nuttx/arch/xtensa/src/common/xtensa_testset.c

/****************************************************************************
 * arch/xtensa/src/common/xtensa_testset.c
 *
 *   Copyright (C) 2016 Gregory Nutt. All rights reserved.
 *   Author: Gregory Nutt <gnutt@nuttx.org>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name NuttX nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/config.h>

#include <nuttx/spinlock.h>
#include <arch/spinlock.h>

#include "xtensa.h"

#ifdef CONFIG_SPINLOCK

/****************************************************************************
 * Private Functions
 ****************************************************************************/

/****************************************************************************
 * Name: xtensa_compareset
 *
 * Description:
 *   Wrapper for the Xtensa compare-and-set instruction. This function will
 *   atomically compare *addr to compare, and if it's the same, will set
 *   *addr to set. It will return the old value of *addr.
 *
 *   Warning: From the ISA docs: in some (unspecified) cases, the s32c1i
 *   instruction may return the *bitwise inverse* of the old mem if the
 *   mem wasn't written. This doesn't seem to happen on the ESP32, though.
 *   (Would show up directly if it did because the magic wouldn't match.)
 *
 ****************************************************************************/

static inline uint32_t xtensa_compareset(FAR volatile uint32_t *addr,
                                         uint32_t compare,
                                         uint32_t set)
{
  __asm__ __volatile__
  (
    "WSR    %2, SCOMPARE1\n" /* Initialize SCOMPARE1 */
    "ISYNC\n"                /* Wait sync */
    "S32C1I %0, %1, 0\n"     /* Store id into the lock, if the lock is the
                              * same as comparel. Otherwise, no write-access */
    : "=r"(set) : "r"(addr), "r"(compare), "0"(set)
  );

  return set;
}

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Name: up_testset
 *
 * Description:
 *   Perform an atomic test and set operation on the provided spinlock.
 *
 *   This function must be provided via the architecture-specific logic.
 *
 * Input Parameters:
 *   lock - The address of spinlock object.
 *
 * Returned Value:
 *   The spinlock is always locked upon return.  The value of previous value
 *   of the spinlock variable is returned, either SP_LOCKED if the spinlock
 *   was previously locked (meaning that the test-and-set operation failed to
 *   obtain the lock) or SP_UNLOCKED if the spinlock was previously unlocked
 *   (meaning that we successfully obtained the lock)
 *
 ****************************************************************************/

spinlock_t up_testset(volatile FAR spinlock_t *lock)
{
  spinlock_t prev;

  /* Perform the 32-bit compare and set operation */

  prev = xtensa_compareset((FAR volatile uint32_t *)lock,
                           SP_UNLOCKED, SP_LOCKED);

  /* xtensa_compareset() should return either SP_UNLOCKED if the spinlock
   * was locked or SP_LOCKED or possibly ~SP_UNLOCKED if the spinlock was
   * not locked:
   *
   * "In the RE-2013.0 release and after, there is a slight change in the
   *  semantics of the S32C1I instruction.  Nothing is changed about the
   *  operation on memory.  In rare cases the resulting value in register
   *  at can be different in this and later releases. The rule still holds
   *  that memory has been written if and only if the register result
   *  equals SCOMPARE1.
   *
   * "The difference is that in some cases where memory has not been
   *  written, the instruction returns ~SCOMPARE1 instead of the current
   *  value of memory.  Although this change can, in principle, affect
   *  the operation of code, scanning all internal Cadence code produced
   *  no examples where this change would change the operation of the
   *  code."
   *
   * In any case, the return value of SP_UNLOCKED can be trusted and will
   * always mean that the spinlock was set.
   */

  return (prev == SP_UNLOCKED) ? SP_UNLOCKED : SP_LOCKED;
}

#endif /* CONFIG_SPINLOCK */