Re: [PATCH 1/2] drm/i915: Fix modesetting in case of unexpected AUX timeouts

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Apr 13, 2021 at 02:24:12AM +0300, Imre Deak wrote:
> In case AUX failures happen unexpectedly during a modeset, the driver
> should still complete the modeset. In particular the driver should
> perform the link training sequence steps even in case of an AUX failure,
> as this sequence also includes port initialization steps. Not doing that
> can leave the port/pipe in a broken state and lead for instance to a
> flip done timeout.
> 
> Fix this by continuing with link training (in a no-LTTPR mode) if the
> DPRX DPCD readout failed for some reason at the beginning of link
> training. After a successful connector detection we already have the
> DPCD read out and cached, so the failed repeated read for it should not
> cause a problem. Note that a partial AUX read could in theory partly
> overwrite the cached DPCD (and return error) but this overwrite should
> not happen if the returned values are corrupted (due to a timeout or
> some other IO error).
> 
> Kudos to Ville to root cause the problem.
> 
> Fixes: 7dffbdedb96a ("drm/i915: Disable LTTPR support when the DPCD rev < 1.4")
> References: https://gitlab.freedesktop.org/drm/intel/-/issues/3308
> Cc: stable@vger.kernel.org # 5.11
> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> Signed-off-by: Imre Deak <imre.deak@intel.com>

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_dp_link_training.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> index 5e9c3c74310ca..cbcfb0c4c3708 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -882,7 +882,8 @@ void intel_dp_start_link_train(struct intel_dp *intel_dp,
>  	int lttpr_count = intel_dp_init_lttpr_and_dprx_caps(intel_dp);
>  
>  	if (lttpr_count < 0)
> -		return;
> +		/* Still continue with enabling the port and link training. */
> +		lttpr_count = 0;
>  
>  	if (!intel_dp_link_train_all_phys(intel_dp, crtc_state, lttpr_count))
>  		intel_dp_schedule_fallback_link_training(intel_dp, crtc_state);
> -- 
> 2.27.0

-- 
Ville Syrjälä
Intel