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authorPeter Maydell <peter.maydell@linaro.org>2021-01-29 17:55:28 +0000
committerPeter Maydell <peter.maydell@linaro.org>2021-02-15 13:16:12 +0000
commit605a1d459f4fed3540f6d6d6d31b25b429b03e13 (patch)
tree65f79aa0f77a7de8612e7b6f20b95377cb521f3d
parent168ebc00e72c686aea046ec3b993610ff678e9dc (diff)
downloadqemu-arm-605a1d459f4fed3540f6d6d6d31b25b429b03e13.tar.gz
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 <peter.maydell@linaro.org> Reviewed-by: Luc Michel <luc@lmichel.fr> Reviewed-by: Hao Wu <wuhaotsh@google.com> Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
-rw-r--r--docs/devel/clocks.rst12
-rw-r--r--include/hw/clock.h41
2 files changed, 53 insertions, 0 deletions
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
-----------------------
diff --git a/include/hw/clock.h b/include/hw/clock.h
index 2ba44e1442..a7187eab95 100644
--- a/include/hw/clock.h
+++ b/include/hw/clock.h
@@ -287,6 +287,47 @@ static inline uint64_t clock_ticks_to_ns(const Clock *clk, uint64_t ticks)
}
/**
+ * clock_ns_to_ticks:
+ * @clk: the clock to query
+ * @ns: duration in nanoseconds
+ *
+ * Returns the number of ticks this clock would make in the given
+ * number of nanoseconds. Because a clock can have a period which
+ * is not a whole number of nanoseconds, it is important to use this
+ * function rather than attempting to obtain a "period in nanoseconds"
+ * value and then dividing the duration by that value.
+ *
+ * If the clock is stopped (ie it has period zero), returns 0.
+ *
+ * For some inputs the result could overflow a 64-bit value (because
+ * the clock's period is short and the duration is long). In these
+ * cases we truncate the result to a 64-bit value. This is on the
+ * assumption that generally the result is going to be used to report
+ * a 32-bit or 64-bit guest register value, so wrapping either cannot
+ * happen or is the desired behaviour.
+ */
+static inline uint64_t clock_ns_to_ticks(const Clock *clk, uint64_t ns)
+{
+ /*
+ * ticks = duration_in_ns / period_in_ns
+ * = ns / (period / 2^32)
+ * = (ns * 2^32) / period
+ * The hi, lo inputs to divu128() are (ns << 32) as a 128 bit value.
+ */
+ uint64_t lo = ns << 32;
+ uint64_t hi = ns >> 32;
+ if (clk->period == 0) {
+ return 0;
+ }
+ /*
+ * Ignore divu128() return value as we've caught div-by-zero and don't
+ * need different behaviour for overflow.
+ */
+ divu128(&lo, &hi, clk->period);
+ return lo;
+}
+
+/**
* clock_is_enabled:
* @clk: a clock
*