From cd3a53b727d2f86e9db795cee69cc142332ca079 Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Fri, 19 Feb 2021 14:45:36 +0000 Subject: clock: Add clock_ns_to_ticks() function Add a clock_ns_to_ticks() function which does the opposite of clock_ticks_to_ns(): given a duration in nanoseconds, it returns the number of clock ticks that would happen in that time. This is useful for devices that have a free running counter register whose value can be calculated when it is read. Signed-off-by: Peter Maydell Reviewed-by: Luc Michel Reviewed-by: Hao Wu Tested-by: Philippe Mathieu-Daudé Message-id: 20210219144617.4782-4-peter.maydell@linaro.org --- docs/devel/clocks.rst | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'docs') diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst index f0391e76b4..956bd147ea 100644 --- a/docs/devel/clocks.rst +++ b/docs/devel/clocks.rst @@ -360,6 +360,18 @@ rather than simply passing it to a QEMUTimer function like ``timer_mod_ns()`` then you should be careful to avoid overflow in those calculations, of course.) +Obtaining tick counts +--------------------- + +For calculations where you need to know the number of ticks in +a given duration, use ``clock_ns_to_ticks()``. This function handles +possible non-whole-number-of-nanoseconds periods and avoids +potential rounding errors. It will return '0' if the clock is stopped +(i.e. it has period zero). If the inputs imply a tick count that +overflows a 64-bit value (a very long duration for a clock with a +very short period) the output value is truncated, so effectively +the 64-bit output wraps around. + Changing a clock period ----------------------- -- cgit v1.2.3-55-g7522