Hi Thomas, On Thu, Nov 28, 2013 at 03:18:50PM +0100, Thomas Gleixner wrote: > On Thu, 28 Nov 2013, Daniel Lezcano wrote: > > > On 11/27/2013 02:04 AM, Soren Brinkmann wrote: > > > To ensure that the timer interrupt is properly enabled/disabled across > > > the whole CPU cluster use enable/disable_irq() instead of > > > local_irq_disable(). > > > > > > Signed-off-by: Soren Brinkmann <soren.brinkmann@xxxxxxxxxx> > > > --- > > > drivers/clocksource/cadence_ttc_timer.c | 6 ++---- > > > 1 file changed, 2 insertions(+), 4 deletions(-) > > > > > > diff --git a/drivers/clocksource/cadence_ttc_timer.c > > > b/drivers/clocksource/cadence_ttc_timer.c > > > index a92350b55d32..246d018d1e63 100644 > > > --- a/drivers/clocksource/cadence_ttc_timer.c > > > +++ b/drivers/clocksource/cadence_ttc_timer.c > > > @@ -322,18 +322,16 @@ static int ttc_rate_change_clockevent_cb(struct > > > notifier_block *nb, > > > switch (event) { > > > case POST_RATE_CHANGE: > > > { > > > - unsigned long flags; > > > - > > > /* > > > * clockevents_update_freq should be called with IRQ disabled > > > on > > > * the CPU the timer provides events for. The timer we use is > > > * common to both CPUs, not sure if we need to run on both > > > * cores. > > Can we adjust that bogus comment as well, please? I can remove that comment. I'll see if including it in one of the existing patches or adding another patch for it makes sense. > > > > */ > > > - local_irq_save(flags); > > > + disable_irq(ttcce->ce.irq); > > > clockevents_update_freq(&ttcce->ce, > > > ndata->new_rate / PRESCALE); > > > - local_irq_restore(flags); > > > + enable_irq(ttcce->ce.irq); > > > > > > /* update cached frequency */ > > > ttc->freq = ndata->new_rate; > > > > > > > I am worried about the 'disable_irq' function calling 'synchronize_irq'. Isn't > > possible to deadlock with the ondemand cpufreq governor ? I added Viresh in > > Cc, he knows better than me the code path. > > disable_irq() will only deadlock if called from the interrupt handler > of the device interrupt itself or when the calling code is holding > locks which prevent the function to proceed. > > But what's more important is, that the patch violates the calling > convention of clockevents_update_freq(). It must be called with > interrupts disabled. > > Now the problem with this device is that it is not a per cpu > device. It's a global device, so this update can conflict with a > parallel access on the other CPU. Now the disable_irq() only prevents > that the other CPU can handle a device interrupt from that timer. But > it does not prevent any parallel access from e.g. the idle code path > which will try to reprogram it. Does that mean interrupts need to be disabled globally? Also, does the cpuidle path depend on interrupts or can it interfere no matter what? > > Soren, is that timer used as the broadcast device ? Yes, this is the only broadcast capable timer on Zynq, AFAIK. Other than the TTC we only have the arm_global_timer and smp_twd timers, which both are per_cpu devices and thus not broadcast capable. Sören -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html