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

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Message-ID: <20090528025258.GB1464@yookeroo.seuss>
Date:	Thu, 28 May 2009 12:52:58 +1000
From:	David Gibson <david@...son.dropbear.id.au>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jon Smirl <jonsmirl@...il.com>,
	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 08:21:16PM +0100, Russell King - ARM Linux wrote:
> On Wed, May 27, 2009 at 03:13:55PM -0400, Jon Smirl wrote:
> > On Wed, May 27, 2009 at 3:08 PM, Scott Wood <scottwood@...escale.com> 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.
> > >
> > > But to restate, having external glue to create platform devices from the
> > > device tree is fine if that's what you want to do.  We used to do that, but
> > > it was a pain compared to keeping everything in one place.  Your experience
> > > may differ.
> > 
> > Could 'struct platform_device' and 'struct of_platform_device" be
> > unified into a single structure? It's personal preference whether the
> > internal representation of the hardware is done via a device tree or
> > snippets of platform code, but do we need to have to different device
> > types?
> 
> That's a damned good question - platform devices have been around since
> the dawn of the device model, so the real question which needs to be
> asked is: what was the reason that of_platform_device created rather
> than unifying it with the already provided platform_device ?
> 
> BTW, I can find no such struct "of_platform_device" in include/linux.
> Is it specific to each and every OF implementation?

They should be unified.  I believe the problem was at the time
of_platform devices came into existence there was no arch-specific
field in the device structure that could be used to hold a reference
to the devtree node.

Since then, the fiddliness of doing the conversion has always just
outweighed the impetus to do so.  The of_platform bus model is
conceptually completely broken, but in practice only slightly broken
for all common cases.

I've been meaning at several times to replace the of_platform bus
infrastructure with a system to traverse the OF tree and construct
platform devices (or other un-probeable bus devices, e.g. i2c) based
on a table of constructor functions.  It's just always been edged out
by other work.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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