rockbox/firmware/kernel/semaphore.c

/***************************************************************************
 *             __________               __   ___.
 *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
 *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
 *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
 *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
 *                     \/            \/     \/    \/            \/
 * $Id$
 *
 * Copyright (C) 2002 by Björn Stenberg
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ****************************************************************************/
#include "kernel-internal.h"
#include "semaphore.h"

/****************************************************************************
 * Simple semaphore functions ;)
 ****************************************************************************/

/* Initialize the semaphore object.
 * max = maximum up count the semaphore may assume (max >= 1)
 * start = initial count of semaphore (0 <= count <= max) */
void semaphore_init(struct semaphore *s, int max, int start)
{
    KERNEL_ASSERT(max > 0 && start >= 0 && start <= max,
                  "semaphore_init->inv arg\n");
    wait_queue_init(&s->queue);
    s->max = max;
    s->count = start;
    corelock_init(&s->cl);
}

/* Down the semaphore's count or wait for 'timeout' ticks for it to go up if
 * it is already 0. 'timeout' as TIMEOUT_NOBLOCK (0) will not block and may
 * safely be used in an ISR. */
int semaphore_wait(struct semaphore *s, int timeout)
{
    int ret = OBJ_WAIT_TIMEDOUT;

    int oldlevel = disable_irq_save();
    corelock_lock(&s->cl);

    int count = s->count;
    if(LIKELY(count > 0))
    {
        /* count is not zero; down it */
        s->count = count - 1;
        ret = OBJ_WAIT_SUCCEEDED;
    }
    else if(timeout != 0)
    {
        /* too many waits - block until count is upped... */
        struct thread_entry *current = __running_self_entry();

        block_thread(current, timeout, &s->queue, NULL);
        corelock_unlock(&s->cl);

        /* ...and turn control over to next thread */
        switch_thread();

        /* if explicit wake indicated; do no more */
        if(LIKELY(!wait_queue_ptr(current)))
            return OBJ_WAIT_SUCCEEDED;

        disable_irq();
        corelock_lock(&s->cl);

        /* see if anyone got us after the expired wait */
        if(wait_queue_try_remove(current))
        {
            count = s->count;
            if(count > 0)
            {
                /* down it lately */
                s->count = count - 1;
                ret = OBJ_WAIT_SUCCEEDED;
            }
        }
    }
    /* else just polling it */

    corelock_unlock(&s->cl);
    restore_irq(oldlevel);

    return ret;
}

/* Up the semaphore's count and release any thread waiting at the head of the
 * queue. The count is saturated to the value of the 'max' parameter specified
 * in 'semaphore_init'. */
void semaphore_release(struct semaphore *s)
{
    unsigned int result = THREAD_NONE;

    int oldlevel = disable_irq_save();
    corelock_lock(&s->cl);

    struct thread_entry *thread = WQ_THREAD_FIRST(&s->queue);
    if(LIKELY(thread != NULL))
    {
        /* a thread was queued - wake it up and keep count at 0 */
        KERNEL_ASSERT(s->count == 0,
            "semaphore_release->threads queued but count=%d!\n", s->count);
        result = wakeup_thread(thread, WAKEUP_DEFAULT);
    }
    else
    {
        int count = s->count;
        if(count < s->max)
        {
            /* nothing waiting - up it */
            s->count = count + 1;
        }
    }

    corelock_unlock(&s->cl);
    restore_irq(oldlevel);

#if defined(HAVE_PRIORITY_SCHEDULING) && defined(is_thread_context)
    /* No thread switch if not thread context */
    if((result & THREAD_SWITCH) && is_thread_context())
        switch_thread();
#endif
    (void)result;
}