[PATCH v2 3/4] USB: EHCI: improve interrupt qh unlink

[Alan Stern]

On Mon, 24 Jun 2013, Ming Lei wrote:

> Given interrupt URB will be resubmitted from tasklet context which
> is scheduled by ehci hardware interrupt handler, and commonly only
> one interrupt URB is scheduled on qh, so the qh may be unlinked
> immediately once qh_completions() returns from ehci_irq(), then
> the intr URB to be resubmitted in complete() can only be scheduled
> and linked to hardware until the qh unlink is completed.
> 
> This patch improves this above situation, and the qh will wait for 5
> milliseconds before being unlinked from hardware, if one URB is submitted
> during the period, the qh is move out of unlink wait list and the
> interrupt transfer can be scheduled immediately, not like before: the
> transfer is only linked to hardware until previous unlink is completed.

This description is very hard to understand.  I suggest rewriting it, 
something like this:

ehci-hcd currently unlinks an interrupt QH when it becomes empty, that
is, after its last URB completes.  This works well because in almost
all cases, the completion handler for an interrupt URB resubmits the
URB; therefore the QH doesn't become empty and doesn't get unlinked.

When we start using tasklets for URB completion, this scheme won't work
as well.  The resubmission won't occur until the tasklet runs, which
will be some time after the completion is queued with the tasklet.  
During that delay, the QH will be empty and so will be unlinked 
unnecessarily.

To prevent this problem, this patch adds a 5-ms time delay before empty
interrupt QHs are unlinked.  Most often, during that time the interrupt
URB will be resubmitted and thus we can avoid unlinking the QH.

> diff --git a/drivers/usb/host/ehci-sched.c b/drivers/usb/host/ehci-sched.c
> index f80d033..5bf67e2 100644
> --- a/drivers/usb/host/ehci-sched.c
> +++ b/drivers/usb/host/ehci-sched.c
> @@ -601,12 +601,24 @@ static void qh_unlink_periodic(struct ehci_hcd *ehci, struct ehci_qh *qh)
>  	list_del(&qh->intr_node);
>  }
>  
> -static void start_unlink_intr(struct ehci_hcd *ehci, struct ehci_qh *qh)
> +/* must be called with holding ehci->lock */

The comment should be:

/* caller must hold ehci->lock */

But in fact you can leave it out.  Almost all the code in this file 
runs with the lock held.

> +static void cancel_unlink_wait_intr(struct ehci_hcd *ehci, struct ehci_qh *qh)
>  {
> -	/* If the QH isn't linked then there's nothing we can do. */
> -	if (qh->qh_state != QH_STATE_LINKED)
> +	if (qh->qh_state != QH_STATE_LINKED || list_empty(&qh->unlink_node))
>  		return;
>  
> +	list_del_init(&qh->unlink_node);
> +
> +	/* avoid unnecessary CPU wakeup */
> +	if (list_empty(&ehci->intr_unlink_wait))
> +		ehci_disable_event(ehci, EHCI_HRTIMER_START_UNLINK_INTR);

If you don't mind, can we leave out ehci_disable_event() for now and
add it after the rest of this series is merged?  It will keeps things a
little simpler, and then we'll be able to use ehci_disable_event() for
the IAA watchdog timer event as well as using it here.

> +}
> +
> +static void start_do_unlink_intr(struct ehci_hcd *ehci, struct ehci_qh *qh)
> +{
> +	/* if the qh is waitting for unlink, cancel it now */

s/waitt/wait/

> +	cancel_unlink_wait_intr(ehci, qh);
> +
>  	qh_unlink_periodic (ehci, qh);
>  
>  	/* Make sure the unlinks are visible before starting the timer */
> @@ -632,6 +644,45 @@ static void start_unlink_intr(struct ehci_hcd *ehci, struct ehci_qh *qh)
>  	}
>  }
>  
> +/*
> + * It is common only one intr URB is scheduled on one qh, and
> + * given complete() is run in tasklet context, introduce a bit
> + * delay to avoid unlink qh too early.
> + */
> +static void __start_unlink_intr(struct ehci_hcd *ehci, struct ehci_qh *qh,
> +				bool wait)
> +{
> +	/* If the QH isn't linked then there's nothing we can do. */
> +	if (qh->qh_state != QH_STATE_LINKED)
> +		return;
> +
> +	if (!wait)
> +		return start_do_unlink_intr(ehci, qh);
> +
> +	qh->unlink_cycle = ehci->intr_unlink_wait_cycle;
> +
> +	/* New entries go at the end of the intr_unlink_wait list */
> +	list_add_tail(&qh->unlink_node, &ehci->intr_unlink_wait);
> +
> +	if (ehci->rh_state < EHCI_RH_RUNNING)
> +		ehci_handle_start_intr_unlinks(ehci);
> +	else if (ehci->intr_unlink_wait.next == &qh->unlink_node) {
> +		ehci_enable_event(ehci, EHCI_HRTIMER_START_UNLINK_INTR, true);
> +		++ehci->intr_unlink_wait_cycle;
> +	}
> +}
> +
> +static void start_unlink_intr(struct ehci_hcd *ehci, struct ehci_qh *qh)
> +{
> +	__start_unlink_intr(ehci, qh, false);
> +}
> +
> +static void start_unlink_intr_wait(struct ehci_hcd *ehci,
> +				   struct ehci_qh *qh)
> +{
> +	__start_unlink_intr(ehci, qh, true);
> +}

This is not what I had in mind.

Instead, copy the qh->qh_state test into the beginning of
start_unlink_intr() and move the rest of start_do_unlink_intr() there.  
Then you can rename __start_unlink_intr() to start_unlink_intr_wait()
and get rid of the "wait" parameter.

> @@ -881,6 +932,9 @@ static int intr_submit (
>  			goto done;
>  	}
>  
> +	/* put back qh from unlink wait list */
> +	cancel_unlink_wait_intr(ehci, qh);

It might be better to change this line into an "else" clause for the 
"if (qh->qh_state == QH_STATE_IDLE)" statement after the BUG_ON below.

> +
>  	/* then queue the urb's tds to the qh */
>  	qh = qh_append_tds(ehci, urb, qtd_list, epnum, &urb->ep->hcpriv);
>  	BUG_ON (qh == NULL);

> +static void ehci_handle_start_intr_unlinks(struct ehci_hcd *ehci)
> +{
> +	bool		stopped = (ehci->rh_state < EHCI_RH_RUNNING);
> +
> +	/*
> +	 * Process all the QHs on the intr_unlink list that were added
> +	 * before the current unlink cycle began.  The list is in
> +	 * temporal order, so stop when we reach the first entry in the
> +	 * current cycle.  But if the root hub isn't running then
> +	 * process all the QHs on the list.
> +	 */
> +	while (!list_empty(&ehci->intr_unlink_wait)) {
> +		struct ehci_qh	*qh;
> +
> +		qh = list_first_entry(&ehci->intr_unlink_wait,
> +				      struct ehci_qh, unlink_node);

As I mentioned before, this line should be indented by two tab stops 
beyond the preceding line.

> +		if (!stopped &&
> +				(qh->unlink_cycle ==

This doesn't need to be on a separate line at all.

> +					ehci->intr_unlink_wait_cycle))

And this should also be indented by two tab stops beyond the preceding 
line.

> --- a/drivers/usb/host/ehci.h
> +++ b/drivers/usb/host/ehci.h
> @@ -88,6 +88,7 @@ enum ehci_hrtimer_event {
>  	EHCI_HRTIMER_POLL_DEAD,		/* Wait for dead controller to stop */
>  	EHCI_HRTIMER_UNLINK_INTR,	/* Wait for interrupt QH unlink */
>  	EHCI_HRTIMER_FREE_ITDS,		/* Wait for unused iTDs and siTDs */
> +	EHCI_HRTIMER_START_UNLINK_INTR, /* wait for new intr schedule */

The comment should be:			/* Unlink empty interrupt QHs */

>  	EHCI_HRTIMER_ASYNC_UNLINKS,	/* Unlink empty async QHs */
>  	EHCI_HRTIMER_IAA_WATCHDOG,	/* Handle lost IAA interrupts */
>  	EHCI_HRTIMER_DISABLE_PERIODIC,	/* Wait to disable periodic sched */
> @@ -143,6 +144,8 @@ struct ehci_hcd {			/* one per controller */
>  	unsigned		i_thresh;	/* uframes HC might cache */
>  
>  	union ehci_shadow	*pshadow;	/* mirror hw periodic table */
> +	struct list_head	intr_unlink_wait;
> +	unsigned		intr_unlink_wait_cycle;
>  	struct list_head	intr_unlink;
>  	unsigned		intr_unlink_cycle;
>  	unsigned		now_frame;	/* frame from HC hardware */

To avoid an alignment gap on 64-bit systems, put intr_unlink_wait_cycle
after intr_unlink.

Alan Stern