Re: [PATCH] USB: Fix: Don't skip endpoint descriptors with maxpacket=0

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, 6 Jan 2020, Johan Hovold wrote:

> On Mon, Jan 06, 2020 at 10:43:42AM -0500, Alan Stern wrote:
> > It turns out that even though endpoints with a maxpacket length of 0
> > aren't useful for data transfer, the descriptors do serve other
> > purposes.  In particular, skipping them will also skip over other
> > class-specific descriptors for classes such as UVC.  This unexpected
> > side effect has caused some UVC cameras to stop working.
> > 
> > In addition, the USB spec requires that when isochronous endpoint
> > descriptors are present in an interface's altsetting 0 (which is true
> > on some devices), the maxpacket size _must_ be set to 0.  Warning
> > about such things seems like a bad idea.
> > 
> > This patch updates an earlier commit which would log a warning and
> > skip these endpoint descriptors.  Now we only log a warning, and we
> > don't even do that for isochronous endpoints in altsetting 0.
> > 
> > We don't need to worry about preventing endpoints with maxpacket = 0
> > from ever being used for data transfers; usb_submit_urb() already
> > checks for this.
> > 
> > Reported-and-tested-by: Roger Whittaker <Roger.Whittaker@suse.com>
> > Fixes: d482c7bb0541 ("USB: Skip endpoints with 0 maxpacket length")
> > Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
> > CC: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
> > Link: https://marc.info/?l=linux-usb&m=157790377329882&w=2
> 
> Acked-by: Johan Hovold <johan@kernel.org>
> 
> We also need
> 
> Cc: stable <stable@vger.kernel.org>
> 
> as d482c7bb0541 ("USB: Skip endpoints with 0 maxpacket length") ended up
> being (auto- ?) selected for stable.

Absolutely -- I had intended to add that CC: but it slipped my mind 
when the email was being prepared.

Alan Stern