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

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Date:	Mon, 04 Apr 2011 14:57:25 +0100
From:	Marc Zyngier <Marc.Zyngier@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	david@...g.hm, Arnd Bergmann <arnd@...db.de>,
	Nicolas Pitre <nico@...xnic.net>,
	Tony Lindgren <tony@...mide.com>,
	Catalin Marinas <catalin.marinas@....com>,
	lkml <linux-kernel@...r.kernel.org>,
	Detlef Vollmann <dv@...lmann.ch>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-omap@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	linux-arm-kernel@...ts.infradead.org,
	David Brown <davidb@...eaurora.org>
Subject: Re: [GIT PULL] omap changes for v2.6.39 merge window

On Mon, 2011-04-04 at 14:31 +0100, Russell King - ARM Linux wrote:
> On Mon, Apr 04, 2011 at 02:24:17PM +0100, Marc Zyngier wrote:
> > On Mon, 2011-04-04 at 12:21 +0100, Russell King - ARM Linux wrote:
> > > Whether its worth it or not is questionable - the above is more lines
> > > of code than many of the existing implementations, and we're not going
> > > to shrink the existing implementations by much (maybe one to three
> > > lines.)  The only thing we gain is the ability to select an implementation
> > > at runtime.
> > 
> > I believe this last point to be rather important if we plan to have this
> > mythical single kernel covering several architectures. It's also nice
> > for the A15 to be able to use some default sched_clock() implementation
> > as a fallback if the generic timers are not available for some reason.
> 
> If ARM are going to architect a set of timers into the hardware, let's
> make sure that all such hardware has them so we can dig ourselves out
> of this crappy mess that we find ourselves in today.

As far as I know, A15 always has a set of generic timers.

It may be that they are not available (frequency not programmed into the
CNTFREQ register), or that someone decided to use a better alternative
(for some particular interpretation of "better").

Overall, it seems like we need some degree of flexibility to have
several sched_clock() implementations within a single image, whether it
is to support multiple platforms, or to allow a single architecture to
pick the best alternative given a set of initial conditions.

	M.
-- 
Reality is an implementation detail.


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