linux-kernel - Re: [RFC] [PATCH] Device Tree on ARM platform

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Date:	Thu, 28 May 2009 14:43:07 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Russell King <rmk+lkml@....linux.org.uk>,
	devicetree-discuss <devicetree-discuss@...abs.org>,
	linux-kernel@...r.kernel.org, Timur Tabi <timur@...escale.com>,
	Scott Wood <scottwood@...escale.com>,
	Janboe Ye <yuan-bo.ye@...orola.com>,
	linux-arm-kernel@...ts.arm.linux.org.uk
Subject: Re: [RFC] [PATCH] Device Tree on ARM platform

On Wed, May 27, 2009 at 03:05:58PM -0600, Grant Likely wrote:
> On Wed, May 27, 2009 at 2:52 PM, Mark Brown
> <broonie@...nsource.wolfsonmicro.com> wrote:
> > On Wed, May 27, 2009 at 08:29:10PM +0100, Russell King wrote:
> >> On Wed, May 27, 2009 at 02:08:42PM -0500, Scott Wood wrote:
> >
> >> > I'm not talking about platform specific code, I'm talking about code to
> >> > retrieve information about a device from the device tree.  There would
> >> > not be separate instances of this for "platforms X, Y and Z", just one
> >> > of_platform binding in each driver.  It's no different than having a
> >> > platform bus binding, except in the data structures used.
> >
> >> I really don't see what OF buys us then, apart from additional dependencies
> >> that have to be correct for the kernel to work.  I can only see disadvantages
> >> if all OF is, is a way to pass some file to the kernel to (effectively) tell
> >> it which drivers to use.
> >
> > The main selling points of the device tree AFAICT are that some
> > platforms have to use it it anyway due to the native OS and firmware for
> > the platform use it, the possibility of using the same device tree with
> > more than one OS (modulo unrepresentable holes) and the fact that some
> > people find it more convenient to use than straight data tables
> > (personally I find the two approaches to be much of a muchness there).
> > Perhaps I'm missing something, though?
> 
> Here are some that I've find useful:
> 
> There is the advantage that it decouples the machine description from
> the kernel code, which in turn seems to encourage code reuse.  There
> has been a significant decrease in the amount of platform specific
> code in powerpc since the switch to FDT booting.
> 
> There is the advantage of easy multiplatform support.  I regularly
> build a single kernel image which boots on all my MPC5200 boards, and
> on my MPC83xx boards.

That is not necessarily an advantage of a device tree. On ARM you can
also build a kernel which runs on 20+ PXA platforms at the same time.
(And I'm sure it can be done to even support say i.MX and PXA at the
same time, but this is another story)

Sascha

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