• Thomas Gleixner's avatar
    usb: musb: Ensure that cppi41 timer gets armed on premature DMA TX irq · c58d80f5
    Thomas Gleixner authored
    Some TI chips raise the DMA complete interrupt before the actual
    transfer has been completed. The code tries to busy wait for a few
    microseconds and if that fails it arms an hrtimer to recheck. So far
    so good, but that has the following issue:
    
    CPU 0					CPU1
    
    start_next_transfer(RQ1);
    
    DMA interrupt
      if (premature_irq(RQ1))
        if (!hrtimer_active(timer))
           hrtimer_start(timer);
    
    hrtimer expires
      timer->state = CALLBACK_RUNNING;
      timer->fn()
        cppi41_recheck_tx_req()
          complete_request(RQ1);
          if (requests_pending())
            start_next_transfer(RQ2);
    
    					DMA interrupt
    					  if (premature_irq(RQ2))
    					    if (!hrtimer_active(timer))
    					       hrtimer_start(timer);
      timer->state = INACTIVE;
    
    The premature interrupt of request2 on CPU1 does not arm the timer and
    therefor the request completion never happens because it checks for
    !hrtimer_active(). hrtimer_active() evaluates:
    
      timer->state != HRTIMER_STATE_INACTIVE
    
    which of course evaluates to true in the above case as timer->state is
    CALLBACK_RUNNING.
    
    That's clearly documented:
    
     * A timer is active, when it is enqueued into the rbtree or the
     * callback function is running or it's in the state of being migrated
     * to another cpu.
    
    But that's not what the code wants to check. The code wants to check
    whether the timer is queued, i.e. whether its armed and waiting for
    expiry.
    
    We have a helper function for this: hrtimer_is_queued(). This
    evaluates:
    
      timer->state & HRTIMER_STATE_QUEUED
    
    So in the above case this evaluates to false and therefor forces the
    DMA interrupt on CPU1 to call hrtimer_start().
    
    Use hrtimer_is_queued() instead of hrtimer_active() and evrything is
    good.
    Reported-by: default avatarTorben Hohn <torbenh@linutronix.de>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarFelipe Balbi <balbi@ti.com>
    c58d80f5
musb_cppi41.c 19.3 KB