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

[Pantelis Antoniou]

Hi Frank,

> On Feb 19, 2015, at 04:08 , Frank Rowand <frowand.list@xxxxxxxxx> 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.
> 

Naming things is hard to do. Suggestions?

> 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.
> 

There is a fundamental shift going on about what hardware is. It is nowhere
as static as it used to be. It is time for the kernel to keep up.

> 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.
> 

Yes.

> 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.
> 

I don’t see what the big problem with the validator is. The ‘quirk’
are easily identified by the presence of the __overlay__ nodes and
the validator can apply each overlay and perform the validation check 
at each resultant tree.
 
> 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.
> 

Disabling a device is the most common case, but other options are desired
too. For instance changing OPPs by a command line option, etc.

> 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.
> 

OK

> -Frank

Regards

— Pantelis

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