Every system has at least one clock counter. As with any hardware device in a machine, this counter too is represented and managed by a structure. Hardware abstraction is provided by struct clocksource, defined in the include/linux/clocksource.h header file. This structure provides callbacks to access and handle power management on the counter through the read, enable, disable, suspend, and resume routines:

struct clocksource {
        u64 (*read)(struct clocksource *cs);
        u64 mask;
        u32 mult;
        u32 shift;
        u64 max_idle_ns;
        u32 maxadj;
#ifdef CONFIG_ARCH_CLOCKSOURCE_DATA
        struct arch_clocksource_data archdata;
#endif
        u64 max_cycles;
        const char *name;
        struct list_head list;
        int rating;
        int (*enable)(struct clocksource *cs);
        void (*disable)(struct clocksource *cs);
        unsigned long flags;
        void (*suspend)(struct clocksource *cs);
        void (*resume)(struct clocksource *cs);
        void (*mark_unstable)(struct clocksource *cs);
        void (*tick_stable)(struct clocksource *cs);

        /* private: */
#ifdef CONFIG_CLOCKSOURCE_WATCHDOG
        /* Watchdog related data, used by the framework */
        struct list_head wd_list;
        u64 cs_last;
        u64 wd_last;
#endif
        struct module *owner;
};

Members mult and shift are useful in obtaining elapsed time in relevant units.