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

[Johan Hovold <johan@kernel.org>]

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.

Johan