Re: [PATCH 1/2] tick: broadcast: Deny per-cpu clockevents from being broadcast sources

["Sören Brinkmann" <soren.brinkmann@xilinx.com>]

Hi Thomas,

On Thu, Sep 12, 2013 at 10:30:15PM +0200, Thomas Gleixner wrote:
> On Thu, 12 Sep 2013, Soren Brinkmann wrote:
> > From: Stephen Boyd <sboyd@codeaurora.org>
> > 
> > On most ARM systems the per-cpu clockevents are truly per-cpu in
> > the sense that they can't be controlled on any other CPU besides
> > the CPU that they interrupt. If one of these clockevents were to
> > become a broadcast source we will run into a lot of trouble
> > because the broadcast source is enabled on the first CPU to go
> > into deep idle (if that CPU suffers from FEAT_C3_STOP) and that
> > could be a different CPU than what the clockevent is interrupting
> > (or even worse the CPU that the clockevent interrupts could be
> > offline).
> > 
> > Theoretically it's possible to support per-cpu clockevents as the
> > broadcast source but so far we haven't needed this and supporting
> > it is rather complicated. Let's just deny the possibility for now
> > until this becomes a reality (let's hope it never does!).
> 
> Well, we can't do it this way. There are globally accessible clock
> event devices which deliver only to cpu0. So the mask check might be
> causing failure here.
> 
> Just add a feature flag CLOCK_EVT_FEAT_PERCPU to the clock event
> device and check for it.

I gave it a shot. Is this what you imagine:
diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
index b66c1f3..c639b1a 100644
--- a/drivers/clocksource/arm_global_timer.c
+++ b/drivers/clocksource/arm_global_timer.c
@@ -169,7 +169,8 @@ static int gt_clockevents_init(struct clock_event_device *clk)
        int cpu = smp_processor_id();
 
        clk->name = "arm_global_timer";
-       clk->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT;
+       clk->features = CLOCK_EVT_FEAT_PERIODIC | CLOCK_EVT_FEAT_ONESHOT |
+               CLOCK_EVT_FEAT_PERCPU;
        clk->set_mode = gt_clockevent_set_mode;
        clk->set_next_event = gt_clockevent_set_next_event;
        clk->cpumask = cpumask_of(cpu);
diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
index 0857922..493aa02 100644
--- a/include/linux/clockchips.h
+++ b/include/linux/clockchips.h
@@ -60,6 +60,7 @@ enum clock_event_mode {
  * Core shall set the interrupt affinity dynamically in broadcast mode
  */
 #define CLOCK_EVT_FEAT_DYNIRQ          0x000020
+#define CLOCK_EVT_FEAT_PERCPU          0x000040
 
 /**
  * struct clock_event_device - clock event device descriptor
diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index d3539e5..de4c5d8 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -70,16 +70,14 @@ static bool tick_check_broadcast_device(struct clock_event_device *curdev,
                                        struct clock_event_device *newdev)
 {
        if ((newdev->features & CLOCK_EVT_FEAT_DUMMY) ||
-           (newdev->features & CLOCK_EVT_FEAT_C3STOP))
+           (newdev->features & CLOCK_EVT_FEAT_C3STOP) ||
+           (newdev->features & CLOCK_EVT_FEAT_PERCPU))
                return false;
 
        if (tick_broadcast_device.mode == TICKDEV_MODE_ONESHOT &&
            !(newdev->features & CLOCK_EVT_FEAT_ONESHOT))
                return false;
 
-       if (cpumask_equal(newdev->cpumask, cpumask_of(smp_processor_id())))
-               return false;
-
        return !curdev || newdev->rating > curdev->rating;
 }

If this is the way to go, I can prepare this in a v2.

	Thanks,
	Sören