[Intel-gfx] [PATCH 2/2] drm/i915: Fix the MST PBN divider calculation

[Ville Syrjälä]

On Mon, Jan 25, 2021 at 07:36:36PM +0200, Imre Deak wrote:
> Atm the driver will calculate a wrong MST timeslots/MTP (aka time unit)
> value for MST streams if the link parameters (link rate or lane count)
> are limited in a way independent of the sink capabilities (reported by
> DPCD).
> 
> One example of such a limitation is when a MUX between the sink and
> source connects only a limited number of lanes to the display and
> connects the rest of the lanes to other peripherals (USB).
> 
> Another issue is that atm MST core calculates the divider based on the
> backwards compatible DPCD (at address 0x0000) vs. the extended
> capability info (at address 0x2200). This can result in leaving some
> part of the MST BW unused (For instance in case of the WD19TB dock).
> 
> Fix the above two issues by calculating the PBN divider value based on
> the rate and lane count link parameters that the driver uses for all
> other computation.
> 
> Bugzilla: https://gitlab.freedesktop.org/drm/intel/-/issues/2977
> Cc: Lyude Paul <lyude at redhat.com>
> Cc: Ville Syrjala <ville.syrjala at intel.com>
> Cc: <stable at vger.kernel.org>
> Signed-off-by: Imre Deak <imre.deak at intel.com>

Looks all right to fix some of the immediate problems.

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

> ---
>  drivers/gpu/drm/i915/display/intel_dp_mst.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index d6a1b961a0e8..b4621ed0127e 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -68,7 +68,9 @@ static int intel_dp_mst_compute_link_config(struct intel_encoder *encoder,
>  
>  		slots = drm_dp_atomic_find_vcpi_slots(state, &intel_dp->mst_mgr,
>  						      connector->port,
> -						      crtc_state->pbn, 0);
> +						      crtc_state->pbn,
> +						      drm_dp_get_vc_payload_bw(crtc_state->port_clock,
> +									       crtc_state->lane_count));
>  		if (slots == -EDEADLK)
>  			return slots;
>  		if (slots >= 0)
> -- 
> 2.25.1
> 
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-- 
Ville Syrjälä
Intel