usb: fix first descriptor fetch error handling

When fetching the first descriptor from a new device, only validate that
we received at least 8 bytes, not that we received the entire descriptor.
The reasoning is:
- The code only uses fields in the first 8 bytes, so that's all we need
  to have fetched at this stage.
- The smallest maxpacket size is 8 bytes. Before we know the actual
  maxpacket the device uses, the USB controller may only accept a single
  packet (see the DWC2 note in the comment added in the commit).
  Consequently we are only guaranteed to receive 1 packet (at least 8
  bytes) even in a non-error case.

Fixes: 1a7758044b04 ("usb: Early failure when the first descriptor read
fails or is invalid")
Cc: Paul Kocialkowski <contact@paulk.fr>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>

diff --git a/common/usb.c b/common/usb.c
index f6488475a7f22c5db1ef8d61ccd4043299751023..bf76c4159341042a846efda075abe365c12f9833 100644 (file)
--- a/common/usb.c
+++ b/common/usb.c
@@ -956,7 +956,22 @@ int usb_new_device(struct usb_device *dev)
         */
 #ifndef CONFIG_USB_XHCI
        err = usb_get_descriptor(dev, USB_DT_DEVICE, 0, desc, 64);
-       if (err < sizeof(struct usb_device_descriptor)) {
+       /*
+        * Validate we've received only at least 8 bytes, not that we've
+        * received the entire descriptor. The reasoning is:
+        * - The code only uses fields in the first 8 bytes, so that's all we
+        *   need to have fetched at this stage.
+        * - The smallest maxpacket size is 8 bytes. Before we know the actual
+        *   maxpacket the device uses, the USB controller may only accept a
+        *   single packet. Consequently we are only guaranteed to receive 1
+        *   packet (at least 8 bytes) even in a non-error case.
+        *
+        * At least the DWC2 controller needs to be programmed with the number
+        * of packets in addition to the number of bytes. A request for 64
+        * bytes of data with the maxpacket guessed as 64 (above) yields a
+        * request for 1 packet.
+        */
+       if (err < 8) {
                debug("usb_new_device: usb_get_descriptor() failed\n");
                return -EIO;
        }