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kinetis: lptmr reload instead of spinning

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Spinning for the correct time has the side effect that it may cause
infinite recursion if the callback function calls timer_set.

timer_set->callback->...->timer_set->callback->...->timer_set-> infinity

In theory, the drawback is that the callback for very short timeouts
(<2 lptmr ticks) may be delayed up to 2 lptmr ticks (61 µs)

In practice however, tests performed using tests/bench_timers shows that
this change only affects the accuracy of the timer target when timer_set
is called with a timeout of 0, which results in a delay of 30 µs.
This commit is contained in:
Joakim Nohlgård 2018-09-22 07:29:42 +02:00
parent b709e63581
commit fa3b9168a0
1 changed files with 8 additions and 20 deletions
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28
cpu/kinetis/periph/timer.c
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@ -379,38 +379,26 @@ static inline uint16_t lptmr_read(uint8_t dev)
} }
/** /**
* @brief Reload the timer with the given timeout, or spin if timeout is too small * @brief Reload the timer with the given timeout
* *
* @pre IRQs masked, timer running * @pre IRQs masked, timer running
*/ */
static inline void lptmr_reload_or_spin(uint8_t dev, uint16_t timeout) static inline void lptmr_reload(uint8_t dev, uint16_t timeout)
{ {
LPTMR_Type *hw = lptmr_config[dev].dev; LPTMR_Type *hw = lptmr_config[dev].dev;
/* Disable timer and set target, 1 to 2 ticks will be dropped by the /* Disable timer and set target, 1 to 2 ticks will be dropped by the
* hardware during the disable-enable cycle */ * hardware during the disable-enable cycle */
/* Disable the timer interrupt first */ /* Disable the timer interrupt first */
hw->CSR = LPTMR_CSR_TEN_MASK | LPTMR_CSR_TFC_MASK; hw->CSR = LPTMR_CSR_TEN_MASK | LPTMR_CSR_TFC_MASK;
if (timeout <= LPTMR_RELOAD_OVERHEAD) { if (timeout >= LPTMR_RELOAD_OVERHEAD) {
/* we spin if the timeout is too short to reload the timer */ timeout -= LPTMR_RELOAD_OVERHEAD;
hw->CNR = 0;
uint16_t cnr_begin = hw->CNR;
while ((hw->CNR - cnr_begin) <= timeout) {
hw->CNR = 0;
}
/* Emulate IRQ handler behaviour */
lptmr[dev].running = 0;
if (lptmr[dev].isr_ctx.cb != NULL) {
lptmr[dev].isr_ctx.cb(lptmr[dev].isr_ctx.arg, 0);
}
thread_yield_higher();
return;
} }
/* Update reference */ /* Update reference */
hw->CNR = 0; hw->CNR = 0;
lptmr[dev].cnr += hw->CNR + LPTMR_RELOAD_OVERHEAD; lptmr[dev].cnr += hw->CNR + LPTMR_RELOAD_OVERHEAD;
/* Disable timer */ /* Disable timer */
hw->CSR = 0; hw->CSR = 0;
hw->CMR = timeout - LPTMR_RELOAD_OVERHEAD; hw->CMR = timeout;
/* Enable timer and IRQ */ /* Enable timer and IRQ */
hw->CSR = LPTMR_CSR_TEN_MASK | LPTMR_CSR_TFC_MASK | LPTMR_CSR_TIE_MASK; hw->CSR = LPTMR_CSR_TEN_MASK | LPTMR_CSR_TFC_MASK | LPTMR_CSR_TIE_MASK;
} }
@ -431,7 +419,7 @@ static inline int lptmr_set(uint8_t dev, uint16_t timeout)
lptmr[dev].cmr = 0; lptmr[dev].cmr = 0;
} }
} }
else if (hw->CSR & LPTMR_CSR_TCF_MASK) { else if ((timeout > 0) && (hw->CSR & LPTMR_CSR_TCF_MASK)) {
/* TCF is set, safe to update CMR live */ /* TCF is set, safe to update CMR live */
hw->CNR = 0; hw->CNR = 0;
hw->CMR = timeout + hw->CNR; hw->CMR = timeout + hw->CNR;
@ -442,7 +430,7 @@ static inline int lptmr_set(uint8_t dev, uint16_t timeout)
hw->CSR = LPTMR_CSR_TEN_MASK | LPTMR_CSR_TFC_MASK | LPTMR_CSR_TIE_MASK; hw->CSR = LPTMR_CSR_TEN_MASK | LPTMR_CSR_TFC_MASK | LPTMR_CSR_TIE_MASK;
} }
else { else {
lptmr_reload_or_spin(dev, timeout); lptmr_reload(dev, timeout);
} }
irq_restore(mask); irq_restore(mask);
return 1; return 1;
@ -476,7 +464,7 @@ static inline int lptmr_set_absolute(uint8_t dev, uint16_t target)
} }
else { else {
uint16_t timeout = target - lptmr_read(dev); uint16_t timeout = target - lptmr_read(dev);
lptmr_reload_or_spin(dev, timeout); lptmr_reload(dev, timeout);
} }
irq_restore(mask); irq_restore(mask);
return 1; return 1;