Re: schedule_timeout sleeps too long after dividing CPU frequency

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, May 14, 2015 at 07:29:38PM +0530, Viresh Kumar wrote:
> On 14 May 2015 at 18:36, Mason <slash.tmp@free.fr> wrote:
> > When I execute "echo 18500 > scaling_max_freq"
> > the system is supposed to change the CPU frequency to 18.5 MHz
> > (I might have a bug lurking there) and PERIPHCLK is 1/2 of that,
> > i.e 9.25 MHz.
> 
> So at least we are on the right path. But it looks to me that this
> call is not getting propagated well.
> 
> >From the attachment you gave initially, the event handler for
> twd-timers is: tick_handle_periodic(). i.e. you are running in
> periodic mode and not onshot...
> 
> why ?

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.

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