Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift

[Anthony Liguori]

On 02/07/2011 08:57 AM, Jan Kiszka wrote:
> There should rather be a special vmstate struct for PeriodicTimer, just
> like we already have for normal timers.
>    

Agreed.

> > It's convenient because then if we lose ticks in the PeriodicTimer
> > layer, the devices have instance access to that info.  When you do a
> > read() from timerfd, it returns the number of coalesced events.  That's
> > the interface I had in my mind.
> >
> > We could just add a getter for PeriodicTimer and it would serve the same
> > purpose.
> >      
> I'm still not sure what the device model is supposed to do with that
> information. I think at could remain private to the PeriodicTimer
> implementation (unless we want to dump some stats or such).
>    

Yeah, I've been thinking about it too and I think I agree with you.

So here's the new proposal:

typedef struct PeriodicTimer PeriodicTimer;

/**
 * @accumulated_ticks:  the number of unacknowledged ticks in total 
since the creation of the timer
 **/
typedef void (PeriodicTimerFunc)(void *opaque);

PeriodicTimer *periodic_timer_new(PeriodicTimerFunc *cb, void *opaque);

void periodic_timer_mod(PeriodicTimer *timer, int64_t interval, TimeUnit 
unit);

/**
 * @policy: the drift catch-up policy
 *                DRIFT_COMP_FAST, deliver next tick as soon as any 
tick is acknowledged if accumulated_ticks > 1
 *                DRIFT_COMP_NONE, do not change interval regardless of 
accumulated ticks
 *                DRIFT_COMP_GRADUAL, shorten interval by half until 
accumulated_ticks <= 1
 */
void periodic_timer_set_policy(PeriodicTimer *timer, 
DriftCompensationPolicy policy);

/**
 * @ticks: number of ticks to acknowledge that are currently outstanding.
 **/
void periodic_timer_ack(PeriodicTimer *timer, int ticks);

int periodic_timer_get_accumulated_ticks(PeriodicTimer *timer);

Regards,

Anthony Liguori

> Jan
>
>