schedule_timeout sleeps too long after dividing CPU frequency

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Fri, May 15, 2015 at 11:29:34AM +0200, Mason wrote:
> On 14/05/2015 16:42, Russell King - ARM Linux wrote:
> 
> > If it's in periodic mode, the update should still be propagated to the
> > hardware, assuming the generic time keeping code doesn't produce an
> > error.
> > 
> > twd_update_frequency
> > `-clockevents_update_freq
> >   `-__clockevents_update_freq
> >     `-__clockevents_set_state(, CLOCK_EVT_STATE_PERIODIC)
> >       `-dev->set_mode (twd_set_mode)
> > 
> > That re-writes the TWD_TIMER_LOAD register based on twd_timer_rate,
> > which would have been updated by twd_update_frequency().
> > 
> > The question I posed earlier remains: is clockevents_update_freq()
> > failing?  We don't know, because we never check its return value.
> > 
> > Another thing to look at is whether we reach twd_set_mode().
> > 
> > Lastly, printing the values of the TWD_TIMER_LOAD and TWD_TIMER_COUNTER
> > after twd_set_mode() has written TWD_TIMER_LOAD might provide some
> > hints as to what's going on.
> 
> I'll have access to the board on Monday.
> 
> I'll add printk in strategic places, and report back ASAP.

That would be good.

> (I'm considering dropping TWD, and using platform timers.)

As you don't say which kernel version you're using, for all we know, you
might be using a version which omits some fixes in this area, such as
this one which you really must have if your timer is operating in
period mode:

fe79a9ba1196 clockevents: Adjust timer interval when frequency changes



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