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.mayd...@linaro.org>
---
The 64x64->128 multiply is a bit painful for 32-bit and I
guess in theory since we know we only want bits [95:32]
of the result we could special-case it, but TBH I don't
think 32-bit hosts merit much optimization effort these days.
---
docs/devel/clocks.rst | 15 +++++++++++++++
include/hw/clock.h | 29 +++++++++++++++++++++++++++++
2 files changed, 44 insertions(+)
diff --git a/docs/devel/clocks.rst b/docs/devel/clocks.rst
index e5da28e2111..aebeedbb95e 100644
--- a/docs/devel/clocks.rst
+++ b/docs/devel/clocks.rst
@@ -258,6 +258,21 @@ 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.
+
Changing a clock period
-----------------------
diff --git a/include/hw/clock.h b/include/hw/clock.h
index 81bcf3e505a..a9425d9fb14 100644
--- a/include/hw/clock.h
+++ b/include/hw/clock.h
@@ -16,6 +16,7 @@
#include "qom/object.h"
#include "qemu/queue.h"
+#include "qemu/host-utils.h"
#define TYPE_CLOCK "clock"
OBJECT_DECLARE_SIMPLE_TYPE(Clock, CLOCK)
@@ -218,6 +219,34 @@ static inline unsigned clock_get_ns(Clock *clk)
return CLOCK_PERIOD_TO_NS(clock_get(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.
+ */
+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);
+ return ns_low >> 32 | ns_high << 32;
+}
+
/**
* clock_is_enabled:
* @clk: a clock
--
2.20.1
