[PATCH 2/2] usb: xhci: fix CFC sequential scheduling lost on ring drain race

[Nicola Lunghi <nick83ola@gmail.com>]

On CFC-capable xHCI controllers, xhci_queue_isoc_tx_prepare() uses
next_frame_id for sequential isochronous scheduling only when the
endpoint ring is non-empty and the endpoint is running. If all TDs are
returned before the next URB is submitted (a ring drain race made more
likely by fast TD-skip on xHCI 1.1+ controllers), the condition fails
and the URB is scheduled ASAP — potentially many frames ahead of where
the stream should continue, creating a gap.

Extend the condition to also use next_frame_id when a periodic
completion is in progress for the endpoint, covering the transient
window where the ring is empty but the stream has not actually stopped.
Guard with URB_ISO_ASAP to preserve explicit ASAP scheduling.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=220748
Assisted-by: Claude:claude-sonnet-4-6 sparse checkpatch
Signed-off-by: Nicola Lunghi <nick83ola@gmail.com>
---
 drivers/usb/host/xhci-ring.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 03e47db82092..116a2dcd0bb2 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -4308,14 +4308,24 @@ int xhci_queue_isoc_tx_prepare(struct xhci_hcd *xhci, gfp_t mem_flags,
 	 */
 	check_interval(urb, ep_ctx);
 
-	/* Calculate the start frame and put it in urb->start_frame. */
-	if ((xhci->hcc_params & HCC_CFC) && !list_empty(&ep_ring->td_list)) {
-		if (GET_EP_CTX_STATE(ep_ctx) ==	EP_STATE_RUNNING) {
-			urb->start_frame = xep->next_frame_id;
-			goto skip_start_over;
-		}
+	/*
+	 * Calculate the start frame and put it in urb->start_frame.
+	 * On CFC-capable controllers, use sequential scheduling from
+	 * next_frame_id whenever the stream is running or a completion
+	 * is in progress (ring transiently empty due to drain race).
+	 * Skip this for explicit URB_ISO_ASAP requests.
+	 */
+	if ((xhci->hcc_params & HCC_CFC) &&
+	    !(urb->transfer_flags & URB_ISO_ASAP) &&
+	    (!list_empty(&ep_ring->td_list) ||
+	     hcd_periodic_completion_in_progress(xhci_to_hcd(xhci), urb->ep))) {
+		urb->start_frame = xep->next_frame_id;
+		goto skip_start_over;
 	}
 
+	xhci_dbg(xhci, "isoc: CFC sequential skipped for slot %u ep %u (ring_empty=%d), using ASAP\n",
+		 slot_id, ep_index, list_empty(&ep_ring->td_list));
+
 	start_frame = readl(&xhci->run_regs->microframe_index);
 	start_frame &= 0x3fff;
 	/*
-- 
2.51.0