Re: [PATCH] drm/i915/dp_mst: Fix encoder HW state readout for UHBR MST

[Jani Nikula <jani.nikula@intel.com>]

On Mon, 24 Feb 2025, Imre Deak <imre.deak@intel.com> wrote:
> The encoder HW/SW state verification should use a SW state which stays
> unchanged while the encoder/output is active. The intel_dp::is_mst flag
> used during state computation to choose between the DP SST/MST modes can
> change while the output is active, if the sink gets disconnected or the
> MST topology is removed for another reason. A subsequent state
> verification using intel_dp::is_mst leads then to a mismatch if the
> output is disabled/re-enabled without recomputing its state.
>
> Use the encoder's active MST link count instead, which will be always
> non-zero for an active MST output and will be zero for SST.
>
> Fixes: 35d2e4b75649 ("drm/i915/ddi: start distinguishing 128b/132b SST and MST at state readout")
> Fixes: 40d489fac0e8 ("drm/i915/ddi: handle 128b/132b SST in intel_ddi_read_func_ctl()")
> Cc: Jani Nikula <jani.nikula@intel.com>
> Signed-off-by: Imre Deak <imre.deak@intel.com>

Do we have a bug for this?

> ---
>  drivers/gpu/drm/i915/display/intel_ddi.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c
> index 7937f4de66cb4..8c8b53414da67 100644
> --- a/drivers/gpu/drm/i915/display/intel_ddi.c
> +++ b/drivers/gpu/drm/i915/display/intel_ddi.c
> @@ -890,7 +890,7 @@ static void intel_ddi_get_encoder_pipes(struct intel_encoder *encoder,
>  			    encoder->base.base.id, encoder->base.name);
>  
>  	if (!mst_pipe_mask && dp128b132b_pipe_mask) {
> -		struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
> +		struct intel_digital_port *dig_port = enc_to_dig_port(encoder);
>  
>  		/*
>  		 * If we don't have 8b/10b MST, but have more than one
> @@ -902,7 +902,8 @@ static void intel_ddi_get_encoder_pipes(struct intel_encoder *encoder,
>  		 * we don't expect MST to have been enabled at that point, and
>  		 * can assume it's SST.
>  		 */
> -		if (hweight8(dp128b132b_pipe_mask) > 1 || intel_dp->is_mst)
> +		if (hweight8(dp128b132b_pipe_mask) > 1 ||
> +		    intel_dp_mst_encoder_active_links(dig_port))

I think the reasoning here is valid, but I think we should probably do
something about the intel_dp_mst_encoder_active_links() naming as
follow-up. This is confusing. What is an "mst encoder"? The above
expects the primary digital port, i.e. it assumes enc_to_dig_port()
already did the right thing. It's all really subtle. Also wrt which
encoders the intel_ddi_get_encoder_pipes() gets called on.

Maybe we need s/intel_dp_mst_encoder/intel_dp_mst_primary/ for this and
some other functions? They're not about the "fake" mst stream encoders.

Anyway,

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


>  			mst_pipe_mask = dp128b132b_pipe_mask;
>  	}
>  
> @@ -4131,13 +4132,13 @@ static void intel_ddi_read_func_ctl(struct intel_encoder *encoder,
>  	} else if (ddi_mode == TRANS_DDI_MODE_SELECT_DP_MST) {
>  		intel_ddi_read_func_ctl_dp_mst(encoder, pipe_config, ddi_func_ctl);
>  	} else if (ddi_mode == TRANS_DDI_MODE_SELECT_FDI_OR_128B132B && HAS_DP20(display)) {
> -		struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
> +		struct intel_digital_port *dig_port = enc_to_dig_port(encoder);
>  
>  		/*
>  		 * If this is true, we know we're being called from mst stream
>  		 * encoder's ->get_config().
>  		 */
> -		if (intel_dp->is_mst)
> +		if (intel_dp_mst_encoder_active_links(dig_port))
>  			intel_ddi_read_func_ctl_dp_mst(encoder, pipe_config, ddi_func_ctl);
>  		else
>  			intel_ddi_read_func_ctl_dp_sst(encoder, pipe_config, ddi_func_ctl);

-- 
Jani Nikula, Intel