linux-kernel - Re: [PATCH] sched: Support current clocksource handling in fallback sched

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Date:	Fri, 29 May 2009 04:34:58 +0900
From:	Paul Mundt <lethal@...ux-sh.org>
To:	Daniel Walker <dwalker@...o99.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Walleij <linus.ml.walleij@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Victor <linux@...im.org.za>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	John Stultz <johnstul@...ux.vnet.ibm.com>
Subject: Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock().

On Thu, May 28, 2009 at 12:04:23PM -0700, Daniel Walker wrote:
> On Fri, 2009-05-29 at 03:27 +0900, Paul Mundt wrote:
> 
> > Ah, ok, I suppose I could have explained that better. There are a couple
> > of different considerations really. The timer blocks are often in
> > different clock and power domains, meaning that only certain ones can be
> > used for wakeups. These tend not to be ideal for general use, and so we
> > only switch to them when we have to.
> > 
> > To make matters more convoluted, the availability of different timer
> > channels on different CPUs will depend on current pin state, and more
> > importantly, what sort of states we are able to achieve. It is not
> > uncommon to have some of the pins required by these channels locked down
> > by other drivers during regular operation, which we in turn need to
> > unload before the pin state can be reconfigured and the timer can
> > succeed, all which needs to happen before certain power state transitions
> > can take place. We implement dynamic pinmux for most of the SH CPUs
> > precisely to deal with these sorts of problems. In the case of some of
> > the microcontrollers that are timer heavy and low on pincount, it is not
> > uncommon to have upwards of 16 different functions per pin.
> 
> I'm still a little confused how kernel modules fit in here.. Are you
> saying a user would unload some certain driver which has a pin locked
> down and prevents the clocksource from working. Then the user would load
> the clocksource module which would now function, and that all would have
> to happen in order to enter a certain power state?
> 
Yes.
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