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