On Thu, 6 Jun 2019, Bollinger, Seth wrote:
> Hello All,
>
> Recently we saw a problem where the device reset will fail due to a
> configuration descriptor check in hub.c:5600.
>
> if (memcmp(buf, udev->rawdescriptors[index], old_length)
> != 0) {
> dev_dbg(&udev->dev, "config index %d changed (#%d)\n",
> index,
> ((struct usb_config_descriptor *) buf)->
> bConfigurationValue);
> changed = 1;
> break;
> }
>
> The descriptors returned from the device have a different iInterface.
> I checked the usb spec and couldn’t find anything that says
> iInterface can’t change. I don’t have the source for the device,
> but I think it’s probably generating the interface string each
> reset and returning a different index for it (“ADB interface”).
>
> Has anyone else seen this? Does the spec guarantee that iInterface
> should never change between device resets?
I have not seen this, and the spec doesn't really guarantee anything
about what happens between device resets.
On the other hand, saying the reset failed in this case is not
unreasonable. The end result is that the device will be re-enumerated
with its new iInterface value.
If this is really a problem we can change the code so that the
iManufacturer, iProduct, iSerialNumber, iConfiguration, and iInterface
descriptor values are exempt from the change check. It would be a
little difficult, though, because we would have to parse the
descriptors to find out where the iInterface values are.
Alan Stern
