drm/i915/fdi: Improve FDI BW sharing between pipe B and C

At the moment modesetting pipe C on IVB will fail if pipe B uses 4 FDI
lanes. Make the BW sharing more dynamic by trying to reduce pipe B's
link bpp in this case, until pipe B uses only up to 2 FDI lanes.

For this instead of the encoder compute config retry loop - which
reduced link bpp only for the encoder's pipe - reduce the maximum link
bpp for pipe B/C as required after all CRTC states are computed and
recompute the CRTC states with the new bpp limit.

Atm, all FDI encoder's compute config function returns an error if a BW
constrain prevents increasing the pipe bpp value. The corresponding
crtc_state->bw_constrained check can be replaced with checking
crtc_state->max_link_bpp_x16, add TODO comments for this. SDVO is an
exception where this case is only handled in the outer config retry
loop, failing the modeset with a WARN, add a FIXME comment to handle
this in the encoder code similarly to other encoders.

v2:
- Don't assume that a CRTC is already in the atomic state, while
  reducing its link bpp.
- Add DocBook description to intel_fdi_atomic_check_link().
v3:
- Enable BW management for FDI links in a separate patch. (Ville)
v4: (Ville)
- Fail the SDVO encoder config computation if it doesn't support the
  link bpp limit.
- Add TODO: comments about checking link_bpp_x16 instead of
  bw_constrained.
v5:
- Replace link bpp limit check with a FIXME: comment in
  intel_sdvo_compute_config(). (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[Amended commit message wrt. changes in v5]
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230921195159.2646027-11-imre.deak@intel.com

1 files changed, 7 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_crt.c b/drivers/gpu/drm/i915/display/intel_crt.c
index 595e65f1c13a..d4bad0ddff41 100644
--- a/drivers/gpu/drm/i915/display/intel_crt.c
+++ b/drivers/gpu/drm/i915/display/intel_crt.c
@@ -413,6 +413,9 @@ static int pch_crt_compute_config(struct intel_encoder *encoder,
 		return -EINVAL;
 
 	pipe_config->has_pch_encoder = true;
+	if (!intel_fdi_compute_pipe_bpp(pipe_config))
+		return -EINVAL;
+
 	pipe_config->output_format = INTEL_OUTPUT_FORMAT_RGB;
 
 	return 0;
@@ -435,10 +438,14 @@ static int hsw_crt_compute_config(struct intel_encoder *encoder,
 		return -EINVAL;
 
 	pipe_config->has_pch_encoder = true;
+	if (!intel_fdi_compute_pipe_bpp(pipe_config))
+		return -EINVAL;
+
 	pipe_config->output_format = INTEL_OUTPUT_FORMAT_RGB;
 
 	/* LPT FDI RX only supports 8bpc. */
 	if (HAS_PCH_LPT(dev_priv)) {
+		/* TODO: Check crtc_state->max_link_bpp_x16 instead of bw_constrained */
 		if (pipe_config->bw_constrained && pipe_config->pipe_bpp < 24) {
 			drm_dbg_kms(&dev_priv->drm,
 				    "LPT only supports 24bpp\n");