diff options
Diffstat (limited to 'drivers/clocksource/i8253.c')
| -rw-r--r-- | drivers/clocksource/i8253.c | 40 |
1 files changed, 34 insertions, 6 deletions
diff --git a/drivers/clocksource/i8253.c b/drivers/clocksource/i8253.c index 64f6490740d7..b603c25f3dfa 100644 --- a/drivers/clocksource/i8253.c +++ b/drivers/clocksource/i8253.c @@ -1,3 +1,4 @@ +// SPDX-License-Identifier: GPL-2.0 /* * i8253 PIT clocksource */ @@ -100,18 +101,45 @@ int __init clocksource_i8253_init(void) #endif #ifdef CONFIG_CLKEVT_I8253 -static int pit_shutdown(struct clock_event_device *evt) +void clockevent_i8253_disable(void) { - if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt)) - return 0; - - raw_spin_lock(&i8253_lock); + guard(raw_spinlock_irqsave)(&i8253_lock); + /* + * Writing the MODE register should stop the counter, according to + * the datasheet. This appears to work on real hardware (well, on + * modern Intel and AMD boxes; I didn't dig the Pegasos out of the + * shed). + * + * However, some virtual implementations differ, and the MODE change + * doesn't have any effect until either the counter is written (KVM + * in-kernel PIT) or the next interrupt (QEMU). And in those cases, + * it may not stop the *count*, only the interrupts. Although in + * the virt case, that probably doesn't matter, as the value of the + * counter will only be calculated on demand if the guest reads it; + * it's the interrupts which cause steal time. + * + * Hyper-V apparently has a bug where even in mode 0, the IRQ keeps + * firing repeatedly if the counter is running. But it *does* do the + * right thing when the MODE register is written. + * + * So: write the MODE and then load the counter, which ensures that + * the IRQ is stopped on those buggy virt implementations. And then + * write the MODE again, which is the right way to stop it. + */ outb_p(0x30, PIT_MODE); outb_p(0, PIT_CH0); outb_p(0, PIT_CH0); - raw_spin_unlock(&i8253_lock); + outb_p(0x30, PIT_MODE); +} + +static int pit_shutdown(struct clock_event_device *evt) +{ + if (!clockevent_state_oneshot(evt) && !clockevent_state_periodic(evt)) + return 0; + + clockevent_i8253_disable(); return 0; } |