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

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Message-ID: <20150220081658.GM32600@odux.rfo.atmel.com>
Date:	Fri, 20 Feb 2015 09:16:58 +0100
From:	Ludovic Desroches <ludovic.desroches@...el.com>
To:	Rob Herring <robherring2@...il.com>
CC:	Frank Rowand <frowand.list@...il.com>,
	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 Thu, Feb 19, 2015 at 12:01:14PM -0600, Rob Herring wrote:
> 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. :)

I don't think it is the goal. It should allow to deal with board
revisions, for example some components put on an other i2c bus to
isolate an i2c device corrupting the bus in some cases. It should
concern plugable h/w which is only needed at boot time for instance a
display panel if we want to show a splash screen.

Ludovic
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