[PATCH v2 2/2] usb: gadget: dummy_hcd: set no_sg_constraint only on the SuperSpeed bus

[Igor Skalkin]

dummy_hcd registers two separate root hubs/buses: a HighSpeed one
emulating an EHCI-class controller, and a SuperSpeed one emulating an
xHCI-class controller.  usb_submit_urb() rejects SG URBs whose
non-final segments are not a multiple of the endpoint maxpacket size,
unless the bus has no_sg_constraint set.

For SuperSpeed bulk endpoints maxpacket is 1024 bytes; usbtest
generates SG lists with varying segment sizes (e.g. 512 bytes) that
are valid transfers but not maxpacket-aligned, causing usb_submit_urb
to return -EINVAL before the URB reaches the host controller.

xHCI-class controllers have no such DMA alignment requirement, so set
no_sg_constraint on the SS bus to match.  Leave the HS bus unchanged:
EHCI-class controllers do have this requirement in hardware, and
dummy_hcd's HS roothub is meant to emulate that behaviour, so the
constraint should stay enforced there.

Suggested-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Assisted-by: OpenCode:claude-sonnet-5
Signed-off-by: Igor Skalkin <igor.skalkin@xxxxxxxxxxxxxxxx>
---
 drivers/usb/gadget/udc/dummy_hcd.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c
index 29f671c7b319..0ae600159478 100644
--- a/drivers/usb/gadget/udc/dummy_hcd.c
+++ b/drivers/usb/gadget/udc/dummy_hcd.c
@@ -2578,6 +2578,15 @@ static int dummy_setup(struct usb_hcd *hcd)
 		dum->ss_hcd->dum = dum;
 		hcd->speed = HCD_USB3;
 		hcd->self.root_hub->speed = USB_SPEED_SUPER;
+
+		/*
+		 * xHCI-class SuperSpeed controllers have no maxpacket
+		 * alignment requirement for non-final SG segments.  Only
+		 * relax the constraint on the SS bus; the HS bus keeps
+		 * emulating the alignment requirement of EHCI-class
+		 * controllers.
+		 */
+		hcd->self.no_sg_constraint = 1;
 	}
 	return 0;
 }
-- 
2.49.0