/*-
* Copyright (c) 2020 Jason R. Thorpe
* Copyright (c) 2018 Naruaki Etomi
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/*
* Most of the following was adapted from the Linux/ia64 cycle counter
* synchronization algorithm:
*
* IA-64 Linux Kernel: Design and Implementation p356-p361
* (Hewlett-Packard Professional Books)
*
* Here's a rough description of how it works.
*
* The primary CPU is the reference monotonic counter. Each secondary
* CPU is responsible for knowing the offset of its own cycle counter
* relative to the primary's. When the time counter is read, the CC
* value is adjusted by this delta.
*
* Calibration happens periodically, and works like this:
*
* Secondary CPU Primary CPU
* Send IPI to publish reference CC
* --------->
* Indicate Primary Ready
* <----------------------------
* T0 = local CC
* Indicate Secondary Ready
* ----------------->
* (assume this happens at Tavg) Publish reference CC
* Indicate completion
* <------------------------
* Notice completion
* T1 = local CC
*
* Tavg = (T0 + T1) / 2
*
* Delta = Tavg - Published primary CC value
*
* "Notice completion" is performed by waiting for the primary to set
* the calibration state to FINISHED. This is a little unfortunate,
* because T0->Tavg involves a single store-release on the secondary, and
* Tavg->T1 involves a store-relaxed and a store-release. It would be
* better to simply wait for the reference CC to transition from 0 to
* non-0 (i.e. just wait for a single store-release from Tavg->T1), but
* if the cycle counter just happened to read back as 0 at that instant,
* we would never break out of the loop.
*
* We trigger calibration roughly once a second; the period is actually
* skewed based on the CPU index in order to avoid lock contention. The
* calibration interval does not need to be precise, and so this is fine.
*/
/*
* Initialize cycle counter timecounter calibration data on a secondary
* CPU. Must be called on that secondary CPU.
*/
void
cc_init_secondary(struct cpu_info * const ci)
{
KASSERT(!CPU_IS_PRIMARY(curcpu()));
KASSERT(ci == curcpu());
ci->ci_cc.cc_ticks = 0;
/*
* It's not critical that calibration be performed in
* precise intervals, so skew when calibration is done
* on each secondary CPU based on it's CPU index to
* avoid contending on the calibration lock.
*/
ci->ci_cc.cc_cal_ticks = hz - cpu_index(ci);
KASSERT(ci->ci_cc.cc_cal_ticks);
cc_calibrate_cpu(ci);
}
/*
* pick up tick count scaled to reference tick count
*/
u_int
cc_get_timecount(struct timecounter *tc)
{
#if defined(MULTIPROCESSOR)
int64_t rcc;
long pctr;
do {
pctr = lwp_pctr();
/* N.B. the delta is always 0 on the primary. */
rcc = cpu_counter32() - curcpu()->ci_cc.cc_delta;
} while (pctr != lwp_pctr());
for (;;) {
if (atomic_load_acquire(&cc_calibration_state) ==
CC_CAL_FINISHED) {
break;
}
}
t1 = cpu_counter32();
if (t1 < t0) {
/* Overflow! */
return false;
}
/* average t0 and t1 without overflow: */
tcenter = (t0 >> 1) + (t1 >> 1);
if ((t0 & 1) + (t1 & 1) == 2)
tcenter++;
ci->ci_cc.cc_delta = tcenter - cc_primary;
return true;
}
#endif /* MULTIPROCESSOR */
/*
* Called on secondary CPUs to calibrate their cycle counter offset
* relative to the primary CPU.
*/
void
cc_calibrate_cpu(struct cpu_info * const ci)
{
#if defined(MULTIPROCESSOR)
KASSERT(!CPU_IS_PRIMARY(ci));
