zephyr/drivers/timer/loapic_timer.c

/*
 * Copyright (c) 2011-2015 Wind River Systems, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/**
 * @file
 * @brief Intel Local APIC timer driver
 *
 * This module implements a kernel device driver for the Intel local APIC
 * timer device. It provides the standard "system clock driver" interfaces for
 * use with P6 (PentiumPro, II, III) and P7 (Pentium4) family processors.
 * The local APIC timer contains a 32-bit programmable down counter that
 * generates an interrupt for use by the local processor when it reaches zero.
 * The time base is derived from the processor's bus clock, divided by a value
 * specified in the divide configuration register. After reset, the timer is
 * initialized to zero.
 *
 * Typically, the local APIC timer operates in periodic mode. That is, after
 * its down counter reaches zero and triggers a timer interrupt, it is reset
 * to its initial value and the down counting continues.
 *
 * If the TICKLESS_IDLE kernel configuration option is enabled, the timer may
 * be programmed to wake the system in N >= TICKLESS_IDLE_THRESH ticks.  The
 * kernel invokes _timer_idle_enter() to program the down counter in one-shot
 * mode to trigger an interrupt in N ticks.  When the timer expires or when
 * another interrupt is detected, the kernel's interrupt stub invokes
 * _timer_idle_exit() to leave the tickless idle state.
 *
 * @internal
 * Factors that increase the driver's complexity:
 *
 * 1. As the down-counter is a 32-bit value, the number of ticks for which the
 * system can be in tickless idle is limited to 'max_system_ticks'; This
 * corresponds to 'cycles_per_max_ticks' (as the timer is programmed in cycles).
 *
 * 2. When the request to enter tickless arrives, any remaining cycles until
 * the next tick must be accounted for to maintain accuracy.
 *
 * 3. The act of entering tickless idle may potentially straddle a tick
 * boundary. Thus the number of remaining cycles to the next tick read from
 * the down counter is suspect as it could occur before or after the tick
 * boundary (thus before or after the counter is reset). If the tick is
 * straddled, the following will occur:
 *    a. Enter tickless idle in one-shot mode
 *    b. Immediately leave tickless idle
 *    c. Process the tick event in the _timer_int_handler() and revert
 *       to periodic mode.
 *    d. Re-run the scheduler and possibly re-enter tickless idle
 *
 * 4. Tickless idle may be prematurely aborted due to a straddled tick.  See
 * previous factor.
 *
 * 5. Tickless idle may be prematurely aborted due to a non-timer interrupt.
 * Its handler may make a task or fiber ready to run, so any elapsed ticks
 * must be accounted for and the timer must also expire at the end of the
 * next logical tick so _timer_int_handler() can put it back in periodic mode.
 * This can only be distinguished from the previous factor by the executiion of
 * _timer_int_handler().
 *
 * 6. Tickless idle may end naturally.  The down counter should be zero in
 * this case. However, some targets do not implement the local APIC timer
 * correctly and the down-counter continues to decrement.
 * @endinternal
 */

#include <nanokernel.h>
#include <toolchain.h>
#include <sections.h>
#include <sys_clock.h>
#include <drivers/system_timer.h>
#include <drivers/loapic.h> /* LOAPIC registers */

#ifdef CONFIG_MICROKERNEL
#include <microkernel.h>
#endif /* CONFIG_MICROKERNEL */

#include <board.h>

/* Local APIC Timer Bits */

#define LOAPIC_TIMER_DIVBY_2 0x0	 /* Divide by 2 */
#define LOAPIC_TIMER_DIVBY_4 0x1	 /* Divide by 4 */
#define LOAPIC_TIMER_DIVBY_8 0x2	 /* Divide by 8 */
#define LOAPIC_TIMER_DIVBY_16 0x3	/* Divide by 16 */
#define LOAPIC_TIMER_DIVBY_32 0x8	/* Divide by 32 */
#define LOAPIC_TIMER_DIVBY_64 0x9	/* Divide by 64 */
#define LOAPIC_TIMER_DIVBY_128 0xa       /* Divide by 128 */
#define LOAPIC_TIMER_DIVBY_1 0xb	 /* Divide by 1 */
#define LOAPIC_TIMER_DIVBY_MASK 0xf      /* mask bits */
#define LOAPIC_TIMER_PERIODIC 0x00020000 /* Timer Mode: Periodic */


#if defined(CONFIG_MICROKERNEL) && defined(CONFIG_TICKLESS_IDLE)
#define TIMER_SUPPORTS_TICKLESS
#endif /* CONFIG_MICROKERNEL && CONFIG_TICKLESS_IDLE */

/* Helpful macros and inlines for programming timer */
#define _REG_TIMER ((volatile uint32_t *) \
					(CONFIG_LOAPIC_BASE_ADDRESS + LOAPIC_TIMER))
#define _REG_TIMER_ICR ((volatile uint32_t *) \
						(CONFIG_LOAPIC_BASE_ADDRESS + LOAPIC_TIMER_ICR))
#define _REG_TIMER_CCR ((volatile uint32_t *) \
						(CONFIG_LOAPIC_BASE_ADDRESS + LOAPIC_TIMER_CCR))
#define _REG_TIMER_CFG ((volatile uint32_t *) \
						(CONFIG_LOAPIC_BASE_ADDRESS + LOAPIC_TIMER_CONFIG))

#if defined(TIMER_SUPPORTS_TICKLESS)
#define TIMER_MODE_ONE_SHOT     0
#define TIMER_MODE_PERIODIC     1
#else /* !TIMER_SUPPORTS_TICKLESS */
#define tickless_idle_init() \
	do {/* nothing */              \
	} while (0)
#endif /* !TIMER_SUPPORTS_TICKLESS */
#if defined(TIMER_SUPPORTS_TICKLESS)
extern int32_t _sys_idle_elapsed_ticks;
#endif /* TIMER_SUPPORTS_TICKLESS */

IRQ_CONNECT_STATIC(loapic, CONFIG_LOAPIC_TIMER_IRQ,
			CONFIG_LOAPIC_TIMER_IRQ_PRIORITY,
			_timer_int_handler, 0);

static uint32_t __noinit cycles_per_tick; /* computed counter 0
							  initial count value */
static uint32_t accumulated_cycle_count;

#if defined(TIMER_SUPPORTS_TICKLESS)
static uint32_t programmed_cycles;
static uint32_t programmed_full_ticks;
static uint32_t __noinit max_system_ticks;
static uint32_t __noinit cycles_per_max_ticks;
static bool timer_known_to_have_expired;
static unsigned char timer_mode = TIMER_MODE_PERIODIC;
#endif /* TIMER_SUPPORTS_TICKLESS */

/* externs */

#ifdef CONFIG_MICROKERNEL
extern struct nano_stack _k_command_stack;
#endif /* CONFIG_MICROKERNEL */

/**
 *
 * @brief Set the timer for periodic mode
 *
 * This routine sets the timer for periodic mode.
 *
 * @return N/A
 *
 * \NOMANUAL
 */

static inline void periodic_mode_set(void)
{
	*_REG_TIMER |= LOAPIC_TIMER_PERIODIC;
}

#if defined(TIMER_SUPPORTS_TICKLESS) ||              \
	defined(LOAPIC_TIMER_PERIODIC_WORKAROUND) || \
	defined(CONFIG_SYSTEM_TIMER_DISABLE)
/**
 *
 * @brief Mask the timer interrupt
 *
 * This routine disables the LOAPIC timer by masking it.
 *
 * @return N/A
 *
 * \NOMANUAL
 */

static inline void timer_interrupt_mask(void)
{
	*_REG_TIMER |= LOAPIC_LVT_MASKED;
}
#endif

#if defined(TIMER_SUPPORTS_TICKLESS) || \
	defined(LOAPIC_TIMER_PERIODIC_WORKAROUND)
/**
 *
 * @brief Unmask the timer interrupt
 *
 * This routine enables the LOAPIC timer by unmasking it.
 *
 * @return N/A
 *
 * \NOMANUAL
 */

static inline void timer_interrupt_unmask(void)
{
	*_REG_TIMER &= ~LOAPIC_LVT_MASKED;
}
#endif

/**
 *
 * @brief Set the initial count register
 *
 * This routine sets value from which the timer will count down.
 * Note that setting the value to zero stops the timer.
 *
 * @return N/A
 *
 * \NOMANUAL
 */

static inline void initial_count_register_set(
	uint32_t count /* count from which timer is to count down */
	)
{
	*_REG_TIMER_ICR = count;
}

#if defined(TIMER_SUPPORTS_TICKLESS)
/**
 *
 * @brief Set the timer for one shot mode
 *
 * This routine sets the timer for one shot mode.
 *
 * @return N/A
 *
 * \NOMANUAL
 */

static inline void one_shot_mode_set(void)
{
	*_REG_TIMER &= ~LOAPIC_TIMER_PERIODIC;
}
#endif /* TIMER_SUPPORTS_TICKLESS */

/**
 *
 * @brief Set the rate at which the timer is decremented
 *
 * This routine sets rate at which the timer is decremented to match the
 * external bus frequency.
 *
 * @return N/A
 *
 * \NOMANUAL
 */

static inline void divide_configuration_register_set(void)
{
	*_REG_TIMER_CFG = (*_REG_TIMER_CFG & ~0xf) | LOAPIC_TIMER_DIVBY_1;
}

/**
 *
 * @brief Get the value from the current count register
 *
 * This routine gets the value from the timer's current count register.  This
 * value is the 'time' remaining to decrement before the timer triggers an
 * interrupt.
 *
 * @return N/A
 *
 * \NOMANUAL
 */
static inline uint32_t current_count_register_get(void)
{
	return *_REG_TIMER_CCR;
}

#if defined(TIMER_SUPPORTS_TICKLESS)
/**
 *
 * @brief Get the value from the initial count register
 *
 * This routine gets the value from the initial count register.
 *
 * @return N/A
 *
 * \NOMANUAL
 */
static inline uint32_t initial_count_register_get(void)
{
	return *_REG_TIMER_ICR;
}
#endif /* TIMER_SUPPORTS_TICKLESS */

/**
 *
 * @brief System clock tick handler
 *
 * This routine handles the system clock tick interrupt.  A TICK_EVENT event
 * is pushed onto the microkernel stack.
 *
 * @return N/A
 */

void _timer_int_handler(void *unused /* parameter is not used */
				 )
{
	ARG_UNUSED(unused);

#ifdef TIMER_SUPPORTS_TICKLESS
	if (timer_mode == TIMER_MODE_ONE_SHOT) {
		if (!timer_known_to_have_expired) {
			uint32_t  cycles;

			/*
			 * The timer fired unexpectedly. This is due to one of two cases:
			 *   1. Entering tickless idle straddled a tick.
			 *   2. Leaving tickless idle straddled the final tick.
			 * Due to the timer reprogramming in _timer_idle_exit(), case #2
			 * can be handled as a fall-through.
			 *
			 * NOTE: Although the cycle count is supposed to stop decrementing
			 * once it hits zero in one-shot mode, not all targets implement
			 * this properly (and continue to decrement).  Thus, we have to
			 * perform a second comparison to check for wrap-around.
			 */

			cycles = current_count_register_get();
			if ((cycles > 0) && (cycles < programmed_cycles)) {
				/* Case 1 */
				_sys_idle_elapsed_ticks = 0;
			}
		}

		/* Return the timer to periodic mode */
		initial_count_register_set(cycles_per_tick - 1);
		periodic_mode_set();
		timer_known_to_have_expired = false;
		timer_mode = TIMER_MODE_PERIODIC;
	}

	/*
	 * Increment the tick because _timer_idle_exit() does not account
	 * for the tick due to the timer interrupt itself. Also, if not in
	 * one-shot mode, _sys_idle_elapsed_ticks will be 0.
	 */
	_sys_idle_elapsed_ticks++;

	/* track the accumulated cycle count */
	accumulated_cycle_count += cycles_per_tick * _sys_idle_elapsed_ticks;

	/*
	 * If we transistion from 0 elapsed ticks to 1 we need to announce the
	 * tick event to the microkernel. Other cases will have already been
	 * covered by _timer_idle_exit().
	 */

	if (_sys_idle_elapsed_ticks == 1) {
		_sys_clock_tick_announce();
	}
#else
	/* track the accumulated cycle count */
	accumulated_cycle_count += cycles_per_tick;

#if defined(CONFIG_MICROKERNEL)
	_sys_clock_tick_announce();
#endif

#endif /*TIMER_SUPPORTS_TICKLESS*/

#if defined(CONFIG_NANOKERNEL)
	_sys_clock_tick_announce();
#endif /*  CONFIG_NANOKERNEL */

#ifdef LOAPIC_TIMER_PERIODIC_WORKAROUND
	/*
	 * On platforms where the LOAPIC timer periodic mode is broken,
	 * re-program the ICR register with the initial count value. This
	 * is only a temporary workaround.
	 */

	initial_count_register_set(cycles_per_tick - 1);
	periodic_mode_set();
#endif /* LOAPIC_TIMER_PERIODIC_WORKAROUND */
}

#if defined(TIMER_SUPPORTS_TICKLESS)
/**
 *
 * @brief Initialize the tickless idle feature
 *
 * This routine initializes the tickless idle feature.  Note that the maximum
 * number of ticks that can elapse during a "tickless idle" is limited by
 * <cycles_per_tick>.  The larger the value (the lower the tick frequency),
 * the fewer elapsed ticks during a "tickless idle".  Conversely, the smaller
 * the value (the higher the tick frequency), the more elapsed ticks during a
 * "tickless idle".
 *
 * @return N/A
 *
 * \NOMANUAL
 */

static void tickless_idle_init(void)
{
	/*
	 * Calculate the maximum number of system ticks less one. This
	 * guarantees that an overflow will not occur when any remaining
	 * cycles are added to <cycles_per_max_ticks> when calculating
	 * <programmed_cycles>.
	 */
	max_system_ticks = (0xffffffff / cycles_per_tick) - 1;
	cycles_per_max_ticks = max_system_ticks * cycles_per_tick;
}

/**
 *
 * @brief Place system timer into idle state
 *
 * Re-program the timer to enter into the idle state for the given number of
 * ticks. It is placed into one shot mode where it will fire in the number of
 * ticks supplied or the maximum number of ticks that can be programmed into
 * hardware. A value of -1 means inifinite number of ticks.
 *
 * @return N/A
 */

void _timer_idle_enter(int32_t ticks /* system ticks */
				)
{
	uint32_t  cycles;

	/*
	 * Although interrupts are disabled, the LOAPIC timer is still counting
	 * down. Take a snapshot of current count register to get the number of
	 * cycles remaining in the timer before it signals an interrupt and apply
	 * that towards the one-shot calculation to maintain accuracy.
	 *
	 * NOTE: If entering tickless idle straddles a tick, 'programmed_cycles'
	 * and 'programmmed_full_ticks' may be incorrect as we do not know which
	 * side of the tick the snapshot occurred.  This is not a problem as the
	 * values will be corrected once the straddling is detected.
	 */

	cycles = current_count_register_get();

	if ((ticks == TICKS_UNLIMITED) || (ticks > max_system_ticks)) {
		/*
		 * The number of cycles until the timer must fire next might not fit
		 * in the 32-bit counter register. To work around this, program
		 * the counter to fire in the maximum number of ticks (plus any
		 * remaining cycles).
		 */

		programmed_full_ticks = max_system_ticks;
		programmed_cycles = cycles + cycles_per_max_ticks;
	} else {
		programmed_full_ticks = ticks - 1;
		programmed_cycles = cycles + (programmed_full_ticks * cycles_per_tick);
	}

	/* Set timer to one-shot mode */
	initial_count_register_set(programmed_cycles);
	one_shot_mode_set();
	timer_mode = TIMER_MODE_ONE_SHOT;
}

/**
 *
 * @brief Handling of tickless idle when interrupted
 *
 * The routine is responsible for taking the timer out of idle mode and
 * generating an interrupt at the next tick interval.
 *
 * Note that in this routine, _sys_idle_elapsed_ticks must be zero because the
 * ticker has done its work and consumed all the ticks. This has to be true
 * otherwise idle mode wouldn't have been entered in the first place.
 *
 * Called in _IntEnt()
 *
 * @return N/A
 */

void _timer_idle_exit(void)
{
	uint32_t remaining_cycles;
	uint32_t remaining_full_ticks;

	/*
	 * Interrupts are locked and idling has ceased. The cause of the cessation
	 * is unknown. It may be due to one of three cases.
	 *  1. The timer, which was previously placed into one-shot mode has
	 *     counted down to zero and signaled an interrupt.
	 *  2. A non-timer interrupt occurred. Note that the LOAPIC timer will
	 *     still continue to decrement and may yet signal an interrupt.
	 *  3. The LOAPIC timer signaled an interrupt while the timer was being
	 *     programmed for one-shot mode.
	 *
	 * NOTE: Although the cycle count is supposed to stop decrementing once it
	 * hits zero in one-shot mode, not all targets implement this properly
	 * (and continue to decrement).  Thus a second comparison is required to
	 * check for wrap-around.
	 */

	remaining_cycles = current_count_register_get();

	if ((remaining_cycles == 0) ||
		(remaining_cycles >= programmed_cycles)) {
		/*
		 * The timer has expired. The handler _timer_int_handler() is
		 * guaranteed to execute. Track the number of elapsed ticks. The
		 * handler _timer_int_handler() will account for the final tick.
		 */

		_sys_idle_elapsed_ticks = programmed_full_ticks;

		/*
		 * Announce elapsed ticks to the microkernel. Note we are guaranteed
		 * that the timer ISR will execute before the tick event is serviced.
		 * (The timer ISR reprograms the timer for the next tick.)
		 */

		_sys_clock_tick_announce();

		timer_known_to_have_expired = true;

		return;
	}

	timer_known_to_have_expired = false;

	/*
	 * Either a non-timer interrupt occurred, or we straddled a tick when
	 * entering tickless idle. It is impossible to determine which occurred
	 * at this point. Regardless of the cause, ensure that the timer will
	 * expire at the end of the next tick in case the ISR makes any tasks
	 * and/or fibers ready to run.
	 *
	 * NOTE #1: In the case of a straddled tick, the '_sys_idle_elapsed_ticks'
	 * calculation below may result in either 0 or 1. If 1, then this may
	 * result in a harmless extra call to _sys_clock_tick_announce().
	 *
	 * NOTE #2: In the case of a straddled tick, it is assumed that when the
	 * timer is reprogrammed, it will be reprogrammed with a cycle count
	 * sufficiently close to one tick that the timer will not expire before
	 * _timer_int_handler() is executed.
	 */

	remaining_full_ticks = remaining_cycles / cycles_per_tick;

	_sys_idle_elapsed_ticks = programmed_full_ticks - remaining_full_ticks;

	if (_sys_idle_elapsed_ticks > 0) {
		_sys_clock_tick_announce();
	}

	if (remaining_full_ticks > 0) {
		/*
		 * Re-program the timer (still in one-shot mode) to fire at the end of
		 * the tick, being careful to not program zero thus stopping the timer.
		 */

		programmed_cycles = 1 + ((remaining_cycles - 1) % cycles_per_tick);

		initial_count_register_set(programmed_cycles);
	}
}
#endif /* TIMER_SUPPORTS_TICKLESS */

/**
 *
 * @brief Initialize and enable the system clock
 *
 * This routine is used to program the timer to deliver interrupts at the
 * rate specified via the 'sys_clock_us_per_tick' global variable.
 *
 * @return 0
 */

int _sys_clock_driver_init(struct device *device)
{
	ARG_UNUSED(device);

	/* determine the timer counter value (in timer clock cycles/system tick)
	 */

	cycles_per_tick = sys_clock_hw_cycles_per_tick;

	tickless_idle_init();

	divide_configuration_register_set();
	initial_count_register_set(cycles_per_tick - 1);
	periodic_mode_set();

	/*
	 * Although the stub has already been "connected", the vector number
	 * still
	 * has to be programmed into the interrupt controller.
	 */
	IRQ_CONFIG(loapic, CONFIG_LOAPIC_TIMER_IRQ, 0);

	/* Everything has been configured. It is now safe to enable the
	 * interrupt */
	irq_enable(CONFIG_LOAPIC_TIMER_IRQ);

	return 0;
}

/**
 *
 * @brief Read the platform's timer hardware
 *
 * This routine returns the current time in terms of timer hardware clock
 * cycles.
 *
 * @return up counter of elapsed clock cycles
 */

uint32_t _sys_clock_cycle_get(void)
{
	uint32_t val; /* system clock value */

	/*
	 * The LOAPIC timer counter is a down counter.  Thus to get the number
	 * of elapsed cycles since 'accumlated_cycle_count' was last updated,
	 * subtract the value in the Current Count Register (CCR) from the value
	 * in the Initial Count Register (ICR).
	 */

#if !defined(TIMER_SUPPORTS_TICKLESS)
	/* The value in the ICR always matches cycles_per_tick. */
	val = accumulated_cycle_count - current_count_register_get() +
			cycles_per_tick;
#else
	/* The value in the ICR may vary.  Read from the register. */
	val = accumulated_cycle_count - current_count_register_get() +
	      initial_count_register_get();
#endif

	return val;
}

FUNC_ALIAS(_sys_clock_cycle_get, nano_cycle_get_32, uint32_t);
FUNC_ALIAS(_sys_clock_cycle_get, task_cycle_get_32, uint32_t);

#if defined(CONFIG_SYSTEM_TIMER_DISABLE)
/**
 *
 * @brief Stop announcing ticks into the kernel
 *
 * This routine simply disables the LOAPIC counter such that interrupts are no
 * longer delivered.
 *
 * @return N/A
 */

void timer_disable(void)
{
	unsigned int key; /* interrupt lock level */

	key = irq_lock();

	timer_interrupt_mask();
	initial_count_register_set(0);

	irq_unlock(key);

	/* disable interrupt in the interrupt controller */

	irq_disable(CONFIG_LOAPIC_TIMER_IRQ);
}

#endif /* CONFIG_SYSTEM_TIMER_DISABLE */