Re: [PATCH 04/17] drm/i915/dp: Rework pipe joiner logic in mode_valid

[Imre Deak <imre.deak@intel.com>]

On Mon, Feb 02, 2026 at 02:54:25PM +0530, Nautiyal, Ankit K wrote:
> 
> On 2/2/2026 2:10 PM, Imre Deak wrote:
> > On Fri, Jan 30, 2026 at 01:47:59PM +0530, Ankit Nautiyal wrote:
> > > Currently in intel_dp_mode_valid(), we compute the number of joined pipes
> > > required before deciding whether DSC is needed. This ordering prevents us
> > > from accounting for DSC-related overhead when determining pipe
> > > requirements.
> > > 
> > > It is not possible to first decide whether DSC is needed and then compute
> > > the required number of joined pipes, because the two depend on each other:
> > > 
> > >   - DSC need is a function of the pipe count (e.g., 4‑pipe always requires
> > >     DSC; 2‑pipe may require it if uncompressed joiner is unavailable).
> > > 
> > >   - Whether a given pipe‑join configuration is sufficient depends on
> > >     effective bandwidth, which itself changes when DSC is used.
> > > 
> > > As a result, the only correct approach is to iterate candidate pipe counts.
> > > 
> > > So, refactor the logic to start with a single pipe and incrementally try
> > > additional pipes only if needed. While DSC overhead is not yet computed
> > > here, this restructuring prepares the code to support that in a follow-up
> > > changes.
> > > 
> > > If a forced joiner configuration is present, we just check for that
> > > configuration. If it fails, we bailout and return instead of trying with
> > > other joiner configurations.
> > > 
> > > v2:
> > >   - Iterate over number of pipes to be joined instead of joiner
> > >     candidates. (Jani)
> > >   - Document the rationale of iterating over number of joined pipes.
> > >     (Imre)
> > > v3:
> > >   - In case the force joiner configuration doesn't work, do not fallback
> > >     to the normal routine, bailout instead of trying other joiner
> > >     configurations. (Imre)
> > > v4:
> > >   - Use num_joined_pipes instead of num_pipes. (Imre)
> > >   - Inititialize status before the loops starts. (Imre)
> > > 
> > > Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> > > Reviewed-by: Imre Deak <imre.deak@intel.com>
> > There is still one issue, see below.
> > 
> > > ---
> > >   drivers/gpu/drm/i915/display/intel_dp.c | 135 ++++++++++++++++--------
> > >   1 file changed, 89 insertions(+), 46 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> > > index 4c3a1b6d0015..dbe63efc1694 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > > @@ -1434,6 +1434,23 @@ bool intel_dp_has_dsc(const struct intel_connector *connector)
> > >   	return true;
> > >   }
> > > +static
> > > +bool intel_dp_can_join(struct intel_display *display,
> > > +		       int num_joined_pipes)
> > > +{
> > > +	switch (num_joined_pipes) {
> > > +	case 1:
> > > +		return true;
> > > +	case 2:
> > > +		return HAS_BIGJOINER(display) ||
> > > +		       HAS_UNCOMPRESSED_JOINER(display);
> > > +	case 4:
> > > +		return HAS_ULTRAJOINER(display);
> > > +	default:
> > > +		return false;
> > > +	}
> > > +}
> > > +
> > >   static enum drm_mode_status
> > >   intel_dp_mode_valid(struct drm_connector *_connector,
> > >   		    const struct drm_display_mode *mode)
> > > @@ -1445,7 +1462,6 @@ intel_dp_mode_valid(struct drm_connector *_connector,
> > >   	const struct drm_display_mode *fixed_mode;
> > >   	int target_clock = mode->clock;
> > >   	int max_rate, mode_rate, max_lanes, max_link_clock;
> > > -	int max_dotclk = display->cdclk.max_dotclk_freq;
> > >   	u16 dsc_max_compressed_bpp = 0;
> > >   	u8 dsc_slice_count = 0;
> > >   	enum drm_mode_status status;
> > > @@ -1488,66 +1504,93 @@ intel_dp_mode_valid(struct drm_connector *_connector,
> > >   					   target_clock, mode->hdisplay,
> > >   					   link_bpp_x16, 0);
> > > -	num_joined_pipes = intel_dp_num_joined_pipes(intel_dp, connector,
> > > -						     mode->hdisplay, target_clock);
> > > -	max_dotclk *= num_joined_pipes;
> > > +	/*
> > > +	 * We cannot determine the required pipe‑join count before knowing whether
> > > +	 * DSC is needed, nor can we determine DSC need without knowing the pipe
> > > +	 * count.
> > > +	 * Because of this dependency cycle, the only correct approach is to iterate
> > > +	 * over candidate pipe counts and evaluate each combination.
> > > +	 */
> > > +	status = MODE_CLOCK_HIGH;
> > > +	for (num_joined_pipes = 1; num_joined_pipes <= I915_MAX_PIPES; num_joined_pipes++) {
> > > +		int max_dotclk = display->cdclk.max_dotclk_freq;
> > > -	if (target_clock > max_dotclk)
> > > -		return MODE_CLOCK_HIGH;
> > > +		if (connector->force_joined_pipes &&
> > > +		    num_joined_pipes != connector->force_joined_pipes)
> > > +			continue;
> > > -	status = intel_pfit_mode_valid(display, mode, output_format, num_joined_pipes);
> > > -	if (status != MODE_OK)
> > > -		return status;
> > > +		if (!intel_dp_can_join(display, num_joined_pipes))
> > > +			continue;
> > > -	if (intel_dp_has_dsc(connector)) {
> > > -		int pipe_bpp;
> > > +		if (mode->hdisplay > num_joined_pipes * intel_dp_max_hdisplay_per_pipe(display))
> > > +			continue;
> > > -		/*
> > > -		 * TBD pass the connector BPC,
> > > -		 * for now U8_MAX so that max BPC on that platform would be picked
> > > -		 */
> > > -		pipe_bpp = intel_dp_dsc_compute_max_bpp(connector, U8_MAX);
> > > +		status = intel_pfit_mode_valid(display, mode, output_format, num_joined_pipes);
> > > +		if (status != MODE_OK)
> > > +			continue;
> > I missed it in my review of this particular patch, even though
> > I did mention the similar issue elsewhere:
> > 
> > status is guaranteed to be MODE_OK at this point and then ...
> 
> 
> Oh yes this was not a problem earlier as I was setting status =
> MODE_CLOCK_HIGH inside the loop.

It was a problem even then, if this continue happened in the last
iteration.

> 
> Thanks for catching this, will fix this in this patch and the patch#8 and
> re-send.
> 
> 
> Regards,
> 
> Ankit
> 
> > 
> > > -		/*
> > > -		 * Output bpp is stored in 6.4 format so right shift by 4 to get the
> > > -		 * integer value since we support only integer values of bpp.
> > > -		 */
> > > -		if (intel_dp_is_edp(intel_dp)) {
> > > -			dsc_max_compressed_bpp =
> > > -				drm_edp_dsc_sink_output_bpp(connector->dp.dsc_dpcd) >> 4;
> > > +		if (intel_dp_has_dsc(connector)) {
> > > +			int pipe_bpp;
> > > -			dsc_slice_count =
> > > -				intel_dp_dsc_get_slice_count(connector,
> > > -							     target_clock,
> > > -							     mode->hdisplay,
> > > -							     num_joined_pipes);
> > > +			/*
> > > +			 * TBD pass the connector BPC,
> > > +			 * for now U8_MAX so that max BPC on that platform would be picked
> > > +			 */
> > > +			pipe_bpp = intel_dp_dsc_compute_max_bpp(connector, U8_MAX);
> > > -			dsc = dsc_max_compressed_bpp && dsc_slice_count;
> > > -		} else if (drm_dp_sink_supports_fec(connector->dp.fec_capability)) {
> > > -			unsigned long bw_overhead_flags = 0;
> > > +			/*
> > > +			 * Output bpp is stored in 6.4 format so right shift by 4 to get the
> > > +			 * integer value since we support only integer values of bpp.
> > > +			 */
> > > +			if (intel_dp_is_edp(intel_dp)) {
> > > +				dsc_max_compressed_bpp =
> > > +					drm_edp_dsc_sink_output_bpp(connector->dp.dsc_dpcd) >> 4;
> > > -			if (!drm_dp_is_uhbr_rate(max_link_clock))
> > > -				bw_overhead_flags |= DRM_DP_BW_OVERHEAD_FEC;
> > > +				dsc_slice_count =
> > > +					intel_dp_dsc_get_slice_count(connector,
> > > +								     target_clock,
> > > +								     mode->hdisplay,
> > > +								     num_joined_pipes);
> > > -			dsc = intel_dp_mode_valid_with_dsc(connector,
> > > -							   max_link_clock, max_lanes,
> > > -							   target_clock, mode->hdisplay,
> > > -							   num_joined_pipes,
> > > -							   output_format, pipe_bpp,
> > > -							   bw_overhead_flags);
> > > +				dsc = dsc_max_compressed_bpp && dsc_slice_count;
> > > +			} else if (drm_dp_sink_supports_fec(connector->dp.fec_capability)) {
> > > +				unsigned long bw_overhead_flags = 0;
> > > +
> > > +				if (!drm_dp_is_uhbr_rate(max_link_clock))
> > > +					bw_overhead_flags |= DRM_DP_BW_OVERHEAD_FEC;
> > > +
> > > +				dsc = intel_dp_mode_valid_with_dsc(connector,
> > > +								   max_link_clock, max_lanes,
> > > +								   target_clock, mode->hdisplay,
> > > +								   num_joined_pipes,
> > > +								   output_format, pipe_bpp,
> > > +								   bw_overhead_flags);
> > > +			}
> > >   		}
> > > +
> > > +		if (intel_dp_joiner_needs_dsc(display, num_joined_pipes) && !dsc)
> > > +			continue;
> > ... this will continue with status == MODE_OK and the loop can terminate
> > like that. So need a status = MODE_CLOCK_HIGH before continue.
> > 
> > > +
> > > +		if (mode_rate > max_rate && !dsc)
> > This needs a status = MODE_CLOCK_HIGH as well.
> > 
> > With the above fixed:
> > Reviewed-by: Imre Deak <imre.deak@intel.com>
> > 
> > > +			continue;
> > > +
> > > +		status = intel_mode_valid_max_plane_size(display, mode, num_joined_pipes);
> > > +		if (status != MODE_OK)
> > > +			continue;
> > > +
> > > +		max_dotclk *= num_joined_pipes;
> > > +
> > > +		if (target_clock > max_dotclk) {
> > > +			status = MODE_CLOCK_HIGH;
> > > +			continue;
> > > +		}
> > > +
> > > +		break;
> > >   	}
> > > -	if (intel_dp_joiner_needs_dsc(display, num_joined_pipes) && !dsc)
> > > -		return MODE_CLOCK_HIGH;
> > > -
> > > -	status = intel_mode_valid_max_plane_size(display, mode, num_joined_pipes);
> > >   	if (status != MODE_OK)
> > >   		return status;
> > > -	if (mode_rate > max_rate && !dsc)
> > > -		return MODE_CLOCK_HIGH;
> > > -
> > >   	return intel_dp_mode_valid_downstream(connector, mode, target_clock);
> > >   }
> > > -- 
> > > 2.45.2
> > >