Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock().

[Paul Mundt <lethal@linux-sh.org>]

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.