linux-kernel - Re: [GIT PULL] omap changes for v2.6.39 merge window

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Date:	Mon, 4 Apr 2011 02:14:16 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Detlef Vollmann <dv@...lmann.ch>, david@...g.hm,
	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	Nicolas Pitre <nico@...xnic.net>,
	Tony Lindgren <tony@...mide.com>,
	Catalin Marinas <catalin.marinas@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	linux-omap@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Brown <davidb@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [GIT PULL] omap changes for v2.6.39 merge window

On Monday 04 April 2011, Benjamin Herrenschmidt wrote:
> On Fri, 2011-04-01 at 16:28 +0200, Detlef Vollmann wrote:
> > >   * No board files
> > Where do you put code that needs to run very early (e.g. pinging the
> > watchdog)? 
> 
> Even on powerpc I keep board files :-)
> 
> The main thing is:
> 
> - The generic -> board linkage must not be hard (ie, no
> platform_restart, but a board_ops.restart() etc....)
> 
> - An average board file is a few hundreds line long, that's it, mostly
> it hooks up to generically provided functions, tho it gets the choice of
> _which_ ones to hookup.

I believe a machine_type is more general than a board file, i.e. what
gets described as a machine in powerpc would often currently correspond
to multiple board files, if I am not mistaken.

The fact that we have a more diverse set of hardware on ARM, and that
it's growing quicker than powerpc also means that we should try harder
to reduce duplication than is necessary there.

> - It can still quirk/fixup a thing or two if needed, I thinkt it's
> useful to keep that around, as long as such "quirks" remain small and
> few. At the end of the day, if dealing with one board special case gives
> you the choice between changing a ton of infrastructure/core to
> introduce a new abstraction to deal with -that- special case vs. having
> a one liner fixup in the platform code, the later is the most sensible
> option. The hard part of course is to have sensible maintainers to make
> sure this doesn't grow back to the old mess.

I guess quirks are fine, as long as it's not required to have a them
for each board. We can have a function that gets called for any matching
"compatible" property of the root node, but I think the default should
be not to need it eventually.

This is one area where I think I can illustrate how a gradual change
from the status quo differs from a parallel new platform implementation:
To gradually change one board file, you would convert the existing
machine description to match the compatible property of the device
tree root node and possibly at a later stage remove that code again
once it's possible to work without it.
When starting out with a fresh implementation, we first need to
change all device drivers that are used on the board to work
without a machine description, but then would not have to change
any code twice, and the work for a similar board is almost done.

	Arnd
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