[PATCH v3] usb: gadget: epautoconf: claim smallest endpoints first

[Ruslan Bilovol <ruslan.bilovol@gmail.com>]

UDC hardware may have endpoints with different maxpacket
size. Current endpoint matching code takes first matching
endpoint from the list.

It's always possible that gadget allocates endpoints for
small transfers (maxpacket size) first, then larger ones.
That works fine if all matching UDC endpoints have same
maxpacket size or are big enough to serve that allocation.

However, some UDCs have first endpoints in the list with
bigger maxpacket size, whereas last endpoints are much
smaller. In this case endpoint allocation will fail for
the gadget (which allocates smaller endpoints first) on
final endpoint allocations.

To make endpoint allocation fair, pick up smallest
matching endpoints first, leaving bigger ones for
heavier applications.

Keel old behavior when "wMaxPacketSize == 0" because
it's a special case. In this case a gadget driver wants
to use a whole available MaxPacketSize of claimed
endpoint. Since it doesn't know what MaxPacketSize
may be in a particular UDC endpoint, it just
relies on epautoconf core and gets what's available

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
---

v3: updated commit msg, rebased onto latest gregkh/usb-next
v2: rebased onto latest balbi/next branch
v1: https://lore.kernel.org/lkml/20200629200551.27040-1-ruslan.bilovol@gmail.com/

 drivers/usb/gadget/epautoconf.c | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/drivers/usb/gadget/epautoconf.c b/drivers/usb/gadget/epautoconf.c
index ed5a92c474e5..086bb46e3f5a 100644
--- a/drivers/usb/gadget/epautoconf.c
+++ b/drivers/usb/gadget/epautoconf.c
@@ -66,7 +66,7 @@ struct usb_ep *usb_ep_autoconfig_ss(
 	struct usb_ss_ep_comp_descriptor *ep_comp
 )
 {
-	struct usb_ep	*ep;
+	struct usb_ep	*ep, *ep_min = NULL;
 
 	if (gadget->ops->match_ep) {
 		ep = gadget->ops->match_ep(gadget, desc, ep_comp);
@@ -74,14 +74,27 @@ struct usb_ep *usb_ep_autoconfig_ss(
 			goto found_ep;
 	}
 
-	/* Second, look at endpoints until an unclaimed one looks usable */
+	/*
+	 * Second, look at endpoints until an unclaimed one looks usable.
+	 * Try to find one with smallest maxpacket limit, leaving larger
+	 * endpoints for heavier applications
+	 */
 	list_for_each_entry (ep, &gadget->ep_list, ep_list) {
-		if (usb_gadget_ep_match_desc(gadget, ep, desc, ep_comp))
-			goto found_ep;
+		if (usb_gadget_ep_match_desc(gadget, ep, desc, ep_comp)) {
+			if (desc->wMaxPacketSize == 0)
+				goto found_ep;
+			else if (!ep_min)
+				ep_min = ep;
+			else if (ep->maxpacket_limit < ep_min->maxpacket_limit)
+				ep_min = ep;
+		}
 	}
 
 	/* Fail */
-	return NULL;
+	if (!ep_min)
+		return NULL;
+
+	ep = ep_min;
 found_ep:
 
 	/*
-- 
2.39.2