Re: [PATCH 1/3] clockevents: Introduce CLOCK_EVT_STATE_ONESHOT_STOPPED state

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

On 03/27/2015 10:44 PM, Viresh Kumar wrote:
> When no timers/hrtimers are pending, the expiry time is set to a special value:
> 'KTIME_MAX'. This normally happens with NO_HZ_{IDLE|FULL} in both LOWRES/HIGHRES
> modes.
> 
> When 'expiry == KTIME_MAX', we either cancel the 'tick-sched' hrtimer
> (NOHZ_MODE_HIGHRES) or skip reprogramming clockevent device (NOHZ_MODE_LOWRES).
> But, the clockevent device is already reprogrammed from the tick-handler for
> next tick.
> 
> As the clock event device is programmed in ONESHOT mode it will at least fire
> one more time (unnecessarily). Timers on few implementations (like
> arm_arch_timer, etc.) only support PERIODIC mode and their drivers emulate
> ONESHOT over that. Which means that on these platforms we will get spurious
> interrupts periodically (at last programmed interval rate, normally tick rate).
> 
> In order to avoid spurious interrupts, the clockevent device should be stopped
> or its interrupts should be masked.
> 
> A simple (yet hacky) solution to get this fixed could be: update
> hrtimer_force_reprogram() to always reprogram clockevent device and update
> clockevent drivers to STOP generating events (or delay it to max time) when
> 'expires' is set to KTIME_MAX. But the drawback here is that every clockevent
> driver has to be hacked for this particular case and its very easy for new ones
> to miss this.
> 
> However, Thomas suggested to add an optional state ONESHOT_STOPPED to solve this
> problem: lkml.org/lkml/2014/5/9/508.
> 
> This patch adds support for ONESHOT_STOPPED state in clockevents core. It will
> only be available to drivers that implement the state-specific callbacks instead
> of the legacy ->set_mode() callback.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
>  include/linux/clockchips.h |  7 ++++++-
>  kernel/time/clockevents.c  | 14 +++++++++++++-
>  kernel/time/timer_list.c   |  6 ++++++
>  3 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/clockchips.h b/include/linux/clockchips.h
> index e20232c3320a..33ad247f8662 100644
> --- a/include/linux/clockchips.h
> +++ b/include/linux/clockchips.h
> @@ -51,12 +51,15 @@ enum clock_event_mode {
>   *		reached from DETACHED or SHUTDOWN.
>   * ONESHOT:	Device is programmed to generate event only once. Can be reached
>   *		from DETACHED or SHUTDOWN.
> + * ONESHOT_STOPPED: Device was programmed in ONESHOT mode and is temporarily
> + *		    stopped.
>   */
>  enum clock_event_state {
>  	CLOCK_EVT_STATE_DETACHED = 0,
>  	CLOCK_EVT_STATE_SHUTDOWN,
>  	CLOCK_EVT_STATE_PERIODIC,
>  	CLOCK_EVT_STATE_ONESHOT,
> +	CLOCK_EVT_STATE_ONESHOT_STOPPED,
>  };
> 
>  /*
> @@ -103,6 +106,7 @@ enum clock_event_state {
>   * @set_mode:		legacy set mode function, only for modes <= CLOCK_EVT_MODE_RESUME.
>   * @set_state_periodic:	switch state to periodic, if !set_mode
>   * @set_state_oneshot:	switch state to oneshot, if !set_mode
> + * @set_state_oneshot_stopped: switch state to oneshot_stopped, if !set_mode
>   * @set_state_shutdown:	switch state to shutdown, if !set_mode
>   * @tick_resume:	resume clkevt device, if !set_mode
>   * @broadcast:		function to broadcast events
> @@ -136,12 +140,13 @@ struct clock_event_device {
>  	 * State transition callback(s): Only one of the two groups should be
>  	 * defined:
>  	 * - set_mode(), only for modes <= CLOCK_EVT_MODE_RESUME.
> -	 * - set_state_{shutdown|periodic|oneshot}(), tick_resume().
> +	 * - set_state_{shutdown|periodic|oneshot|oneshot_stopped}(), tick_resume().
>  	 */
>  	void			(*set_mode)(enum clock_event_mode mode,
>  					    struct clock_event_device *);
>  	int			(*set_state_periodic)(struct clock_event_device *);
>  	int			(*set_state_oneshot)(struct clock_event_device *);
> +	int			(*set_state_oneshot_stopped)(struct clock_event_device *);
>  	int			(*set_state_shutdown)(struct clock_event_device *);
>  	int			(*tick_resume)(struct clock_event_device *);
> 
> diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
> index 73689df1e4b8..04f6c3433f8e 100644
> --- a/kernel/time/clockevents.c
> +++ b/kernel/time/clockevents.c
> @@ -138,6 +138,17 @@ static int __clockevents_set_state(struct clock_event_device *dev,
>  			return -ENOSYS;
>  		return dev->set_state_oneshot(dev);
> 
> +	case CLOCK_EVT_STATE_ONESHOT_STOPPED:
> +		/* Core internal bug */
> +		if (WARN_ONCE(dev->state != CLOCK_EVT_STATE_ONESHOT,
> +			      "Current state: %d\n", dev->state))
> +			return -EINVAL;
> +
> +		if (dev->set_state_oneshot_stopped)
> +			return dev->set_state_oneshot_stopped(dev);
> +		else
> +			return -ENOSYS;
> +
>  	default:
>  		return -ENOSYS;
>  	}
> @@ -449,7 +460,8 @@ static int clockevents_sanity_check(struct clock_event_device *dev)
>  	if (dev->set_mode) {
>  		/* We shouldn't be supporting new modes now */
>  		WARN_ON(dev->set_state_periodic || dev->set_state_oneshot ||
> -			dev->set_state_shutdown || dev->tick_resume);
> +			dev->set_state_shutdown || dev->tick_resume ||
> +			dev->set_state_oneshot_stopped);
> 
>  		BUG_ON(dev->mode != CLOCK_EVT_MODE_UNUSED);
>  		return 0;
> diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
> index 05aa5590106a..98d77969ba85 100644
> --- a/kernel/time/timer_list.c
> +++ b/kernel/time/timer_list.c
> @@ -251,6 +251,12 @@ print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
>  			SEQ_printf(m, "\n");
>  		}
> 
> +		if (dev->set_state_oneshot_stopped) {
> +			SEQ_printf(m, " oneshot stopped: ");
> +			print_name_offset(m, dev->set_state_oneshot_stopped);
> +			SEQ_printf(m, "\n");
> +		}
> +
>  		if (dev->tick_resume) {
>  			SEQ_printf(m, " resume:   ");
>  			print_name_offset(m, dev->tick_resume);
> 
With all the precursor patches gone in, this series looks good to me.

Reviewed-by: Preeti U. Murthy <preeti@linux.vnet.ibm.com>