Documentation: document ktime_get_*() APIs

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index f5a66b72f984..989c97cc232a 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -28,6 +28,7 @@  Core utilities
    printk-formats
    circular-buffers
    gfp_mask-from-fs-io
+   timekeeping
 
 Interfaces for kernel debugging
 ===============================
diff --git a/Documentation/core-api/timekeeping.rst b/Documentation/core-api/timekeeping.rst
new file mode 100644
index 000000000000..97dafa69dddf
--- /dev/null
+++ b/Documentation/core-api/timekeeping.rst
@@ -0,0 +1,185 @@ 
+ktime access
+============
+
+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_tai_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_boottime( void )
+		ktime_t ktime_get_coarse_real( void )
+		ktime_t ktime_get_coarse_clocktai( void )
+		ktime_t ktime_get_coarse_raw( 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 * )
+		void ktime_get_coarse_raw_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.
diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index 86bc2026efce..947b1b8d2d01 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -21,6 +21,21 @@  extern int do_sys_settimeofday64(const struct timespec64 *tv,
 				 const struct timezone *tz);
 
 /*
+ * ktime_get() family: read the current time in a multitude of ways,
+ *
+ * The default time reference is CLOCK_MONOTONIC, starting at
+ * boot time but not counting the time spent in suspend.
+ * For other references, use the functions with "real", "clocktai",
+ * "boottime" and "raw" suffixes.
+ *
+ * To get the time in a different format, use the ones wit
+ * "ns", "ts64" and "seconds" suffix.
+ *
+ * See Documentation/core-api/timekeeping.rst for more details.
+ */
+
+
+/*
  * timespec64 based interfaces
  */
 extern void ktime_get_raw_ts64(struct timespec64 *ts);