From 16745946e096e6c7fe26fc73e411bafc8c3e05ff Mon Sep 17 00:00:00 2001
From: Martin Stein <martin.stein@genode-labs.com>
Date: Wed, 23 Aug 2017 18:19:32 +0200
Subject: [PATCH] hw pit: fix precision reduction to milliseconds
Due to the simplicity of the algorithm that translated from timer ticks
to time, we lost microseconds precision although the timer allows for it.
Ref #2400
---
repos/base-hw/src/core/spec/x86_64/timer.cc | 25 ++++++++++++++++++++-
1 file changed, 24 insertions(+), 1 deletion(-)
diff --git a/repos/base-hw/src/core/spec/x86_64/timer.cc b/repos/base-hw/src/core/spec/x86_64/timer.cc
index 7ce390d3bc..9d3375d76d 100644
--- a/repos/base-hw/src/core/spec/x86_64/timer.cc
+++ b/repos/base-hw/src/core/spec/x86_64/timer.cc
@@ -64,6 +64,7 @@ Timer_driver::Timer_driver(unsigned)
/* Calculate timer frequency */
ticks_per_ms = pit_calc_timer_freq();
+ assert(ticks_per_ms >= 1000);
}
@@ -84,7 +85,29 @@ void Timer::_start_one_shot(time_t const ticks) {
time_t Timer::_ticks_to_us(time_t const ticks) const {
- return (ticks / _driver.ticks_per_ms) * 1000; }
+
+ /*
+ * If we would do the translation with one division and
+ * multiplication over the whole argument, we would loose
+ * microseconds granularity although the timer frequency would
+ * allow for such granularity. Thus, we treat the most significant
+ * half and the least significant half of the argument separate.
+ * Each half is shifted to the best bit position for the
+ * translation, then translated, and then shifted back.
+ */
+ enum {
+ HALF_WIDTH = (sizeof(time_t) << 2),
+ MSB_MASK = ~0UL << HALF_WIDTH,
+ LSB_MASK = ~0UL >> HALF_WIDTH,
+ MSB_RSHIFT = 10,
+ LSB_LSHIFT = HALF_WIDTH - MSB_RSHIFT,
+ };
+ time_t const msb = ((((ticks & MSB_MASK) >> MSB_RSHIFT)
+ * 1000) / _driver.ticks_per_ms) << MSB_RSHIFT;
+ time_t const lsb = ((((ticks & LSB_MASK) << LSB_LSHIFT)
+ * 1000) / _driver.ticks_per_ms) >> LSB_LSHIFT;
+ return msb + lsb;
+}
time_t Timer::us_to_ticks(time_t const us) const {