Actually - ignore this patch, I'm going to do a v3 because i just noticed
there is something very silly and broken I just introduced into the disable
codepath

On Thu, 2018-04-05 at 16:36 -0400, Lyude Paul wrote:
> When doing a modeset where the sink is transitioning from D3 to D0 , it
> would sometimes be possible for the initial power_up_phy() to start
> timing out. This would only be observed in the last action before the
> sink went into D3 mode was intel_dp_sink_dpms(DRM_MODE_DPMS_OFF). We
> originally thought this might be an issue with us accidentally shutting
> off the aux block when putting the sink into D3, but since the DP spec
> mandates that sinks must wake up within 1ms while we have 100ms to
> respond to an ESI irq, this didn't really add up. Turns out that the
> problem is more subtle then that:
> 
> It turns out that the timeout is from us not enabling DPMS on the MST
> hub before actually trying to initiate sideband communications. This
> would cause the first sideband communication (power_up_phy()), to start
> timing out because the sink wasn't ready to respond. Afterwards, we
> would call intel_dp_sink_dpms(DRM_MODE_DPMS_ON) in
> intel_ddi_pre_enable_dp(), which would actually result in waking up the
> sink so that sideband requests would work again.
> 
> Since DPMS is what lets us actually bring the hub up into a state where
> sideband communications become functional again, we just need to make
> sure to enable DPMS on the display before attempting to perform sideband
> communications.
> 
> Changes since v1:
> - Remove comment above if (!intel_dp->is_mst) - vsryjala
> - Move intel_dp_sink_dpms() for MST into intel_dp_post_disable_mst() to
>   keep enable/disable paths symmetrical
> - Improve commit message - dhnkrn
> 
> Signed-off-by: Lyude Paul <ly...@redhat.com>
> Cc: Dhinakaran Pandiyan <dhinakaran.pandi...@intel.com>
> Cc: Ville Syrjälä <ville.syrj...@linux.intel.com>
> Cc: Laura Abbott <labb...@redhat.com>
> Cc: sta...@vger.kernel.org
> Fixes: ad260ab32a4d9 ("drm/i915/dp: Write to SET_POWER dpcd to enable MST
> hub.")
> ---
> This email should hopefully actually be picked up by patchwork this
> time, hooray!
> 
>  drivers/gpu/drm/i915/intel_ddi.c    | 6 ++++--
>  drivers/gpu/drm/i915/intel_dp_mst.c | 2 ++
>  2 files changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_ddi.c
> b/drivers/gpu/drm/i915/intel_ddi.c
> index a6672a9abd85..c0bf7419e1c1 100644
> --- a/drivers/gpu/drm/i915/intel_ddi.c
> +++ b/drivers/gpu/drm/i915/intel_ddi.c
> @@ -2324,7 +2324,8 @@ static void intel_ddi_pre_enable_dp(struct
> intel_encoder *encoder,
>               intel_prepare_dp_ddi_buffers(encoder, crtc_state);
>  
>       intel_ddi_init_dp_buf_reg(encoder);
> -     intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_ON);
> +     if (!intel_dp->is_mst)
> +             intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_ON);
>       intel_dp_start_link_train(intel_dp);
>       if (port != PORT_A || INTEL_GEN(dev_priv) >= 9)
>               intel_dp_stop_link_train(intel_dp);
> @@ -2427,7 +2428,8 @@ static void intel_ddi_post_disable_dp(struct
> intel_encoder *encoder,
>        * Power down sink before disabling the port, otherwise we end
>        * up getting interrupts from the sink on detecting link loss.
>        */
> -     intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_OFF);
> +     if (!intel_dp->is_mst)
> +             intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_OFF);
>  
>       intel_disable_ddi_buf(encoder);
>  
> diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c
> b/drivers/gpu/drm/i915/intel_dp_mst.c
> index c3de0918ee13..2493bd1e0e59 100644
> --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> @@ -176,6 +176,7 @@ static void intel_mst_post_disable_dp(struct
> intel_encoder *encoder,
>        */
>       drm_dp_send_power_updown_phy(&intel_dp->mst_mgr, connector->port,
>                                    false);
> +     intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_OFF);
>  
>       intel_dp->active_mst_links--;
>  
> @@ -223,6 +224,7 @@ static void intel_mst_pre_enable_dp(struct intel_encoder
> *encoder,
>  
>       DRM_DEBUG_KMS("active links %d\n", intel_dp->active_mst_links);
>  
> +     intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_ON);
>       drm_dp_send_power_updown_phy(&intel_dp->mst_mgr, connector->port,
> true);
>       if (intel_dp->active_mst_links == 0)
>               intel_dig_port->base.pre_enable(&intel_dig_port->base,
-- 
Cheers,
        Lyude Paul
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