[PATCH] usb: musb: musb_host: Introduce postponed URB giveback

[Matwey V. Kornilov]

This commit changes the order of actions undertaken in
musb_advance_schedule() in order to overcome issue with broken
isochronous transfer [1].

There is no harm to split musb_giveback into two pieces.  The first
unlinks finished urb, the second givebacks it. The issue here that
givebacking may be quite time-consuming due to urb->complete() call.
As it happens in case of pwc-driven web cameras. It may take about 0.5
ms to call __musb_giveback() that calls urb->callback() internally.
Under specific circumstances setting MUSB_RXCSR_H_REQPKT in subsequent
musb_start_urb() for the next urb will be too late to produce physical
IN packet. Since auto req is not used by this module the exchange
would be as the following:

[        ]   7.220456 d=  0.000997 [182   +  3.667] [  3] IN   : 4.5
[    T   ]   7.220459 d=  0.000003 [182   +  7.000] [800] DATA0: [skipped]
[        ]   7.222456 d=  0.001997 [184   +  3.667] [  3] IN   : 4.5
[        ]   7.222459 d=  0.000003 [184   +  7.000] [  3] DATA0: 00 00

It is known that missed IN in isochronous mode makes some
perepherial broken. For instance, pwc-driven or uvc-driven
web cameras.
In order to workaround this issue we postpone calling
urb->callback() after setting MUSB_RXCSR_H_REQPKT for the
next urb if there is the next urb pending in queue.

[1] https://www.spinics.net/lists/linux-usb/msg145747.html

Fixes: f551e1352983 ("Revert "usb: musb: musb_host: Enable HCD_BH flag to handle urb return in bottom half"")
Signed-off-by: Matwey V. Kornilov <matwey@xxxxxxxxxx>
---
 drivers/usb/musb/musb_host.c | 54 +++++++++++++++++++++++++++++++++++++-------
 1 file changed, 46 insertions(+), 8 deletions(-)

diff --git a/drivers/usb/musb/musb_host.c b/drivers/usb/musb/musb_host.c
index ac3a4952abb4..b590c2555dab 100644
--- a/drivers/usb/musb/musb_host.c
+++ b/drivers/usb/musb/musb_host.c
@@ -299,19 +299,24 @@ musb_start_urb(struct musb *musb, int is_in, struct musb_qh *qh)
 	}
 }
 
-/* Context: caller owns controller lock, IRQs are blocked */
-static void musb_giveback(struct musb *musb, struct urb *urb, int status)
+static void __musb_giveback(struct musb *musb, struct urb *urb, int status)
 __releases(musb->lock)
 __acquires(musb->lock)
 {
-	trace_musb_urb_gb(musb, urb);
-
-	usb_hcd_unlink_urb_from_ep(musb->hcd, urb);
 	spin_unlock(&musb->lock);
 	usb_hcd_giveback_urb(musb->hcd, urb, status);
 	spin_lock(&musb->lock);
 }
 
+/* Context: caller owns controller lock, IRQs are blocked */
+static void musb_giveback(struct musb *musb, struct urb *urb, int status)
+{
+	trace_musb_urb_gb(musb, urb);
+
+	usb_hcd_unlink_urb_from_ep(musb->hcd, urb);
+	__musb_giveback(musb, urb, status);
+}
+
 /* For bulk/interrupt endpoints only */
 static inline void musb_save_toggle(struct musb_qh *qh, int is_in,
 				    struct urb *urb)
@@ -346,6 +351,7 @@ static void musb_advance_schedule(struct musb *musb, struct urb *urb,
 	struct musb_hw_ep	*ep = qh->hw_ep;
 	int			ready = qh->is_ready;
 	int			status;
+	int                     postponed_giveback = 0;
 
 	status = (urb->status == -EINPROGRESS) ? 0 : urb->status;
 
@@ -361,9 +367,35 @@ static void musb_advance_schedule(struct musb *musb, struct urb *urb,
 		break;
 	}
 
-	qh->is_ready = 0;
-	musb_giveback(musb, urb, status);
-	qh->is_ready = ready;
+	usb_hcd_unlink_urb_from_ep(musb->hcd, urb);
+
+	/* It may take about 0.5 ms to call __musb_giveback() that
+	 * calls urb->callback() internally. Under specific circumstances
+	 * setting MUSB_RXCSR_H_REQPKT in subsequent musb_start_urb() for the
+	 * next urb will be too late to produce physical IN packet. Since
+	 * auto req is not used by this module the exchange would be as the
+	 * following:
+	 *
+	 * [        ]   7.220456 d=  0.000997 [182   +  3.667] [  3] IN   : 4.5
+	 * [    T   ]   7.220459 d=  0.000003 [182   +  7.000] [800] DATA0: [skipped]
+	 * [        ]   7.222456 d=  0.001997 [184   +  3.667] [  3] IN   : 4.5
+	 * [        ]   7.222459 d=  0.000003 [184   +  7.000] [  3] DATA0: 00 00
+	 *
+	 * It is known that missed IN in isochronous mode makes some
+	 * perepherial broken. For instance, pwc-driven or uvc-driven
+	 * web cameras.
+	 * In order to workaround this issue we postpone calling
+	 * urb->callback() after setting MUSB_RXCSR_H_REQPKT for the
+	 * next urb if there is the next urb pending in queue.
+	 */
+	if (is_in && qh->type == USB_ENDPOINT_XFER_ISOC
+	    && !list_empty(&qh->hep->urb_list)) {
+		postponed_giveback = 1;
+	} else {
+		qh->is_ready = 0;
+		__musb_giveback(musb, urb, status);
+		qh->is_ready = ready;
+	}
 
 	/* reclaim resources (and bandwidth) ASAP; deschedule it, and
 	 * invalidate qh as soon as list_empty(&hep->urb_list)
@@ -428,6 +460,12 @@ static void musb_advance_schedule(struct musb *musb, struct urb *urb,
 		    hw_ep->epnum, is_in ? 'R' : 'T', next_urb(qh));
 		musb_start_urb(musb, is_in, qh);
 	}
+
+	if (postponed_giveback) {
+		qh->is_ready = 0;
+		__musb_giveback(musb, urb, status);
+		qh->is_ready = ready;
+	}
 }
 
 static u16 musb_h_flush_rxfifo(struct musb_hw_ep *hw_ep, u16 csr)
-- 
2.12.0