|Drivers||Manage hardware to generate requested time events.|
|sys/kern/kern_clocksource.c uses event timers to supply kernel with hardclock, statclock and profclock time events.|
|Glue code||sys/sys/timeet.h, sys/kern/kern_et.c provide APIs for event timer drivers and consumers.|
Driver API is built around eventtimer structure. To register its functionality driver allocates that structure and calls et_register. Driver should fill following fields there:
et_name Unique name of the event timer for management purposes. et_flags Set of flags, describing timer capabilities:
ET_FLAGS_PERIODIC Periodic mode supported. ET_FLAGS_ONESHOT One-shot mode supported. ET_FLAGS_PERCPU Timer is per-CPU. ET_FLAGS_C3STOP Timer may stop in CPU sleep state. ET_FLAGS_POW2DIV Timer supports only 2^n divisors.
et_quality Abstract value to certify whether this timecounter is better than the others. Higher value means better. et_frequency Timer oscillators base frequency, if applicable and known. Used by consumers to predict set of possible frequencies that could be obtained by dividing it. Should be zero if not applicable or unknown. et_min_period, et_max_period Minimal and maximal reliably programmable time periods. et_start Drivers timer start function pointer. et_stop Drivers timer stop function pointer. et_priv Drivers private data storage.
After the event timer functionality is registered, it is controlled via et_start and et_stop methods. et_start method is called to start the specified event timer. The last two arguments are used to specify time when events should be generated. first argument specifies time period before the first event generated. In periodic mode NULL value specifies that first period is equal to the period argument value. period argument specifies the time period between following events for the periodic mode. The NULL value there specifies the one-shot mode. At least one of these two arguments should be not NULL. When event time arrive, driver should call et_event_cb callback function, passing et_arg as the second argument. et_stop method is called to stop the specified event timer. For the per-CPU event timers et_start and et_stop methods control timers associated with the current CPU.
Driver may deregister its functionality by calling et_deregister.
If the frequency of the clock hardware can change while it is running (for example, during power-saving modes), the driver must call et_change_frequency on each change. If the given event timer is the active timer, et_change_frequency stops the timer on all CPUs, updates et->frequency, then restarts the timer on all CPUs so that all current events are rescheduled using the new frequency. If the given timer is not currently active, et_change_frequency simply updates et->frequency.
et_find allows consumer to find available event timer, optionally matching specific name and/or capability flags. Consumer may read returned eventtimer structure, but should not modify it. When wanted event timer is found, et_init should be called for it, submitting event and optionally deregister callbacks functions, and the opaque argument arg. That argument will be passed as argument to the callbacks. Event callback function will be called on scheduled time events. It is called from the hardware interrupt context, so no sleep is permitted there. Deregister callback function may be called to report consumer that the event timer functionality is no longer available. On this call, consumer should stop using event timer before the return.
After the timer is found and initialized, it can be controlled via et_start and et_stop. The arguments are the same as described in driver API. Per-CPU event timers can be controlled only from specific CPUs.
et_ban allows consumer to mark event timer as broken via clearing both one-shot and periodic capability flags, if it was somehow detected. et_free is the opposite to et_init. It releases the event timer for other consumers use.
ET_LOCK and ET_UNLOCK macros should be used to manage mutex(9) lock around et_find, et_init and et_free calls to serialize access to the list of the registered event timers and the pointers returned by et_find. et_start and et_stop calls should be serialized in consumers internal way to avoid concurrent timer hardware access.
.An Alexander Motin Aq mav@FreeBSD.org