[Intel-gfx] [PATCH] drm/i915/dp_mst: Make sure pbn_div is up-to-date after sink reconnect

[Lyude Paul]

Definitely a TODO there that I no longer can remember why I didn't just do
this in the first place, lol. Thank you for the fix

Reviewed-by: Lyude Paul <lyude at redhat.com>

On Fri, 2023-09-29 at 18:49 +0300, Imre Deak wrote:
> Atm the MST encoder config computation may use an out-of-date pbn_div
> value, if the sink is unplugged and a sink is replugged with different
> link rate/lane count capabilities. The current way of reinitializing
> pbn_div depends on pbn_div getting cleared via intel_atomic_check() ->
> drm_dp_mst_atomic_check() ->
> drm_dp_mst_atomic_check_payload_alloc_limits(), however the clearing
> won't happen if the sink got unplugged (and hence
> drm_dp_mst_topology_mgr::mst_state being false).
> 
> To fix the above, simply update pbn_div unconditionally during config
> computation, making pbn_div always match the link rate and lane count.
> 
> Cc: Lyude Paul <lyude at redhat.com>
> Signed-off-by: Imre Deak <imre.deak at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp_mst.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> index 648cf37e02a87..42ddfc85d72d6 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> @@ -94,12 +94,9 @@ static int intel_dp_mst_find_vcpi_slots_for_bpp(struct intel_encoder *encoder,
>  	crtc_state->lane_count = limits->max_lane_count;
>  	crtc_state->port_clock = limits->max_rate;
>  
> -	// TODO: Handle pbn_div changes by adding a new MST helper
> -	if (!mst_state->pbn_div) {
> -		mst_state->pbn_div = drm_dp_get_vc_payload_bw(&intel_dp->mst_mgr,
> -							      crtc_state->port_clock,
> -							      crtc_state->lane_count);
> -	}
> +	mst_state->pbn_div = drm_dp_get_vc_payload_bw(&intel_dp->mst_mgr,
> +						      crtc_state->port_clock,
> +						      crtc_state->lane_count);
>  
>  	for (bpp = max_bpp; bpp >= min_bpp; bpp -= step) {
>  		drm_dbg_kms(&i915->drm, "Trying bpp %d\n", bpp);

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