• Lyude Paul's avatar
    drm/dp_mst: Skip validating ports during destruction, just ref · c54c7374
    Lyude Paul authored
    Jerry Zuo pointed out a rather obscure hotplugging issue that it seems I
    accidentally introduced into DRM two years ago.
    
    Pretend we have a topology like this:
    
    |- DP-1: mst_primary
       |- DP-4: active display
       |- DP-5: disconnected
       |- DP-6: active hub
          |- DP-7: active display
          |- DP-8: disconnected
          |- DP-9: disconnected
    
    If we unplug DP-6, the topology starting at DP-7 will be destroyed but
    it's payloads will live on in DP-1's VCPI allocations and thus require
    removal. However, this removal currently fails because
    drm_dp_update_payload_part1() will (rightly so) try to validate the port
    before accessing it, fail then abort. If we keep going, eventually we
    run the MST hub out of bandwidth and all new allocations will start to
    fail (or in my case; all new displays just start flickering a ton).
    
    We could just teach drm_dp_update_payload_part1() not to drop the port
    ref in this case, but then we also need to teach
    drm_dp_destroy_payload_step1() to do the same thing, then hope no one
    ever adds anything to the that requires a validated port reference in
    drm_dp_destroy_connector_work(). Kind of sketchy.
    
    So let's go with a more clever solution: any port that
    drm_dp_destroy_connector_work() interacts with is guaranteed to still
    exist in memory until we say so. While said port might not be valid we
    don't really care: that's the whole reason we're destroying it in the
    first place! So, teach drm_dp_get_validated_port_ref() to use the all
    mighty current_work() function to avoid attempting to validate ports
    from the context of mgr->destroy_connector_work. I can't see any
    situation where this wouldn't be safe, and this avoids having to play
    whack-a-mole in the future of trying to work around port validation.
    Signed-off-by: default avatarLyude Paul <lyude@redhat.com>
    Fixes: 263efde3 ("drm/dp/mst: Get validated port ref in drm_dp_update_payload_part1()")
    Reported-by: default avatarJerry Zuo <Jerry.Zuo@amd.com>
    Cc: Jerry Zuo <Jerry.Zuo@amd.com>
    Cc: Harry Wentland <Harry.Wentland@amd.com>
    Cc: <stable@vger.kernel.org> # v4.6+
    Reviewed-by: default avatarDave Airlie <airlied@redhat.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20181113224613.28809-1-lyude@redhat.comSigned-off-by: default avatarSean Paul <seanpaul@chromium.org>
    c54c7374
drm_dp_mst_topology.c 90.1 KB