Documentation: document ktime_get_*() APIs

As Dave Chinner points out, we don't have a proper documentation for the
ktime_get() family of interfaces, making it rather unclear which of the
over 30 (!) interfaces one should actually use in a driver or elsewhere
in the kernel.

I wrote up an explanation from how I personally see the interfaces,
documenting what each of the functions do and hopefully making it a bit
clearer which should be used where.

This is the first time I tried writing .rst format documentation, so
in addition to any mistakes in the content, I probably also introduce
nonstandard formatting ;-)

I first tried to add an extra section to
Documentation/timers/timekeeping.txt, but this is currently not included
in the generated API, and it seems useful to have the API docs as part
of what gets generated in
https://www.kernel.org/doc/html/latest/core-api/index.html#core-utilities
instead, so I started a new file there.

I also considered adding the documentation inline in the
include/linux/timekeeping.h header, but couldn't figure out how to do
that in a way that would result both in helpful inline comments as
well as readable html output, so I settled for the latter, with
a small note pointing to it from the header.

Cc: Dave Chinner <david@fromorbit.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index f5a66b7..989c97c 100644 (file)
--- 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 (file)
index 0000000..93cbeb9
--- /dev/null
+++ b/Documentation/core-api/timekeeping.rst
@@ -0,0 +1,185 @@
+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_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 86bc202..947b1b8 100644 (file)
--- 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);