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drivers: timer: arm_arch_timer: Fix round up issue

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Fast hardware with slow timer hardware can trigger and enter an
interrupt and reach 'sys_clock_set_timeout' before the counter has
advanced.

That defeats the "round up" logic such that we end up scheduling
timeouts a tick too soon (e.g. if the kernel requests an interrupt
at the "X" tick, we would end up computing a comparator value
representing the "X-1" tick!).

Choose the bigger one between 1 and "curr_cycle - last_cycle" to
correct.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>
This commit is contained in:
Huifeng Zhang 2022-06-17 10:53:15 +08:00 • committed by Carles Cufí
commit 79d076be7f
1 changed files with 11 additions and 2 deletions
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13
drivers/timer/arm_arch_timer.c
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@ -100,8 +100,17 @@ void sys_clock_set_timeout(int32_t ticks, bool idle)
uint64_t curr_cycle = arm_arch_timer_count();
uint64_t req_cycle = ticks * CYC_PER_TICK;
/* Round up to next tick boundary */
req_cycle += (curr_cycle - last_cycle) + (CYC_PER_TICK - 1);
/*
* Round up to next tick boundary, but an edge case should be handled.
* Fast hardware with slow timer hardware can trigger and enter an
* interrupt and reach this spot before the counter has advanced.
* That defeats the "round up" logic such that we end up scheduling
* timeouts a tick too soon (e.g. if the kernel requests an interrupt
* at the "X" tick, we would end up computing a comparator value
* representing the "X-1" tick!). Choose the bigger one between 1 and
* "curr_cycle - last_cycle" to correct.
*/
req_cycle += MAX(curr_cycle - last_cycle, 1) + (CYC_PER_TICK - 1);
req_cycle = (req_cycle / CYC_PER_TICK) * CYC_PER_TICK;