Re: [Intel-gfx] [PATCH v2 02/11] drm/dp_mst: Fix PBN divider calculation for UHBR rates

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Nov 16, 2023 at 03:18:32PM +0200, Imre Deak wrote:
> The current way of calculating the pbn_div value, the link BW per each
> MTP slot, worked only for DP 1.4 link rates. Fix things up for UHBR
> rates calculating with the correct channel coding efficiency based on
> the link rate.
> 
> v2:
> - Return the fractional pbn_div value from drm_dp_get_vc_payload_bw().
> 
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: dri-devel@lists.freedesktop.org
> Signed-off-by: Imre Deak <imre.deak@intel.com>
> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 13 ++++++++++---
>  include/drm/display/drm_dp_helper.h           | 13 +++++++++++++
>  2 files changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index 000d05e80352a..3fbd5585d5e6c 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -3585,14 +3585,18 @@ static int drm_dp_send_up_ack_reply(struct drm_dp_mst_topology_mgr *mgr,
>  fixed20_12 drm_dp_get_vc_payload_bw(const struct drm_dp_mst_topology_mgr *mgr,
>  				    int link_rate, int link_lane_count)
>  {
> +	int ch_coding_efficiency =
> +		drm_dp_bw_channel_coding_efficiency(drm_dp_is_uhbr_rate(link_rate));
>  	fixed20_12 ret;
>  
>  	if (link_rate == 0 || link_lane_count == 0)
>  		drm_dbg_kms(mgr->dev, "invalid link rate/lane count: (%d / %d)\n",
>  			    link_rate, link_lane_count);
>  
> -	/* See DP v2.0 2.6.4.2, VCPayload_Bandwidth_for_OneTimeSlotPer_MTP_Allocation */
> -	ret.full = dfixed_const(link_rate * link_lane_count / 54000);
> +	/* See DP v2.0 2.6.4.2, 2.7.6.3 VCPayload_Bandwidth_for_OneTimeSlotPer_MTP_Allocation */
> +	ret.full = DIV_ROUND_DOWN_ULL(mul_u32_u32(link_rate * link_lane_count,
> +						  ch_coding_efficiency),
> +				      (1000000ULL * 8 * 5400) >> 12);

Seems to produce the correct numbers.

>  
>  	return ret;
>  }
> @@ -4315,6 +4319,7 @@ int drm_dp_atomic_find_time_slots(struct drm_atomic_state *state,
>  	struct drm_dp_mst_atomic_payload *payload = NULL;
>  	struct drm_connector_state *conn_state;
>  	int prev_slots = 0, prev_bw = 0, req_slots;
> +	fixed20_12 req_slots_fp;
>  
>  	topology_state = drm_atomic_get_mst_topology_state(state, mgr);
>  	if (IS_ERR(topology_state))
> @@ -4342,7 +4347,9 @@ int drm_dp_atomic_find_time_slots(struct drm_atomic_state *state,
>  		}
>  	}
>  
> -	req_slots = DIV_ROUND_UP(pbn, dfixed_trunc(topology_state->pbn_div));
> +	req_slots_fp.full = dfixed_div((fixed20_12)dfixed_init(pbn), topology_state->pbn_div);

dfixed_div() looks super dodgy. It seems to have some kind of hand
rolled round to closest behaviour, which would imply that this can
return a rounded down result.

> +	req_slots_fp.full = dfixed_ceil(req_slots_fp);
> +	req_slots = dfixed_trunc(req_slots_fp);
>  
>  	drm_dbg_atomic(mgr->dev, "[CONNECTOR:%d:%s] [MST PORT:%p] TU %d -> %d\n",
>  		       port->connector->base.id, port->connector->name,
> diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
> index c5f1079acb3b1..863b2e7add29e 100644
> --- a/include/drm/display/drm_dp_helper.h
> +++ b/include/drm/display/drm_dp_helper.h
> @@ -252,6 +252,19 @@ drm_edp_backlight_supported(const u8 edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE])
>  	return !!(edp_dpcd[1] & DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP);
>  }
>  
> +/**
> + * drm_dp_is_uhbr_rate - Determine if a link rate is UHBR
> + * @link_rate: link rate in 10kbits/s units
> + *
> + * Determine if the provided link rate is an UHBR rate.
> + *
> + * Returns: %True if @link_rate is an UHBR rate.
> + */
> +static inline bool drm_dp_is_uhbr_rate(int link_rate)
> +{
> +	return link_rate >= 1000000;
> +}
> +
>  /*
>   * DisplayPort AUX channel
>   */
> -- 
> 2.39.2

-- 
Ville Syrjälä
Intel