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

From: Frank Rowand
Date: Wed Feb 18 2015 - 21:08:35 EST

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.

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.

The stated use of this proposal is to create dynamic DT blobs that can
describe many similar variants of a given system instead of creating
unique DT blobs for each different system.

I obviously have not thought through the architectural implications yet,
but just a quick example. One of the issues we have been trying to fix
is device tree validation. The not yet existent (except as a few proof
of concept attempts) validator would need to validate a device tree
for each dynamic variant. Probably not a big deal, but an example of
the ripple effects this conceptual change implies.

A second function that this patch is proposing is a method to enable
or disable devices via command line options. If I understand
correctly, this is meant to solve a problem with run time overlays
that require disabling a device previously enabled by the DT blob.
If so, it seems like it could easily be implemented in a simpler
generic way than in the board specific code in this patch series.

I share the concerns that Mark Rutland has expressed in his comments
about this series.

< snip >

I have read through the patches and will have comments on the code
later if this proposal is seen as a good idea.

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