/****************************************************************************
 * arch/arm/src/armv7-a/arm_cpupause.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 <stdint.h>

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

#include "up_internal.h"
#include "gic.h"
#include "sched/sched.h"

#ifdef CONFIG_SMP

/****************************************************************************
 * Private Data
 ****************************************************************************/

/* These spinlocks are used in the SMP configuration in order to implement
 * up_cpu_pause().  The protocol for CPUn to pause CPUm is as follows
 *
 * 1. The up_cpu_pause() implementation on CPUn locks both g_cpu_wait[m]
 *    and g_cpu_paused[m].  CPUn then waits spinning on g_cpu_paused[m].
 * 2. CPUm receives the interrupt it (1) unlocks g_cpu_paused[m] and
 *    (2) locks g_cpu_wait[m].  The first unblocks CPUn and the second
 *    blocks CPUm in the interrupt handler.
 *
 * When CPUm resumes, CPUn unlocks g_cpu_wait[m] and the interrupt handler
 * on CPUm continues.  CPUm must, of course, also then unlock g_cpu_wait[m]
 * so that it will be ready for the next pause operation.
 */

static volatile spinlock_t g_cpu_wait[CONFIG_SMP_NCPUS];
static volatile spinlock_t g_cpu_paused[CONFIG_SMP_NCPUS];

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

/****************************************************************************
 * Name: up_cpu_pausereq
 *
 * Description:
 *   Return true if a pause request is pending for this CPU.
 *
 * Input Parameters:
 *   cpu - The index of the CPU to be queried
 *
 * Returned Value:
 *   true   = a pause request is pending.
 *   false = no pasue request is pending.
 *
 ****************************************************************************/

bool up_cpu_pausereq(int cpu)
{
  return spin_islocked(&g_cpu_paused[cpu]);
}

/****************************************************************************
 * Name: up_cpu_paused
 *
 * Description:
 *   Handle a pause request from another CPU.  Normally, this logic is
 *   executed from interrupt handling logic within the architecture-specific
 *   However, it is sometimes necessary necessary to perform the pending
 *   pause operation in other contexts where the interrupt cannot be taken
 *   in order to avoid deadlocks.
 *
 *   This function performs the following operations:
 *
 *   1. It saves the current task state at the head of the current assigned
 *      task list.
 *   2. It waits on a spinlock, then
 *   3. Returns from interrupt, restoring the state of the new task at the
 *      head of the ready to run list.
 *
 * Input Parameters:
 *   cpu - The index of the CPU to be paused
 *
 * Returned Value:
 *   On success, OK is returned.  Otherwise, a negated errno value indicating
 *   the nature of the failure is returned.
 *
 ****************************************************************************/

int up_cpu_paused(int cpu)
{
  FAR struct tcb_s *tcb = this_task();

  /* Update scheduler parameters */

  sched_suspend_scheduler(tcb);

  /* Save the current context at CURRENT_REGS into the TCB at the head
   * of the assigned task list for this CPU.
   */

  up_savestate(tcb->xcp.regs);

  /* Wait for the spinlock to be released */

  spin_unlock(&g_cpu_paused[cpu]);
  spin_lock(&g_cpu_wait[cpu]);

  /* Restore the exception context of the tcb at the (new) head of the
   * assigned task list.
   */

  tcb = this_task();

  /* Reset scheduler parameters */

  sched_resume_scheduler(tcb);

  /* Then switch contexts.  Any necessary address environment changes
   * will be made when the interrupt returns.
   */

  up_restorestate(tcb->xcp.regs);
  spin_unlock(&g_cpu_wait[cpu]);

  return OK;
}

/****************************************************************************
 * Name: arm_pause_handler
 *
 * Description:
 *   This is the handler for SGI2.  It performs the following operations:
 *
 *   1. It saves the current task state at the head of the current assigned
 *      task list.
 *   2. It waits on a spinlock, then
 *   3. Returns from interrupt, restoring the state of the new task at the
 *      head of the ready to run list.
 *
 * Input Parameters:
 *   Standard interrupt handling
 *
 * Returned Value:
 *   Zero on success; a negated errno value on failure.
 *
 ****************************************************************************/

int arm_pause_handler(int irq, FAR void *context)
{
  int cpu = this_cpu();

  /* Check for false alarms.  Such false could occur as a consequence of
   * some deadlock breaking logic that might have already serviced the SG2
   * interrupt by calling up_cpu_paused().  If the pause event has already
   * been processed then g_cpu_paused[cpu] will not be locked.
   */

  if (spin_islocked(&g_cpu_paused[cpu]))
    {
      return up_cpu_paused(cpu);
    }

  return OK;
}

/****************************************************************************
 * Name: up_cpu_pause
 *
 * Description:
 *   Save the state of the current task at the head of the
 *   g_assignedtasks[cpu] task list and then pause task execution on the
 *   CPU.
 *
 *   This function is called by the OS when the logic executing on one CPU
 *   needs to modify the state of the g_assignedtasks[cpu] list for another
 *   CPU.
 *
 * Input Parameters:
 *   cpu - The index of the CPU to be stopped
 *
 * Returned Value:
 *   Zero on success; a negated errno value on failure.
 *
 ****************************************************************************/

int up_cpu_pause(int cpu)
{
  int ret;

  DEBUGASSERT(cpu >= 0 && cpu < CONFIG_SMP_NCPUS && cpu != this_cpu());

  /* Take the both spinlocks.  The g_cpu_wait spinlock will prevent the SGI2
   * handler from returning until up_cpu_resume() is called; g_cpu_paused
   * is a handshake that will prefent this function from returning until
   * the CPU is actually paused.
   */

  DEBUGASSERT(!spin_islocked(&g_cpu_wait[cpu]) &&
              !spin_islocked(&g_cpu_paused[cpu]));

  spin_lock(&g_cpu_wait[cpu]);
  spin_lock(&g_cpu_paused[cpu]);

  /* Execute SGI2 */

  ret = arm_cpu_sgi(GIC_IRQ_SGI2, (1 << cpu));
  if (ret < 0)
    {
      /* What happened?  Unlock the g_cpu_wait spinlock */

      spin_unlock(&g_cpu_wait[cpu]);
    }
  else
    {
      /* Wait for the other CPU to unlock g_cpu_paused meaning that
       * it is fully paused and ready for up_cpu_resume();
       */

      spin_lock(&g_cpu_paused[cpu]);
    }

  spin_unlock(&g_cpu_paused[cpu]);

  /* On successful return g_cpu_wait will be locked, the other CPU will be
   * spinninf on g_cpu_wait and will not continue until g_cpu_resume() is
   * called.  g_cpu_paused will be unlocked in any case.
   */

 return ret;
}

/****************************************************************************
 * Name: up_cpu_resume
 *
 * Description:
 *   Restart the cpu after it was paused via up_cpu_pause(), restoring the
 *   state of the task at the head of the g_assignedtasks[cpu] list, and
 *   resume normal tasking.
 *
 *   This function is called after up_cpu_pause in order resume operation of
 *   the CPU after modifying its g_assignedtasks[cpu] list.
 *
 * Input Parameters:
 *   cpu - The index of the CPU being re-started.
 *
 * Returned Value:
 *   Zero on success; a negated errno value on failure.
 *
 ****************************************************************************/

int up_cpu_resume(int cpu)
{
  DEBUGASSERT(cpu >= 0 && cpu < CONFIG_SMP_NCPUS && cpu != this_cpu());

  /* Release the spinlock.  Releasing the spinlock will cause the SGI2
   * handler on 'cpu' to continue and return from interrupt to the newly
   * established thread.
   */

  DEBUGASSERT(spin_islocked(&g_cpu_wait[cpu]) &&
              !spin_islocked(&g_cpu_paused[cpu]));

  spin_unlock(&g_cpu_wait[cpu]);
  return OK;
}

#endif /* CONFIG_SMP */