Originally by Seth Forshee <seth.forshee@canonical.com>, 2012-10-04:
    During graphics driver initialization it's useful to be able to mux
    only the DDC to the inactive client in order to read the EDID. Add
    a switch_ddc callback to allow capable handlers to provide this
    functionality, and add vga_switcheroo_switch_ddc() to allow DRM
    to mux only the DDC.

Modified by Dave Airlie <airlied@gmail.com>, 2012-12-22:
    I can't figure out why I didn't like this, but I rewrote this [...]
    to lock/unlock the ddc lines [...]. I think I'd prefer something
    like that otherwise the interface got really ugly.

Modified by Lukas Wunner <lukas@wunner.de>, 2015-04 - 2015-10:
    Change semantics of ->switch_ddc handler callback to return previous
    DDC owner. Original version tried to determine previous DDC owner
    with find_active_client() but this fails if the inactive client
    registers before the active client.

    Don't lock vgasr_mutex in _lock_ddc() / _unlock_ddc(), it can cause
    deadlocks because (a) during switch (with vgasr_mutex already held),
    GPU is woken and probes its outputs, tries to re-acquire vgasr_mutex
    to lock DDC lines; (b) Likewise during switch, GPU is suspended and
    calls cancel_delayed_work_sync() to stop output polling, if poll
    task is running at this moment we may wait forever for it to finish.

    Instead, lock mux_hw_lock when unregistering the handler because
    the only reason why we'd want to lock vgasr_mutex in _lock_ddc() /
    _unlock_ddc() is to block the handler from disappearing while DDC
    lines are switched.

    Also acquire mux_hw_lock in stage2 to avoid race condition where
    reading the EDID and switching happens simultaneously. Likewise on
    MIGD / MDIS commands and on runtime suspend.

    v2.1: Overhaul locking, squash commits (Daniel Vetter)

    v2.2: Readability improvements (Thierry Reding)

    v2.3: Overhaul locking once more

    v2.4: Retain semantics of ->switchto handler callback to switch all
          pins, including DDC (Daniel Vetter)

    v5:   Rename ddc_lock to mux_hw_lock: Since we acquire this both
          when calling ->switch_ddc and ->switchto, it protects not just
          access to the DDC lines but to the mux in general. This is in
          line with the DRM convention to use low-level locks to avoid
          concurrent hw access (e.g. i2c, dp_aux) which are often called
          hw_lock (Daniel Vetter)

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=88861
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=61115Tested-by: Lukas Wunner <lukas@wunner.de>
    [MBP  9,1 2012  intel IVB + nvidia GK107  pre-retina  15"]
Cc: Seth Forshee <seth.forshee@canonical.com>
Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/e81ae9722b84c5ed591805fee3ea6dbf5dc6c4b3.1452525860.git.lukas@wunner.de