On Fri, 3 Jan 2020, Laurent Pinchart wrote:
> I've investigated this a bit further.
>
> UVC defines class-specific interface descriptors that are usually
> located right after the standard interface descriptor in altsetting 0.
> The uvcvideo driver accesses those descriptor through
> intf->altsetting[0].extra. However, some devices insert an isochronous
> endpoint descriptor with wMaxPAcketSize set to 0 between the standard
> interface descriptor and the UVC class-specific interface descriptors.
>
> Before your patch, these descriptors were recorded in the extra field of
> the endpoint, as they're located right after it:
>
> static int usb_parse_endpoint(struct device *ddev, int cfgno, int inum,
> int asnum, struct usb_host_interface *ifp, int num_ep,
> unsigned char *buffer, int size)
> {
> ...
> /* Skip over any Class Specific or Vendor Specific descriptors;
> * find the next endpoint or interface descriptor */
> endpoint->extra = buffer;
> i = find_next_descriptor(buffer, size, USB_DT_ENDPOINT,
> USB_DT_INTERFACE, &n);
> endpoint->extralen = i;
> ...
> }
>
> The uvcvideo driver looks at endpoint->extra when altsetting[0] has no
> extra data.
>
> With your patch, the endpoint is skipped, and the class-specific
> interface descriptors are dropped with it. The uvcvideo driver can't
> access them and fails.
Ah, a very tricky and unexpected interaction!
> One solution may be to add to altsetting[0].extra all the extra
> class-specific descriptors, regardless of whether they're located before
> or after endpoints. I however fear we may not always be able to identify
> those descriptors properly, especially with the CS_INTERFACE descriptor
> type being defined in class specifications, not in the USB core
> specification. There's also a risk of breaking working devices if we do
> so (the uvcvideo driver should be able to cope, but other drivers may
> always look for descriptors in the endpoint).
With the patch I posted yesterday, everything should go back to working
the way it used to. Have you had a chance to test it?
Alan Stern
