Re: [Intel-gfx] [PATCH v3] drm/i915/dp: Send DPCD ON for MST before phy_up

["Pandiyan, Dhinakaran" <dhinakaran.pandiyan@intel.com>]

On Thu, 2018-04-05 at 17:19 -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
> Changes since v2:
> - Only send DPMS off when we're disabling the last sink, and only send
>   DPMS on when we're enabling the first sink - dhnkrn
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Cc: Laura Abbott <labbott@redhat.com>
> Cc: stable@vger.kernel.org
> Fixes: ad260ab32a4d9 ("drm/i915/dp: Write to SET_POWER dpcd to enable MST hub.")
> ---
>  drivers/gpu/drm/i915/intel_ddi.c    | 6 ++++--
>  drivers/gpu/drm/i915/intel_dp_mst.c | 8 +++++++-
>  2 files changed, 11 insertions(+), 3 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);



I believe Ville recommended to check for
	is_mst = intel_crtc_has_type(crtc_state, INTEL_OUTPUT_DP_MST)
here. The question is, are there cases where 
intel_dp->is_mst != is_mst? A disconnect in the middle of a modeset
would cause intel_dp->is_mst to be false, wouldn't it?