Linux Kernel Architecture

Chapter 15: Time Management

Clock Event Devices

Clock event devices are defined by the following data structure:

<clockchips.h>

struct clock_event_device {

const char *name;

unsigned int features;

unsigned long max_delta_ns;

unsigned long min_delta_ns;

unsigned long mult;

int shift;

int rating;

int irq;

cpumask_t cpumask;

int (*set_next_event)(unsigned long evt,

struct clock_event_device *);

void (*set_mode)(enum clock_event_mode mode,

struct clock_event_device *);

void (*event_handler)(struct clock_event_device *);

void (*broadcast)(cpumask_t mask);

struct list_head list;

enum clock_event_mode mode;

ktime_t next_event;

};

Recall that clock event devices allow for registering aneventthat is going to happen at a defined point

of time in the future. In comparison to a full-blown timer implementation, however, only a single event

can be stored. The key elements of everyclock_event_deviceareset_next_eventbecause it allows for

setting the time at which the event is going to take place, andevent_handler, which is called when the

event actually happens.

Besides, the elements ofclock_event_devicehave the following purpose:

❑ nameis a human-readable representation for the event device. It shows up in/proc/timerlist.

❑ max_delta_nsandmin_delta_nsspecify the maximum or minimum, respectively, difference

between the current time and the time for the next event. Clocks work with individual frequen-

cies at which device cycles occur, but the generic time subsystem expects a nanosecond value

when the event shall take place. The auxiliary functionclockevent_delta2nshelps to convert

one representation into the other.

Consider, for instance, that the current time is 20,min_delta_nsis 2, andmax_delta_nsis 40 (of

course, the exemplary values do not represent any situation possible in reality). Then the next

event can take place during the time interval [22, 60] where the boundaries are included.

❑ multandshiftare a multiplier and a divider, respectively, used to convert between clock cycles

and nanosecond values.

❑ The function pointed to byevent_handleris called by the hardware interface code (which usu-

ally is architecture-specific) to pass clock events on to the generic layers.