in the case of
quirks, I would expect them to be overlays built into the kernel. It
would be more a way to update old dtbs.

> What this patch series seems to be proposing is a method to apply DT
> overlays as soon as unflatten_device_tree() completes.  In other words,
> making the device tree a dynamic object, that is partially defined by
> the kernel during boot.  Well, to be fair, the kernel chooses among
> several possible alternatives encoded in the DT blob.  So the device
> tree is no longer a static object that describes the hardware of the
> system.  It may not sound like a big deal, but it seems to me to be
> a fundamental shift in what the device tree blob is.  Something that
> should be thought about carefully and not just applied as a patch to
> solve a point problem.

I agree. I would not want to see every board for an SOC become an
overlay for example. I think it has to be limited to truly plugable
h/w (e.g. capes) or minor changes. We just have to define what is
minor. :)

Rob