[Intel-gfx] [PATCH v2 1/2] drm/i915/mst: Do not retrain new links

[Nathan Ciobanu]

On Wed, Jul 18, 2018 at 10:19:42AM -0700, Dhinakaran Pandiyan wrote:
> The short pulse handler checks if channel equalization is okay and
> goes onto retrain a link if there are active MST links. This retraining
> path is not meant for new MST connections, but due to a bug elsewhere, if
> active_mst_links is < 0 the boolean check for active_mst_links passes and
> we proceed to retrain a new link. This results in a sequence of failed link
> training attempts, most likely due to the hardware not setup for link
> training at that point i.e., missing the DDI pre_enable sequence.
> 
> [   80.301272] [drm:intel_dp_check_mst_status] channel EQ not ok, retraining
> [   80.301312] [drm:intel_ddi_prepare_link_retrain] *ERROR* Timeout waiting for DDI BUF C idle bit
> 
> The above error gives us a hint something went wrong before link
> training started.
> 
> Check for a positive value of active_mst_links and throw in a warning for
> invalid active_mst_links as debug aid.
> 
> Cc: Nathan Ciobanu <nathan.d.ciobanu at linux.intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
Tested-by: Nathan Ciobanu <nathan.d.ciobanu at linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index b45b08420c1f..2d61ff01cf51 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -4213,12 +4213,14 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
>  		int ret = 0;
>  		int retry;
>  		bool handled;
> +
> +		WARN_ON_ONCE(intel_dp->active_mst_links < 0);
>  		bret = intel_dp_get_sink_irq_esi(intel_dp, esi);
>  go_again:
>  		if (bret == true) {
>  
>  			/* check link status - esi[10] = 0x200c */
> -			if (intel_dp->active_mst_links &&
> +			if (intel_dp->active_mst_links > 0 &&
>  			    !drm_dp_channel_eq_ok(&esi[10], intel_dp->lane_count)) {
>  				DRM_DEBUG_KMS("channel EQ not ok, retraining\n");
>  				intel_dp_start_link_train(intel_dp);
> -- 
> 2.17.1
>