clock: Introduce clock_ticks_to_ns()

The clock_get_ns() API claims to return the period of a clock in
nanoseconds. Unfortunately since it returns an integer and a
clock's period is represented in units of 2^-32 nanoseconds,
the result is often an approximation, and calculating a clock
expiry deadline by multiplying clock_get_ns() by a number-of-ticks
is unacceptably inaccurate.

Introduce a new API clock_ticks_to_ns() which returns the number
of nanoseconds it takes the clock to make a given number of ticks.
This function can do the complete calculation internally and
will thus give a more accurate result.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20201215150929.30311-2-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

2 files changed, 70 insertions, 0 deletions

diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst
index e5da28e211..c2e70e64db 100644
--- a/docs/devel/clocks.rst
+++ b/docs/devel/clocks.rst
@@ -258,6 +258,35 @@ Here is an example:
                         clock_get_ns(dev->my_clk_input));
     }
 
+Calculating expiry deadlines
+----------------------------
+
+A commonly required operation for a clock is to calculate how long
+it will take for the clock to tick N times; this can then be used
+to set a timer expiry deadline. Use the function ``clock_ticks_to_ns()``,
+which takes an unsigned 64-bit count of ticks and returns the length
+of time in nanoseconds required for the clock to tick that many times.
+
+It is important not to try to calculate expiry deadlines using a
+shortcut like multiplying a "period of clock in nanoseconds" value
+by the tick count, because clocks can have periods which are not a
+whole number of nanoseconds, and the accumulated error in the
+multiplication can be significant.
+
+For a clock with a very long period and a large number of ticks,
+the result of this function could in theory be too large to fit in
+a 64-bit value. To avoid overflow in this case, ``clock_ticks_to_ns()``
+saturates the result to INT64_MAX (because this is the largest valid
+input to the QEMUTimer APIs). Since INT64_MAX nanoseconds is almost
+300 years, anything with an expiry later than that is in the "will
+never happen" category. Callers of ``clock_ticks_to_ns()`` should
+therefore generally not special-case the possibility of a saturated
+result but just allow the timer to be set to that far-future value.
+(If you are performing further calculations on the returned value
+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.)
+
 Changing a clock period
 -----------------------
 
diff --git a/include/hw/clock.h b/include/hw/clock.h
index 81bcf3e505..b5fff6ded8 100644
--- a/include/hw/clock.h
+++ b/include/hw/clock.h
@@ -16,6 +16,8 @@
 
 #include "qom/object.h"
 #include "qemu/queue.h"
+#include "qemu/host-utils.h"
+#include "qemu/bitops.h"
 
 #define TYPE_CLOCK "clock"
 OBJECT_DECLARE_SIMPLE_TYPE(Clock, CLOCK)
@@ -219,6 +221,45 @@ static inline unsigned clock_get_ns(Clock *clk)
 }
 
 /**
+ * clock_ticks_to_ns:
+ * @clk: the clock to query
+ * @ticks: number of ticks
+ *
+ * Returns the length of time in nanoseconds for this clock
+ * to tick @ticks times. Because a clock can have a period
+ * which is not a whole number of nanoseconds, it is important
+ * to use this function when calculating things like timer
+ * expiry deadlines, rather than attempting to obtain a "period
+ * in nanoseconds" value and then multiplying that by a number
+ * of ticks.
+ *
+ * The result could in theory be too large to fit in a 64-bit
+ * value if the number of ticks and the clock period are both
+ * large; to avoid overflow the result will be saturated to INT64_MAX
+ * (because this is the largest valid input to the QEMUTimer APIs).
+ * Since INT64_MAX nanoseconds is almost 300 years, anything with
+ * an expiry later than that is in the "will never happen" category
+ * and callers can reasonably not special-case the saturated result.
+ */
+static inline uint64_t clock_ticks_to_ns(const Clock *clk, uint64_t ticks)
+{
+    uint64_t ns_low, ns_high;
+
+    /*
+     * clk->period is the period in units of 2^-32 ns, so
+     * (clk->period * ticks) is the required length of time in those
+     * units, and we can convert to nanoseconds by multiplying by
+     * 2^32, which is the same as shifting the 128-bit multiplication
+     * result right by 32.
+     */
+    mulu64(&ns_low, &ns_high, clk->period, ticks);
+    if (ns_high & MAKE_64BIT_MASK(31, 33)) {
+        return INT64_MAX;
+    }
+    return ns_low >> 32 | ns_high << 32;
+}
+
+/**
  * clock_is_enabled:
  * @clk: a clock
  *