linux-kernel - Re: [PATCH 04/11] ST SPEAr: Added basic header files for SPEAr platform

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Date:	Thu, 11 Mar 2010 00:42:04 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Paul Mundt <lethal@...ux-sh.org>
cc:	Tony Lindgren <tony@...mide.com>,
	Viresh KUMAR <viresh.kumar@...com>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	armando.visconti@...com, amit.goel@...com, shiraz.hashim@...com,
	vipin.kumar@...com, rajeev-dlh.kumar@...com, deepak.sikri@...com,
	ashish.priyadarshi@...com
Subject: Re: [PATCH 04/11] ST SPEAr: Added basic header files for SPEAr
 platform

On Thu, 11 Mar 2010, Paul Mundt wrote:
> On Wed, Mar 10, 2010 at 02:16:53PM -0800, Tony Lindgren wrote:
> > * Thomas Gleixner <tglx@...utronix.de> [100310 08:38]:
> > > On Wed, 10 Mar 2010, Paul Mundt wrote:
> > > > This is hardly a unique situation for your platform, this is true for
> > > > most platforms. There's no reason why clockevents couldn't just be
> > > > extended and drivers could then just grab unused clockevents and pin them
> > > > accordingly. Most of the infrastructure is already in place for something
> > > > like that, without really having to do anything special.
> > > > 
> > > > Having said that, most drivers have pretty lame reasons for trying to get
> > > > at fixed timer channels, and most of the time they can easily get by with
> > > > an hrtimer instead. There's also the issue that you're effectively
> > > > bypassing nohz by having some timer channel off on the side doing who
> > > > knows what. You would need a pretty compelling reason for why you are
> > > > sidestepping all of the existing infrastructure anyways.
> > > 
> > > Right, and that's exactly the reason why we did not add the few
> > > missing bits to make clock events directly usable from drivers.
> > 
> > Yeah still no direct users so far for the 12 hardware timers on
> > omaps.. I guess the only use I could see is bit banging data
> > over a few GPIO lines using a FIQ handler.
> > 
> > Another thing to consider is that most likely all hardware timers
> > are not able to wake up the system from idle, which would easily
> > cause some mysterious errors for drivers.
> > 
> Even if most drivers shouldn't be touching clockevents directly, there
> are still legitimate cases. In SMP configurations where a single timer
> block is shared across multiple CPUs it would be easier to have the boot
> CPU register all of the timer channels under clockevents and have the
> secondaries grab one at random for setting up their local timers. (Even
> if they're not truly "local" timers, it's still a better situation than
> broadcast IPIs). clockevents would need some minor extension, including
> dealing with unregistration for the CPU hotplug case, but it's a pretty
> good fit for the problem otherwise.

No objections against that, but that's not a use case for drivers and
limited to (arch) core code functionality.

Thanks,

	tglx
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