• Daniel Vetter's avatar
    drm: Rip out totally bogus vga_switcheroo->can_switch locking · fc8fd40e
    Daniel Vetter authored
    So I just wanted to add a new field to struct drm_device and
    accidentally stumbled over something. According to comments
    dev->open_count is protected by dev->count_lock, but that's totally
    not the case. It's protected by drm_global_mutex.
    
    Unfortunately the vga switcheroo callbacks took this comment at face
    value. The problem is that we can't just take the drm_global_mutex
    because:
    - It would lead to a locking inversion with the driver load/unload
      paths.
    - It wouldn't actually protect anything, for that we'd need to wrap
      the entire vga switcheroo code in the drm_global_mutex. And I'm not
      sure whether that would actually solve anything.
    
    What we probably want is a try_to_grab_switcheroo reference kind of
    thing which is used in the driver's ->open callback. Then we could
    move all that ->can_switch madness into the vga switcheroo core where
    it really belongs.
    
    But since that would amount to real work take the easy way out and
    just add a comment. It's definitely not going to make anything worse
    since doing switcheroo state changes while restarting X just isn't
    recommended. Even though the delayed switching code does exactly that.
    
    v2:
    - Simplify the ->can_switch implementations more (Thierry)
    - Fix comment about the dev->open_count locking (Thierry)
    
    Cc: Thierry Reding <treding@nvidia.com>
    Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> (v1)
    Reviewed-by: default avatarThierry Reding <treding@nvidia.com>
    Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
    fc8fd40e
drmP.h 54.2 KB