From b774480cd6ab59aa8b45fec16df1dbb34d0694d5 Mon Sep 17 00:00:00 2001
From: Hans Petter Selasky <hselasky@FreeBSD.org>
Date: Wed, 27 Jun 2018 12:08:12 +0000
Subject: [PATCH] Improve the kernel's USB descriptor reading function. Some
 USB devices does not allow a partial descriptor readout.

Found by:	bz@
MFC after:	1 week
Sponsored by:	Mellanox Technologies
---
 sys/dev/usb/usb_request.c | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/sys/dev/usb/usb_request.c b/sys/dev/usb/usb_request.c
index 29d9368ac413..fa5c469406cb 100644
--- a/sys/dev/usb/usb_request.c
+++ b/sys/dev/usb/usb_request.c
@@ -990,7 +990,7 @@ usbd_req_get_desc(struct usb_device *udev,
     uint8_t retries)
 {
 	struct usb_device_request req;
-	uint8_t *buf;
+	uint8_t *buf = desc;
 	usb_error_t err;
 
 	DPRINTFN(4, "id=%d, type=%d, index=%d, max_len=%d\n",
@@ -1012,6 +1012,32 @@ usbd_req_get_desc(struct usb_device *udev,
 		err = usbd_do_request_flags(udev, mtx, &req,
 		    desc, 0, NULL, 500 /* ms */);
 
+		if (err != 0 && err != USB_ERR_TIMEOUT &&
+		    min_len != max_len) {
+			/* clear descriptor data */
+			memset(desc, 0, max_len);
+
+			/* try to read full descriptor length */
+			USETW(req.wLength, max_len);
+
+			err = usbd_do_request_flags(udev, mtx, &req,
+			    desc, USB_SHORT_XFER_OK, NULL, 500 /* ms */);
+
+			if (err == 0) {
+				/* verify length */
+				if (buf[0] > max_len)
+					buf[0] = max_len;
+				else if (buf[0] < 2)
+					err = USB_ERR_INVAL;
+
+				min_len = buf[0];
+
+				/* enforce descriptor type */
+				buf[1] = type;
+				goto done;
+			}
+		}
+
 		if (err) {
 			if (!retries) {
 				goto done;
@@ -1022,7 +1048,6 @@ usbd_req_get_desc(struct usb_device *udev,
 
 			continue;
 		}
-		buf = desc;
 
 		if (min_len == max_len) {