[PATCH] UHCI: use dummy TDs

This patch (as624) fixes a hardware race in uhci-hcd by adding a dummy
TD to the end of each endpoint's queue.  Without the dummy the host
controller will effectively turn off the queue when it reaches the end,
which happens asynchronously.  This leads to a potential problem when
new transfer descriptors are added to the end of the queue; they may
never get used.

With a dummy TD present the controller never turns off the queue;
instead it just stops at the dummy and leaves the queue on but inactive.
When new TDs are added to the end of the queue, the first new one gets
written over the dummy.  Thus there's never any question about whether
the queue is running or needs to be restarted.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/usb/host/uhci-debug.c b/drivers/usb/host/uhci-debug.c
index 3faccbd6854..6814783adf9 100644
--- a/drivers/usb/host/uhci-debug.c
+++ b/drivers/usb/host/uhci-debug.c
@@ -189,6 +189,11 @@ static int uhci_show_qh(struct uhci_qh *qh, char *buf, int len, int space)
 					space, "", nurbs);
 	}
 
+	if (qh->udev) {
+		out += sprintf(out, "%*s  Dummy TD\n", space, "");
+		out += uhci_show_td(qh->dummy_td, out, len - (out - buf), 0);
+	}
+
 	return out - buf;
 }