[PATCH] USB: ehci-hcd, partial VIA workaround

This patch resolves a FIXME, which happens to make many of
the VIA problems act significantly less severe.  The change
defers unlinking any QH that just became idle, since it's not
unlikely it'll be used again before many milliseconds pass.

It reduces the number of unlink interrupts (IAA), and means
fewer re-activation issues.  Like: newly queued TDs being
all or partially processed before the QH gets de-activated.
The VIA hardware seems to have some problems in those cases.
(Which are extremely common on 2.4 kernels, and less so on
2.5 because usb-storage streams data much better now.)

It also starts tracking the "lost IAA" errors that I see on
at least one VT8235 motherboard.  It shows in the "registers"
sysfs file.  It'd be good to know if it's just my hardware
that has this problem, or if other folk also see it.

drivers/usb/host/ehci-dbg.c

@@ -605,8 +605,10 @@ show_registers (struct device *dev, char *buf)
 	}
 
 #ifdef EHCI_STATS
-	temp = snprintf (next, size, "irq normal %ld err %ld reclaim %ld\n",
-		ehci->stats.normal, ehci->stats.error, ehci->stats.reclaim);
+	temp = snprintf (next, size,
+		"irq normal %ld err %ld reclaim %ld (lost %ld)\n",
+		ehci->stats.normal, ehci->stats.error, ehci->stats.reclaim,
+		ehci->stats.lost_iaa);
 	size -= temp;
 	next += temp;
 

drivers/usb/host/ehci-hcd.c

@@ -260,6 +260,7 @@ static void ehci_watchdog (unsigned long param)
 
 		if (status & STS_IAA) {
 			ehci_vdbg (ehci, "lost IAA\n");
+			COUNT (ehci->stats.lost_iaa);
 			writel (STS_IAA, &ehci->regs->status);
 			ehci->reclaim_ready = 1;
 		}
@@ -547,8 +548,9 @@ static void ehci_stop (struct usb_hcd *hcd)
 	ehci_mem_cleanup (ehci);
 
 #ifdef	EHCI_STATS
-	ehci_dbg (ehci, "irq normal %ld err %ld reclaim %ld\n",
-		ehci->stats.normal, ehci->stats.error, ehci->stats.reclaim);
+	ehci_dbg (ehci, "irq normal %ld err %ld reclaim %ld (lost %ld)\n",
+		ehci->stats.normal, ehci->stats.error, ehci->stats.reclaim,
+		ehci->stats.lost_iaa);
 	ehci_dbg (ehci, "complete %ld unlink %ld\n",
 		ehci->stats.complete, ehci->stats.unlink);
 #endif

drivers/usb/host/ehci-q.c

@@ -800,6 +800,7 @@ static struct ehci_qh *qh_append_tds (
 				&& !usb_pipecontrol (urb->pipe)) {
 			/* "never happens": drivers do stall cleanup right */
 			if (qh->qh_state != QH_STATE_IDLE
+					&& !list_empty (&qh->qtd_list)
 					&& qh->qh_state != QH_STATE_COMPLETING)
 				ehci_warn (ehci, "clear toggle dev%d "
 						"ep%d%s: not idle\n",
@@ -1014,6 +1015,7 @@ static void
 scan_async (struct ehci_hcd *ehci, struct pt_regs *regs)
 {
 	struct ehci_qh		*qh;
+	int			unlink_delay = 0;
 
 	if (!++(ehci->stamp))
 		ehci->stamp++;
@@ -1040,17 +1042,25 @@ scan_async (struct ehci_hcd *ehci, struct pt_regs *regs)
 				}
 			}
 
-			/* unlink idle entries, reducing HC PCI usage as
-			 * well as HCD schedule-scanning costs.
-			 *
-			 * FIXME don't unlink idle entries so quickly; it
-			 * can penalize (common) half duplex protocols.
+			/* unlink idle entries, reducing HC PCI usage as well
+			 * as HCD schedule-scanning costs.  delay for any qh
+			 * we just scanned, there's a not-unusual case that it
+			 * doesn't stay idle for long.
+			 * (plus, avoids some kind of re-activation race.)
 			 */
-			if (list_empty (&qh->qtd_list) && !ehci->reclaim) {
-				start_unlink_async (ehci, qh);
+			if (list_empty (&qh->qtd_list)) {
+				if (qh->stamp == ehci->stamp)
+					unlink_delay = 1;
+				else if (!ehci->reclaim) {
+					start_unlink_async (ehci, qh);
+					unlink_delay = 0;
+				}
 			}
 
 			qh = qh->qh_next.qh;
 		} while (qh);
 	}
+
+	if (unlink_delay && !timer_pending (&ehci->watchdog))
+		mod_timer (&ehci->watchdog, jiffies + EHCI_WATCHDOG_JIFFIES/2);
 }

drivers/usb/host/ehci.h

@@ -27,6 +27,7 @@ struct ehci_stats {
 	unsigned long		normal;
 	unsigned long		error;
 	unsigned long		reclaim;
+	unsigned long		lost_iaa;
 
 	/* termination of urbs from core */
 	unsigned long		complete;