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

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Message-ID: <CAL_JsqKT3+URcad6X=i+73NLRnVstbpvFJxxT=ru7KJmp_XuVQ@mail.gmail.com>
Date:	Thu, 19 Feb 2015 12:01:14 -0600
From:	Rob Herring <robherring2@...il.com>
To:	Frank Rowand <frowand.list@...il.com>
Cc:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Tony Lindgren <tony@...mide.com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ludovic Desroches <ludovic.desroches@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Pantelis Antoniou <panto@...oniou-consulting.com>,
	Matt Porter <matt.porter@...aro.org>,
	Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH 2/4] of: DT quirks infrastructure

On Wed, Feb 18, 2015 at 8:08 PM, Frank Rowand <frowand.list@...il.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
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