| ktime accessors |
| =============== |
| |
| Device drivers can read the current time using ktime_get() and the many |
| related functions declared in linux/timekeeping.h. As a rule of thumb, |
| using an accessor with a shorter name is preferred over one with a longer |
| name if both are equally fit for a particular use case. |
| |
| Basic ktime_t based interfaces |
| ------------------------------ |
| |
| The recommended simplest form returns an opaque ktime_t, with variants |
| that return time for different clock references: |
| |
| |
| .. c:function:: ktime_t ktime_get( void ) |
| |
| CLOCK_MONOTONIC |
| |
| Useful for reliable timestamps and measuring short time intervals |
| accurately. Starts at system boot time but stops during suspend. |
| |
| .. c:function:: ktime_t ktime_get_boottime( void ) |
| |
| CLOCK_BOOTTIME |
| |
| Like ktime_get(), but does not stop when suspended. This can be |
| used e.g. for key expiration times that need to be synchronized |
| with other machines across a suspend operation. |
| |
| .. c:function:: ktime_t ktime_get_real( void ) |
| |
| CLOCK_REALTIME |
| |
| Returns the time in relative to the UNIX epoch starting in 1970 |
| using the Coordinated Universal Time (UTC), same as gettimeofday() |
| user space. This is used for all timestamps that need to |
| persist across a reboot, like inode times, but should be avoided |
| for internal uses, since it can jump backwards due to a leap |
| second update, NTP adjustment settimeofday() operation from user |
| space. |
| |
| .. c:function:: ktime_t ktime_get_clocktai( void ) |
| |
| CLOCK_TAI |
| |
| Like ktime_get_real(), but uses the International Atomic Time (TAI) |
| reference instead of UTC to avoid jumping on leap second updates. |
| This is rarely useful in the kernel. |
| |
| .. c:function:: ktime_t ktime_get_raw( void ) |
| |
| CLOCK_MONOTONIC_RAW |
| |
| Like ktime_get(), but runs at the same rate as the hardware |
| clocksource without (NTP) adjustments for clock drift. This is |
| also rarely needed in the kernel. |
| |
| nanosecond, timespec64, and second output |
| ----------------------------------------- |
| |
| For all of the above, there are variants that return the time in a |
| different format depending on what is required by the user: |
| |
| .. c:function:: u64 ktime_get_ns( void ) |
| u64 ktime_get_boottime_ns( void ) |
| u64 ktime_get_real_ns( void ) |
| u64 ktime_get_clocktai_ns( void ) |
| u64 ktime_get_raw_ns( void ) |
| |
| Same as the plain ktime_get functions, but returning a u64 number |
| of nanoseconds in the respective time reference, which may be |
| more convenient for some callers. |
| |
| .. c:function:: void ktime_get_ts64( struct timespec64 * ) |
| void ktime_get_boottime_ts64( struct timespec64 * ) |
| void ktime_get_real_ts64( struct timespec64 * ) |
| void ktime_get_clocktai_ts64( struct timespec64 * ) |
| void ktime_get_raw_ts64( struct timespec64 * ) |
| |
| Same above, but returns the time in a 'struct timespec64', split |
| into seconds and nanoseconds. This can avoid an extra division |
| when printing the time, or when passing it into an external |
| interface that expects a 'timespec' or 'timeval' structure. |
| |
| .. c:function:: time64_t ktime_get_seconds( void ) |
| time64_t ktime_get_boottime_seconds( void ) |
| time64_t ktime_get_real_seconds( void ) |
| time64_t ktime_get_clocktai_seconds( void ) |
| time64_t ktime_get_raw_seconds( void ) |
| |
| Return a coarse-grained version of the time as a scalar |
| time64_t. This avoids accessing the clock hardware and rounds |
| down the seconds to the full seconds of the last timer tick |
| using the respective reference. |
| |
| Coarse and fast_ns access |
| ------------------------- |
| |
| Some additional variants exist for more specialized cases: |
| |
| .. c:function:: ktime_t ktime_get_coarse( void ) |
| ktime_t ktime_get_coarse_boottime( void ) |
| ktime_t ktime_get_coarse_real( void ) |
| ktime_t ktime_get_coarse_clocktai( void ) |
| |
| .. c:function:: u64 ktime_get_coarse_ns( void ) |
| u64 ktime_get_coarse_boottime_ns( void ) |
| u64 ktime_get_coarse_real_ns( void ) |
| u64 ktime_get_coarse_clocktai_ns( void ) |
| |
| .. c:function:: void ktime_get_coarse_ts64( struct timespec64 * ) |
| void ktime_get_coarse_boottime_ts64( struct timespec64 * ) |
| void ktime_get_coarse_real_ts64( struct timespec64 * ) |
| void ktime_get_coarse_clocktai_ts64( struct timespec64 * ) |
| |
| These are quicker than the non-coarse versions, but less accurate, |
| corresponding to CLOCK_MONONOTNIC_COARSE and CLOCK_REALTIME_COARSE |
| in user space, along with the equivalent boottime/tai/raw |
| timebase not available in user space. |
| |
| The time returned here corresponds to the last timer tick, which |
| may be as much as 10ms in the past (for CONFIG_HZ=100), same as |
| reading the 'jiffies' variable. These are only useful when called |
| in a fast path and one still expects better than second accuracy, |
| but can't easily use 'jiffies', e.g. for inode timestamps. |
| Skipping the hardware clock access saves around 100 CPU cycles |
| on most modern machines with a reliable cycle counter, but |
| up to several microseconds on older hardware with an external |
| clocksource. |
| |
| .. c:function:: u64 ktime_get_mono_fast_ns( void ) |
| u64 ktime_get_raw_fast_ns( void ) |
| u64 ktime_get_boot_fast_ns( void ) |
| u64 ktime_get_real_fast_ns( void ) |
| |
| These variants are safe to call from any context, including from |
| a non-maskable interrupt (NMI) during a timekeeper update, and |
| while we are entering suspend with the clocksource powered down. |
| This is useful in some tracing or debugging code as well as |
| machine check reporting, but most drivers should never call them, |
| since the time is allowed to jump under certain conditions. |
| |
| Deprecated time interfaces |
| -------------------------- |
| |
| Older kernels used some other interfaces that are now being phased out |
| but may appear in third-party drivers being ported here. In particular, |
| all interfaces returning a 'struct timeval' or 'struct timespec' have |
| been replaced because the tv_sec member overflows in year 2038 on 32-bit |
| architectures. These are the recommended replacements: |
| |
| .. c:function:: void ktime_get_ts( struct timespec * ) |
| |
| Use ktime_get() or ktime_get_ts64() instead. |
| |
| .. c:function:: struct timeval do_gettimeofday( void ) |
| struct timespec getnstimeofday( void ) |
| struct timespec64 getnstimeofday64( void ) |
| void ktime_get_real_ts( struct timespec * ) |
| |
| ktime_get_real_ts64() is a direct replacement, but consider using |
| monotonic time (ktime_get_ts64()) and/or a ktime_t based interface |
| (ktime_get()/ktime_get_real()). |
| |
| .. c:function:: struct timespec current_kernel_time( void ) |
| struct timespec64 current_kernel_time64( void ) |
| struct timespec get_monotonic_coarse( void ) |
| struct timespec64 get_monotonic_coarse64( void ) |
| |
| These are replaced by ktime_get_coarse_real_ts64() and |
| ktime_get_coarse_ts64(). However, A lot of code that wants |
| coarse-grained times can use the simple 'jiffies' instead, while |
| some drivers may actually want the higher resolution accessors |
| these days. |
| |
| .. c:function:: struct timespec getrawmonotonic( void ) |
| struct timespec64 getrawmonotonic64( void ) |
| struct timespec timekeeping_clocktai( void ) |
| struct timespec64 timekeeping_clocktai64( void ) |
| struct timespec get_monotonic_boottime( void ) |
| struct timespec64 get_monotonic_boottime64( void ) |
| |
| These are replaced by ktime_get_raw()/ktime_get_raw_ts64(), |
| ktime_get_clocktai()/ktime_get_clocktai_ts64() as well |
| as ktime_get_boottime()/ktime_get_boottime_ts64(). |
| However, if the particular choice of clock source is not |
| important for the user, consider converting to |
| ktime_get()/ktime_get_ts64() instead for consistency. |
