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

[Lyude Paul <lyude@redhat.com>]

On Mon, 2021-01-25 at 19:36 +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@redhat.com>
> Cc: Ville Syrjala <ville.syrjala@intel.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Imre Deak <imre.deak@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,
> +                                                                      

This patch looks fine, however you should take care to also update the
documentation for drm_dp_atomic_find_vcpi_slots() so that it mentiones that
pbn_div should be DSC aware but also is not exclusive to systems supporting DSC
over MST (see the docs for the @pbn_div parameter)

Thank you for doing this! I've been meaning to fix the WD19 issues for a while
now but have been too bogged down by other stuff to spend any time on MST
recently.

>         crtc_state->lane_count));
>                 if (slots == -EDEADLK)
>                         return slots;
>                 if (slots >= 0)

-- 
Sincerely,
   Lyude Paul (she/her)
   Software Engineer at Red Hat
   
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