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>
Message-id: 20210219144617.4782-4-peter.maydell@linaro.org

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
 -----------------------
 

include/hw/clock.h

@@ -286,6 +286,47 @@ static inline uint64_t clock_ticks_to_ns(const Clock *clk, uint64_t ticks)
     return ns_low >> 32 | ns_high << 32;
 }
 
+/**
+ * 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