I got a StarTech SV431USBDDM KVM switch, and it specifically mangles the
string descriptor responses for my keyboard (which otherwise works just
fine) when probed about the vendor string after plugging in or resetting
the device.
After a ton of messing around, I also found out that this problem just
isn't happening at all if OpenBSD only sends a single string descriptor
request with the maximum possible length, rather than two as it
currently does.
According to the USB 2.0 and 3.2 specs, devices that receive a larger
wLength than the descriptor's actual length /should/ just send the full
descriptor in a short packet. Of course, the current behavior isn't
wrong either, if the wLength is shorter, then only the initial bytes or
length (depending on the spec) /should/ be sent.
(So unless there's some deeper bug in OpenBSD, my KVM switch is totally
dropping the ball either way. I'll probably post about it on Misc later,
unless details are desired here...)
But I'm curious if there's a reason for sending two separate packets
when one should be fine. Do certain USB devices expect it that way?
I suppose it seems Windows does it this way too (the only reason the
switch works there is because they neglect to check the vendor string),
so the answer to that could very well be a hard yes...
This is my first post, so sorry in advance if I did anything dumb. Diff
provided for context.
Index: usb_subr.c
===================================================================
RCS file: /cvs/src/sys/dev/usb/usb_subr.c,v
retrieving revision 1.158
diff -u -p -r1.158 usb_subr.c
--- usb_subr.c 16 Feb 2022 06:23:42 -0000 1.158
+++ usb_subr.c 21 Aug 2022 03:33:10 -0000
@@ -125,20 +125,17 @@ usbd_get_string_desc(struct usbd_device
req.bRequest = UR_GET_DESCRIPTOR;
USETW2(req.wValue, UDESC_STRING, sindex);
USETW(req.wIndex, langid);
- USETW(req.wLength, 2); /* size and descriptor type first */
+ USETW(req.wLength, sizeof(*sdesc)); /* size and descriptor type
first */
+
err = usbd_do_request_flags(dev, &req, sdesc, USBD_SHORT_XFER_OK,
&actlen, USBD_DEFAULT_TIMEOUT);
+
if (err)
return (err);
if (actlen < 2)
return (USBD_SHORT_XFER);
- USETW(req.wLength, sdesc->bLength); /* the whole string */
- err = usbd_do_request_flags(dev, &req, sdesc, USBD_SHORT_XFER_OK,
- &actlen, USBD_DEFAULT_TIMEOUT);
- if (err)
- return (err);
if (actlen != sdesc->bLength) {
DPRINTFN(-1, ("%s: expected %d, got %d\n", __func__,
