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    Subject[ 083/221] drm/i915: dont disable disconnected outputs
    Date
    3.7-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Daniel Vetter <daniel.vetter@ffwll.ch>

    commit b0a2658acb5bf9ca86b4aab011b7106de3af0add upstream.

    This piece of neat lore has been ported painstakingly and bug-for-bug
    compatible from the old crtc helper code.

    Imo it's utter nonsense.

    If you disconnected a cable and before you reconnect it, userspace (or
    the kernel) does an set_crtc call, this will result in that connector
    getting disabled. Which will result in a nice black screen when
    plugging in the cable again.

    There's absolutely no reason the kernel does such policy enforcements
    - if userspace tries to set up a mode on something disconnected we
    might fail loudly (since the dp link training fails), but silently
    adjusting the output configuration behind userspace's back is a recipe
    for disaster. Specifically I think that this could explain some of our
    MI_WAIT hangs around suspend, where userspace issues a scanline wait
    on a disable pipe. This mechanisims here could explain how that pipe
    got disabled without userspace noticing.

    Note that this fixes a NULL deref at BIOS takeover when the firmware
    sets up a disconnected output in a clone configuration with a
    connected output on the 2nd pipe: When doing the full modeset we don't
    have a mode for the 2nd pipe and OOPS. On the first pipe this doesn't
    matter, since at boot-up the fbdev helpers will set up the choosen
    configuration on that on first. Since this is now the umptenth bug
    around handling this imo brain-dead semantics correctly, I think it's
    time to kill it and see whether there's any userspace out there which
    relies on this.

    It also nicely demonstrates that we have a tiny window where DP
    hotplug can still kill the driver.

    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=58396
    Tested-by: Peter Ujfalusi <peter.ujfalusi@gmail.com>
    Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
    Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/gpu/drm/i915/intel_display.c | 4 ----
    1 file changed, 4 deletions(-)

    --- a/drivers/gpu/drm/i915/intel_display.c
    +++ b/drivers/gpu/drm/i915/intel_display.c
    @@ -7298,10 +7298,6 @@ intel_modeset_stage_output_state(struct
    DRM_DEBUG_KMS("encoder changed, full mode switch\n");
    config->mode_changed = true;
    }
    -
    - /* Disable all disconnected encoders. */
    - if (connector->base.status == connector_status_disconnected)
    - connector->new_encoder = NULL;
    }
    /* connector->new_encoder is now updated for all connectors. */




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