USB: EHCI: improve interrupt qh unlink

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.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 45 insertions, 2 deletions

diff --git a/drivers/usb/host/ehci-sched.c b/drivers/usb/host/ehci-sched.c
index f80d0330d548..94388738a6f7 100644
--- a/drivers/usb/host/ehci-sched.c
+++ b/drivers/usb/host/ehci-sched.c
@@ -601,12 +601,29 @@ static void qh_unlink_periodic(struct ehci_hcd *ehci, struct ehci_qh *qh)
         list_del(&qh->intr_node);
 }
 
+static void cancel_unlink_wait_intr(struct ehci_hcd *ehci, struct ehci_qh *qh)
+{
+        if (qh->qh_state != QH_STATE_LINKED ||
+                        list_empty(&qh->unlink_node))
+                return;
+
+        list_del_init(&qh->unlink_node);
+
+        /*
+         * TODO: disable the event of EHCI_HRTIMER_START_UNLINK_INTR for
+         * avoiding unnecessary CPU wakeup
+         */
+}
+
 static void start_unlink_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)
                 return;
 
+        /* if the qh is waiting for unlink, cancel it now */
+        cancel_unlink_wait_intr(ehci, qh);
+
         qh_unlink_periodic (ehci, qh);
 
         /* Make sure the unlinks are visible before starting the timer */
@@ -632,6 +649,27 @@ 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_wait(struct ehci_hcd *ehci,
+                                   struct ehci_qh *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 end_unlink_intr(struct ehci_hcd *ehci, struct ehci_qh *qh)
 {
         struct ehci_qh_hw       *hw = qh->hw;
@@ -889,6 +927,9 @@ static int intr_submit (
         if (qh->qh_state == QH_STATE_IDLE) {
                 qh_refresh(ehci, qh);
                 qh_link_periodic(ehci, qh);
+        } else {
+                /* cancel unlink wait for the qh */
+                cancel_unlink_wait_intr(ehci, qh);
         }
 
         /* ... update usbfs periodic stats */
@@ -924,9 +965,11 @@ static void scan_intr(struct ehci_hcd *ehci)
                          * in qh_unlink_periodic().
                          */
                         temp = qh_completions(ehci, qh);
-                        if (unlikely(temp || (list_empty(&qh->qtd_list) &&
-                                        qh->qh_state == QH_STATE_LINKED)))
+                        if (unlikely(temp))
                                 start_unlink_intr(ehci, qh);
+                        else if (unlikely(list_empty(&qh->qtd_list) &&
+                                        qh->qh_state == QH_STATE_LINKED))
+                                start_unlink_intr_wait(ehci, qh);
                 }
         }
 }