[PATCH v5 4/6] drm/i915/dp: Fix link training interrupted by a short HPD pulse

[Jani Nikula]

On Tue, 04 Mar 2025, Imre Deak <imre.deak at intel.com> wrote:
> During Display Port link training the handling of HPD pulses should be
> prevented, as that handling can interfere with the link training:
>
> - Accessing DPCD registers outside the range of link training registers
>   are not allowed by the Standard (see DP Standard v2.1, 3.5.2.16.1,
>   3.6.6.1). The pulse handler reads the DPRX capability registers, which
>   are outside of the allowed range.
> - Switching of the LTTPR transparent/non-transparent mode may reset the
>   LTTPRs on the link, thus aborting any ongoing link training. The pulse
>   handler does set the LTTPR mode, thus it could unexpectedly abort the
>   ongoing link training.
>
> Block/unblock the HPD pulse handling for the duration of the link
> training to prevent the above DPCD register accesses / LTTPR mode
> change.
>
> Apart from the above scenarios, there are other ways a non-link training
> DPCD register could be accessed during link training: via the DRM AUX
> device node, or via DPCD register probing (as performed by
> drm_dp_dpcd_probe()). These will be addressed by a follow-up change.
>
> v2: Rebase on the intel_hpd_suspend/resume -> intel_hpd_block/unblock()
>     rename change.
>
> Signed-off-by: Imre Deak <imre.deak at intel.com>

Reviewed-by: Jani Nikula <jani.nikula at intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_dp_link_training.c | 10 +++++++++-
>  1 file changed, 9 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 2966f5b393922..3906c11acc968 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> @@ -1110,6 +1110,8 @@ intel_dp_128b132b_intra_hop(struct intel_dp *intel_dp,
>  void intel_dp_stop_link_train(struct intel_dp *intel_dp,
>  			      const struct intel_crtc_state *crtc_state)
>  {
> +	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
> +
>  	intel_dp->link_trained = true;
>  
>  	intel_dp_disable_dpcd_training_pattern(intel_dp, DP_PHY_DPRX);
> @@ -1120,6 +1122,8 @@ void intel_dp_stop_link_train(struct intel_dp *intel_dp,
>  	    wait_for(intel_dp_128b132b_intra_hop(intel_dp, crtc_state) == 0, 500)) {
>  		lt_dbg(intel_dp, DP_PHY_DPRX, "128b/132b intra-hop not clearing\n");
>  	}
> +
> +	intel_hpd_unblock(encoder);
>  }
>  
>  static bool
> @@ -1602,7 +1606,11 @@ void intel_dp_start_link_train(struct intel_atomic_state *state,
>  	 * non-transparent mode. During an earlier LTTPR detection this
>  	 * could've been prevented by an active link.
>  	 */
> -	int lttpr_count = intel_dp_init_lttpr_and_dprx_caps(intel_dp);
> +	int lttpr_count;
> +
> +	intel_hpd_block(encoder);
> +
> +	lttpr_count = intel_dp_init_lttpr_and_dprx_caps(intel_dp);
>  
>  	if (lttpr_count < 0)
>  		/* Still continue with enabling the port and link training. */

-- 
Jani Nikula, Intel