On Sat, 14 Aug 2010, Alessio Sangalli wrote:
> On 08/13/2010 06:32 PM, Alan Stern wrote:
>
> > The number of times the larger loop executes depends on how often a
> > periodic URB completes. That might be hundreds of iterations, but if
> > it is then most of the iterations should be very fast. For example, if
> > there's an isochronous URB with 10 packets and a period of 64 uframes,
> > then the loop would execute 640 times but only 10 of those iterations
> > would do any work. This shouldn't take 2 milliseconds.
>
> Ok but I only had a mouse connected here...
>
> > Can you find out how much work is done on each loop iteration? For
> > example, how many times does the "while (q.ptr != NULL) {" inner loop
>
> I will experiment with the code.
I managed to lose track of this thread for a couple of months, sorry.
Here is a patch which ought to reduce the overhead of the loop. It
avoids doing the expensive call to qh_completions() more than once for
each qh.
Alan Stern
Index: usb-2.6/drivers/usb/host/ehci.h
===================================================================
--- usb-2.6.orig/drivers/usb/host/ehci.h
+++ usb-2.6/drivers/usb/host/ehci.h
@@ -117,6 +117,7 @@ struct ehci_hcd { /* one per controlle
struct timer_list watchdog;
unsigned long actions;
unsigned stamp;
+ unsigned periodic_stamp;
unsigned random_frame;
unsigned long next_statechange;
ktime_t last_periodic_enable;
Index: usb-2.6/drivers/usb/host/ehci-q.c
===================================================================
--- usb-2.6.orig/drivers/usb/host/ehci-q.c
+++ usb-2.6/drivers/usb/host/ehci-q.c
@@ -838,6 +838,7 @@ qh_make (
is_input, 0,
hb_mult(maxp) * max_packet(maxp)));
qh->start = NO_FRAME;
+ qh->stamp = ehci->periodic_stamp;
if (urb->dev->speed == USB_SPEED_HIGH) {
qh->c_usecs = 0;
Index: usb-2.6/drivers/usb/host/ehci-sched.c
===================================================================
--- usb-2.6.orig/drivers/usb/host/ehci-sched.c
+++ usb-2.6/drivers/usb/host/ehci-sched.c
@@ -2261,6 +2261,7 @@ scan_periodic (struct ehci_hcd *ehci)
}
clock &= mod - 1;
clock_frame = clock >> 3;
+ ++ehci->periodic_stamp;
for (;;) {
union ehci_shadow q, *q_p;
@@ -2289,10 +2290,14 @@ restart:
temp.qh = qh_get (q.qh);
type = Q_NEXT_TYPE(ehci, q.qh->hw->hw_next);
q = q.qh->qh_next;
- modified = qh_completions (ehci, temp.qh);
- if (unlikely(list_empty(&temp.qh->qtd_list) ||
- temp.qh->needs_rescan))
- intr_deschedule (ehci, temp.qh);
+ if (temp.qh->stamp != ehci->periodic_stamp) {
+ modified = qh_completions(ehci, temp.qh);
+ if (!modified)
+ temp.qh->stamp = ehci->periodic_stamp;
+ if (unlikely(list_empty(&temp.qh->qtd_list) ||
+ temp.qh->needs_rescan))
+ intr_deschedule(ehci, temp.qh);
+ }
qh_put (temp.qh);
break;
case Q_TYPE_FSTN:
@@ -2427,6 +2432,7 @@ restart:
free_cached_lists(ehci);
ehci->clock_frame = clock_frame;
}
+ ++ehci->periodic_stamp;
} else {
now_uframe++;
now_uframe &= mod - 1;
--
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