Jörg-Volker Peetz wrote on 09/08/2014 17:17:
> Since 3.16 the builtin USB video cam of an HP Pavilion dv7 laptop shows only a
> black screen with the application guvcview although the video LED is turned on.
> There is no hint in dmesg. Also the output of guvcview shows no conspicuousness.
> 
> With kernel version 3.15.9 the camera works.
> 
> I tried to bisect between good v3.15 and bad v3.16, which identified
> 
> commit 9262c19d14c433a6a1ba25c3ff897cb89e412309
> Author: Dan Williams <dan.j.williams@intel.com>
> Date:   Tue May 20 18:08:12 2014 -0700
> 
>     usb: disable port power control if not supported in wHubCharacteristics
> 
>     A hub indicates whether it supports per-port power control via the
>     wHubCharacteristics field in its descriptor.  If it is not supported
>     a hub will still emulate ClearPortPower(PORT_POWER) requests by
>     stopping the link state machine.  However, since this does not save
>     power do not bother suspending.
> 
>     This also consolidates support checks into a
>     hub_is_port_power_switchable() helper.
> 
>     Acked-by: Alan Stern <stern@rowland.harvard.edu>
>     Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>     Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> In the bad bisection steps the video device even was not found. Therefore, I'm
> not sure if I identified the right culprit.
> 

As you suggested Dan, I reverted this commit on top of 3.16.0 (the commit could
not be reverted cleanly, I had to do it manually, patch appended). But still I
see a black picture from the camera. So this commit is not the culprit.

How to proceed with the bisection?
-- 
Regards,
Jörg-Volker.