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hw/timer/armv7m_systick: Rewrite to use ptimers

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The armv7m systick timer is a 24-bit decrementing, wrap-on-zero,
clear-on-write counter. Our current implementation has various
bugs and dubious workarounds in it (for instance see
https://bugs.launchpad.net/qemu/+bug/1872237).

We have an implementation of a simple decrementing counter
and we put a lot of effort into making sure it handles the
interesting corner cases (like "spend a cycle at 0 before
reloading") -- ptimer.

Rewrite the systick timer to use a ptimer rather than
a raw QEMU timer.

Unfortunately this is a migration compatibility break,
which will affect all M-profile boards.

Among other bugs, this fixes
https://bugs.launchpad.net/qemu/+bug/1872237 :
now writes to SYST_CVR when the timer is enabled correctly
do nothing; when the timer is enabled via SYST_CSR.ENABLE,
the ptimer code will (because of POLICY_NO_IMMEDIATE_RELOAD)
arrange that after one timer tick the counter is reloaded
from SYST_RVR and then counts down from there, as the
architecture requires.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20201015151829.14656-3-peter.maydell@linaro.org
This commit is contained in:
Peter Maydell 2020-10-15 16:18:29 +01:00
parent 68d59c6d8d
commit 32bd322a01
2 changed files with 54 additions and 73 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

124
hw/timer/armv7m_systick.c
View File

@ -39,26 +39,6 @@ static inline int64_t systick_scale(SysTickState *s)
}
}
static void systick_reload(SysTickState *s, int reset)
{
/* The Cortex-M3 Devices Generic User Guide says that "When the
* ENABLE bit is set to 1, the counter loads the RELOAD value from the
* SYST RVR register and then counts down". So, we need to check the
* ENABLE bit before reloading the value.
*/
trace_systick_reload();
if ((s->control & SYSTICK_ENABLE) == 0) {
return;
}
if (reset) {
s->tick = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
}
s->tick += (s->reload + 1) * systick_scale(s);
timer_mod(s->timer, s->tick);
}
static void systick_timer_tick(void *opaque)
{
SysTickState *s = (SysTickState *)opaque;
@ -70,10 +50,12 @@ static void systick_timer_tick(void *opaque)
/* Tell the NVIC to pend the SysTick exception */
qemu_irq_pulse(s->irq);
}
if (s->reload == 0) {
s->control &= ~SYSTICK_ENABLE;
} else {
systick_reload(s, 0);
if (ptimer_get_limit(s->ptimer) == 0) {
/*
* Timer expiry with SYST_RVR zero disables the timer
* (but doesn't clear SYST_CSR.ENABLE)
*/
ptimer_stop(s->ptimer);
}
}
@ -94,30 +76,11 @@ static MemTxResult systick_read(void *opaque, hwaddr addr, uint64_t *data,
s->control &= ~SYSTICK_COUNTFLAG;
break;
case 0x4: /* SysTick Reload Value. */
val = s->reload;
val = ptimer_get_limit(s->ptimer);
break;
case 0x8: /* SysTick Current Value. */
{
int64_t t;
if ((s->control & SYSTICK_ENABLE) == 0) {
val = 0;
val = ptimer_get_count(s->ptimer);
break;
}
t = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
if (t >= s->tick) {
val = 0;
break;
}
val = ((s->tick - (t + 1)) / systick_scale(s)) + 1;
/* The interrupt in triggered when the timer reaches zero.
However the counter is not reloaded until the next clock
tick. This is a hack to return zero during the first tick. */
if (val > s->reload) {
val = 0;
}
break;
}
case 0xc: /* SysTick Calibration Value. */
val = 10000;
break;
@ -149,39 +112,50 @@ static MemTxResult systick_write(void *opaque, hwaddr addr,
switch (addr) {
case 0x0: /* SysTick Control and Status. */
{
uint32_t oldval = s->control;
uint32_t oldval;
ptimer_transaction_begin(s->ptimer);
oldval = s->control;
s->control &= 0xfffffff8;
s->control |= value & 7;
if ((oldval ^ value) & SYSTICK_ENABLE) {
int64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
if (value & SYSTICK_ENABLE) {
if (s->tick) {
s->tick += now;
timer_mod(s->timer, s->tick);
/*
* Always reload the period in case board code has
* changed system_clock_scale. If we ever replace that
* global with a more sensible API then we might be able
* to set the period only when it actually changes.
*/
ptimer_set_period(s->ptimer, systick_scale(s));
ptimer_run(s->ptimer, 0);
} else {
systick_reload(s, 1);
}
} else {
timer_del(s->timer);
s->tick -= now;
if (s->tick < 0) {
s->tick = 0;
}
ptimer_stop(s->ptimer);
}
} else if ((oldval ^ value) & SYSTICK_CLKSOURCE) {
/* This is a hack. Force the timer to be reloaded
when the reference clock is changed. */
systick_reload(s, 1);
ptimer_set_period(s->ptimer, systick_scale(s));
}
ptimer_transaction_commit(s->ptimer);
break;
}
case 0x4: /* SysTick Reload Value. */
s->reload = value;
ptimer_transaction_begin(s->ptimer);
ptimer_set_limit(s->ptimer, value & 0xffffff, 0);
ptimer_transaction_commit(s->ptimer);
break;
case 0x8: /* SysTick Current Value. Writes reload the timer. */
systick_reload(s, 1);
case 0x8: /* SysTick Current Value. */
/*
* Writing any value clears SYST_CVR to zero and clears
* SYST_CSR.COUNTFLAG. The counter will then reload from SYST_RVR
* on the next clock edge unless SYST_RVR is zero.
*/
ptimer_transaction_begin(s->ptimer);
if (ptimer_get_limit(s->ptimer) == 0) {
ptimer_stop(s->ptimer);
}
ptimer_set_count(s->ptimer, 0);
s->control &= ~SYSTICK_COUNTFLAG;
ptimer_transaction_commit(s->ptimer);
break;
default:
qemu_log_mask(LOG_GUEST_ERROR,
@ -210,10 +184,13 @@ static void systick_reset(DeviceState *dev)
*/
assert(system_clock_scale != 0);
ptimer_transaction_begin(s->ptimer);
s->control = 0;
s->reload = 0;
s->tick = 0;
timer_del(s->timer);
ptimer_stop(s->ptimer);
ptimer_set_count(s->ptimer, 0);
ptimer_set_limit(s->ptimer, 0, 0);
ptimer_set_period(s->ptimer, systick_scale(s));
ptimer_transaction_commit(s->ptimer);
}
static void systick_instance_init(Object *obj)
@ -229,18 +206,21 @@ static void systick_instance_init(Object *obj)
static void systick_realize(DeviceState *dev, Error **errp)
{
SysTickState *s = SYSTICK(dev);
s->timer = timer_new_ns(QEMU_CLOCK_VIRTUAL, systick_timer_tick, s);
s->ptimer = ptimer_init(systick_timer_tick, s,
PTIMER_POLICY_WRAP_AFTER_ONE_PERIOD |
PTIMER_POLICY_NO_COUNTER_ROUND_DOWN |
PTIMER_POLICY_NO_IMMEDIATE_RELOAD |
PTIMER_POLICY_TRIGGER_ONLY_ON_DECREMENT);
}
static const VMStateDescription vmstate_systick = {
.name = "armv7m_systick",
.version_id = 1,
.minimum_version_id = 1,
.version_id = 2,
.minimum_version_id = 2,
.fields = (VMStateField[]) {
VMSTATE_UINT32(control, SysTickState),
VMSTATE_UINT32(reload, SysTickState),
VMSTATE_INT64(tick, SysTickState),
VMSTATE_TIMER_PTR(timer, SysTickState),
VMSTATE_PTIMER(ptimer, SysTickState),
VMSTATE_END_OF_LIST()
}
};

3
include/hw/timer/armv7m_systick.h
View File

@ -14,6 +14,7 @@
#include "hw/sysbus.h"
#include "qom/object.h"
#include "hw/ptimer.h"
#define TYPE_SYSTICK "armv7m_systick"
@ -27,7 +28,7 @@ struct SysTickState {
uint32_t control;
uint32_t reload;
int64_t tick;
QEMUTimer *timer;
ptimer_state *ptimer;
MemoryRegion iomem;
qemu_irq irq;
};