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

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Date:	Thu, 28 May 2009 01:48:01 +0200
From:	Robert Schwebel <r.schwebel@...gutronix.de>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Robert Schwebel <r.schwebel@...gutronix.de>,
	Timur Tabi <timur@...escale.com>,
	Janboe Ye <yuan-bo.ye@...orola.com>,
	devicetree-discuss <devicetree-discuss@...abs.org>,
	linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk, rmk@....linux.org.uk
Subject: Re: [RFC] [PATCH] Device Tree on ARM platform

On Wed, May 27, 2009 at 02:35:11PM -0600, Grant Likely wrote:
> > Unfortunately, it is an incomplete data structure regarding to what
> > the kernel needs.
>
> I don't follow your argument. It's a data structure that uniquely
> describes your hardware in a way which encourages the most code reuse
> possible; but is still independent of kernel internal implementation.
> ie. a FDT blob should be usable not just by Linux, but also by BSD or
> any of the other OS options. It is not an attempt to eliminate
> platform specific code; just to reduce it as much as possible. Weird,
> harry, non-standard stuff probably still needs board specific code to
> handle.

The oftree by design wants to be a complete hardware description. As you
mention above, there are cases where you *nevertheless* need ad-hoc
information about things *not* encoded into the device tree.

This renders the whole concept ad absurdum. You need a machine number
again - and if you need that: why not stay with the ARM model, define
everything with platform data and avoid the whole thing?

Regarding the multi OS argument: I consider it impossible that people
agree on all micro details of a hardware decision. Will it really be
possible to get a common agreement about let's say Russell's SMSC
example between all these people? Remember: if we forget (or don't agree
on) one single aspect which a driver developer *needs*, he will have to
work around it -> machine number land.

My impression is that oftree only works in a perfect world. But we don't
have one, so the fundamental design decision is broken.

rsc
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