[PATCH 3/5] usbdevfs: Don't advertise USBDEVFS_CAP_BULK_CONTINUATION on XHCI attached devs

[Hans de Goede <hdegoede@xxxxxxxxxx>]

As discussed in length on the mailinglist, USBDEVFS_URB_BULK_CONTINUATION
does not work as it should when combined with USBDEVFS_URB_SHORT_NOT_OK
(which is its intended use) on devices attached to an XHCI controller.

Note that this patch only does not advertise the cap, bulk transfers
with USBDEVFS_URB_BULK_CONTINUATION will still be accepted. Returning
-EINVAL for them would break existing apps, and in most cases the
troublesome scenario wrt USBDEVFS_URB_SHORT_NOT_OK urbs on XHCI controllers
will never get hit, so this would break working use cases.

The disadvantage of not returning -EINVAL is that cases were it is causing
real trouble may go undetected / the cause of the trouble may be unclear,
but this is the best we can do.

Signed-off-by: Hans de Goede <hdegoede@xxxxxxxxxx>
---
 drivers/usb/core/devio.c |    3 ++-
 drivers/usb/host/xhci.c  |    2 ++
 include/linux/usb.h      |    5 +++++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index 3807ea6..b3a5e2b 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -1823,7 +1823,8 @@ static int proc_get_capabilities(struct dev_state *ps, void __user *arg)
 	__u32 caps;
 
 	caps = USBDEVFS_CAP_ZERO_PACKET | USBDEVFS_CAP_NO_PACKET_SIZE_LIM;
-	caps |= USBDEVFS_CAP_BULK_CONTINUATION;
+	if (!ps->dev->bus->no_stop_on_short)
+		caps |= USBDEVFS_CAP_BULK_CONTINUATION;
 
 	if (put_user(caps, (__u32 __user *)arg))
 		return -EFAULT;
diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index 36641a7..bde855f 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -3974,6 +3974,8 @@ int xhci_gen_setup(struct usb_hcd *hcd, xhci_get_quirks_t get_quirks)
 
 	/* Accept arbitrarily long scatter-gather lists */
 	hcd->self.sg_tablesize = ~0;
+	/* XHCI controllers don't stop the ep queue on short packets :| */
+	hcd->self.no_stop_on_short = 1;
 
 	if (usb_hcd_is_primary_hcd(hcd)) {
 		xhci = kzalloc(sizeof(struct xhci_hcd), GFP_KERNEL);
diff --git a/include/linux/usb.h b/include/linux/usb.h
index 73b68d1..748ca86 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -331,6 +331,11 @@ struct usb_bus {
 	u8 otg_port;			/* 0, or number of OTG/HNP port */
 	unsigned is_b_host:1;		/* true during some HNP roleswitches */
 	unsigned b_hnp_enable:1;	/* OTG: did A-Host enable HNP? */
+	unsigned no_stop_on_short:1;    /*
+					 * Quirk: some controllers don't stop
+					 * the ep queue on a short transfer
+					 * with the URB_SHORT_NOT_OK flag set.
+					 */
 	unsigned sg_tablesize;		/* 0 or largest number of sg list entries */
 
 	int devnum_next;		/* Next open device number in
-- 
1.7.10.4

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