[PATCH v3 4/5] usb: xhci: Expedite skipping missed isoch TDs on modern HCs

Michal Pecio
2025-02-26T07:05:05Z

xHCI spec rev. 1.0 allowed the TRB pointer of Missed Service events
to be NULL. Having no idea which of the queued TDs were missed and
which are waiting, we can only set a flag to skip missed TDs later.

But HCs are also allowed to give us pointer to the last missed TRB,
and this became mandatory in spec rev. 1.1 and later.

Use this pointer, if available, to immediately skip all missed TDs.
This reduces latency and risk of skipping-related bugs, because we
can now leave the skip flag cleared for future events.

Handle Missed Service Error events as 'error mid TD', if applicable,
because rev. 1.0 spec excplicitly says so in notes to 4.10.3.2 and
later revs in 4.10.3.2 and 4.11.2.5.2. Notes to 4.9.1 seem to apply.

Tested on ASM1142 and ASM3142 v1.1 xHCs which provide TRB pointers.
Tested on AMD, Etron, Renesas v1.0 xHCs which provide TRB pointers.
Tested on NEC v0.96 and VIA v1.0 xHCs which send a NULL pointer.

Change inspired by a discussion about realtime USB audio.

Link: https://lore.kernel.org/linux-usb/76e1a191-020d-4a76-97f6-237f9bd0ede0@gmx.net/T/
Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
---
drivers/usb/host/xhci-ring.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 995f8a9b5b53..ad5f0e439200 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -2481,6 +2481,12 @@ static void process_isoc_td(struct xhci_hcd *xhci, struct xhci_virt_ep *ep,
if (ep_trb != td->end_trb)
td->error_mid_td = true;
break;
+ case COMP_MISSED_SERVICE_ERROR:
+ frame->status = -EXDEV;
+ sum_trbs_for_length = true;
+ if (ep_trb != td->end_trb)
+ td->error_mid_td = true;
+ break;
case COMP_INCOMPATIBLE_DEVICE_ERROR:
case COMP_STALL_ERROR:
frame->status = -EPROTO;
@@ -2791,8 +2797,8 @@ static int handle_tx_event(struct xhci_hcd *xhci,
*/
ep->skip = true;
xhci_dbg(xhci,
- "Miss service interval error for slot %u ep %u, set skip flag\n",
- slot_id, ep_index);
+ "Miss service interval error for slot %u ep %u, set skip flag%s\n",
+ slot_id, ep_index, ep_trb_dma ? ", skip now" : "");
break;
case COMP_NO_PING_RESPONSE_ERROR:
ep->skip = true;
@@ -2841,8 +2847,8 @@ static int handle_tx_event(struct xhci_hcd *xhci,
xhci_dequeue_td(xhci, td, ep_ring, td->status);
}

- /* Missed TDs will be skipped on the next event */
- if (trb_comp_code == COMP_MISSED_SERVICE_ERROR)
+ /* If the TRB pointer is NULL, missed TDs will be skipped on the next event */
+ if (trb_comp_code == COMP_MISSED_SERVICE_ERROR && !ep_trb_dma)
return 0;

if (list_empty(&ep_ring->td_list)) {
--
2.48.1