drm/i915: implement fdi auto-dithering

So on a bunch of setups we only have 2 fdi lanes available, e.g. hsw
VGA or 3 pipes on ivb. And seemingly a lot of modes don't quite fit
into this, among them the default 1080p mode.

The solution is to dither down the pipe a bit so that everything fits,
which this patch implements.

But ports compute their state under the assumption that the bpp they
pick will be the one selected, e.g. the display port bw computations
won't work otherwise. Now we could adjust our code to again up-dither
to the computed DP link parameters, but that's pointless.

So instead when the pipe needs to adjust parameters we need to retry
the pipe_config computation at the encoder stage. Furthermore we need
to inform encoders that they should not increase bandwidth
requirements if possible. This is required for the hdmi code, which
prefers the pipe to up-dither to either of the two possible hdmi bpc
values.

LVDS has a similar requirement, although that's probably only
theoretical in nature: It's unlikely that we'll ever see an 8bpc
high-res lvds panel (which is required to hit the 2 fdi lane limit).

eDP is the only thing which could increase the pipe_bpp setting again,
even when in the retry-loop. This could hit the WARN. Two reasons for
not bothering:
- On many eDP panels we'll get a black screen if the bpp settings
  don't match vbt. So failing the modeset is the right thing to do.
  But since that also means it's the only way to light up the panel,
  it should work. So we shouldn't be able to hit this WARN.
- There are still opens around the eDP panel handling, and maybe we
  need additional tricks. Before that happens it's imo no use trying
  to be too clever.
Worst case we just need to kill that WARN or maybe fail the compute
config stage if the eDP connector can't get the bpp setting it wants.
And since this can only happen with an fdi link in between and so for
pch eDP panels it's rather unlikely to blow up, if ever.

v2: Rebased on top of a bikeshed from Paulo.

v3: Improve commit message around eDP handling with the stuff
things with Imre.
Reviewed-by: NImre Deak <imre.deak@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -4040,13 +4040,16 @@ static bool ironlake_check_fdi_lanes(struct drm_device *dev, enum pipe pipe,
 	}
 }
 
-static bool ironlake_fdi_compute_config(struct intel_crtc *intel_crtc,
-					struct intel_crtc_config *pipe_config)
+#define RETRY 1
+static int ironlake_fdi_compute_config(struct intel_crtc *intel_crtc,
+				       struct intel_crtc_config *pipe_config)
 {
 	struct drm_device *dev = intel_crtc->base.dev;
 	struct drm_display_mode *adjusted_mode = &pipe_config->adjusted_mode;
 	int target_clock, lane, link_bw;
+	bool setup_ok, needs_recompute = false;
 
+retry:
 	/* FDI is a binary signal running at ~2.7GHz, encoding
 	 * each output octet as 10 bits. The actual frequency
 	 * is stored as a divider into a 100MHz clock, and the
@@ -4071,12 +4074,26 @@ static bool ironlake_fdi_compute_config(struct intel_crtc *intel_crtc,
 	intel_link_compute_m_n(pipe_config->pipe_bpp, lane, target_clock,
 			       link_bw, &pipe_config->fdi_m_n);
 
-	return ironlake_check_fdi_lanes(intel_crtc->base.dev,
-					intel_crtc->pipe, pipe_config);
+	setup_ok = ironlake_check_fdi_lanes(intel_crtc->base.dev,
+					    intel_crtc->pipe, pipe_config);
+	if (!setup_ok && pipe_config->pipe_bpp > 6*3) {
+		pipe_config->pipe_bpp -= 2*3;
+		DRM_DEBUG_KMS("fdi link bw constraint, reducing pipe bpp to %i\n",
+			      pipe_config->pipe_bpp);
+		needs_recompute = true;
+		pipe_config->bw_constrained = true;
+
+		goto retry;
+	}
+
+	if (needs_recompute)
+		return RETRY;
+
+	return setup_ok ? 0 : -EINVAL;
 }
 
-static bool intel_crtc_compute_config(struct drm_crtc *crtc,
-				      struct intel_crtc_config *pipe_config)
+static int intel_crtc_compute_config(struct drm_crtc *crtc,
+				     struct intel_crtc_config *pipe_config)
 {
 	struct drm_device *dev = crtc->dev;
 	struct drm_display_mode *adjusted_mode = &pipe_config->adjusted_mode;
@@ -4085,7 +4102,7 @@ static bool intel_crtc_compute_config(struct drm_crtc *crtc,
 		/* FDI link clock is fixed at 2.7G */
 		if (pipe_config->requested_mode.clock * 3
 		    > IRONLAKE_FDI_FREQ * 4)
-			return false;
+			return -EINVAL;
 	}
 
 	/* All interlaced capable intel hw wants timings in frames. Note though
@@ -4099,7 +4116,7 @@ static bool intel_crtc_compute_config(struct drm_crtc *crtc,
 	 */
 	if ((INTEL_INFO(dev)->gen > 4 || IS_G4X(dev)) &&
 		adjusted_mode->hsync_start == adjusted_mode->hdisplay)
-		return false;
+		return -EINVAL;
 
 	if ((IS_G4X(dev) || IS_VALLEYVIEW(dev)) && pipe_config->pipe_bpp > 10*3) {
 		pipe_config->pipe_bpp = 10*3; /* 12bpc is gen5+ */
@@ -4112,7 +4129,7 @@ static bool intel_crtc_compute_config(struct drm_crtc *crtc,
 	if (pipe_config->has_pch_encoder)
 		return ironlake_fdi_compute_config(to_intel_crtc(crtc), pipe_config);
 
-	return true;
+	return 0;
 }
 
 static int valleyview_get_display_clock_speed(struct drm_device *dev)
@@ -7692,7 +7709,8 @@ intel_modeset_pipe_config(struct drm_crtc *crtc,
 	struct drm_encoder_helper_funcs *encoder_funcs;
 	struct intel_encoder *encoder;
 	struct intel_crtc_config *pipe_config;
-	int plane_bpp;
+	int plane_bpp, ret = -EINVAL;
+	bool retry = true;
 
 	pipe_config = kzalloc(sizeof(*pipe_config), GFP_KERNEL);
 	if (!pipe_config)
@@ -7705,6 +7723,7 @@ intel_modeset_pipe_config(struct drm_crtc *crtc,
 	if (plane_bpp < 0)
 		goto fail;
 
+encoder_retry:
 	/* Pass our mode to the connectors and the CRTC to give them a chance to
 	 * adjust it according to limitations or connector properties, and also
 	 * a chance to reject the mode entirely.
@@ -7733,10 +7752,23 @@ intel_modeset_pipe_config(struct drm_crtc *crtc,
 		}
 	}
 
-	if (!(intel_crtc_compute_config(crtc, pipe_config))) {
+	ret = intel_crtc_compute_config(crtc, pipe_config);
+	if (ret < 0) {
 		DRM_DEBUG_KMS("CRTC fixup failed\n");
 		goto fail;
 	}
+
+	if (ret == RETRY) {
+		if (WARN(!retry, "loop in pipe configuration computation\n")) {
+			ret = -EINVAL;
+			goto fail;
+		}
+
+		DRM_DEBUG_KMS("CRTC bw constrained, retrying\n");
+		retry = false;
+		goto encoder_retry;
+	}
+
 	DRM_DEBUG_KMS("[CRTC:%d]\n", crtc->base.id);
 
 	pipe_config->dither = pipe_config->pipe_bpp != plane_bpp;
@@ -7746,7 +7778,7 @@ intel_modeset_pipe_config(struct drm_crtc *crtc,
 	return pipe_config;
 fail:
 	kfree(pipe_config);
-	return ERR_PTR(-EINVAL);
+	return ERR_PTR(ret);
 }
 
 /* Computes which crtcs are affected and sets the relevant bits in the mask. For

drivers/gpu/drm/i915/intel_drv.h

@@ -223,6 +223,13 @@ struct intel_crtc_config {
 	/* Controls for the clock computation, to override various stages. */
 	bool clock_set;
 
+	/*
+	 * crtc bandwidth limit, don't increase pipe bpp or clock if not really
+	 * required. This is set in the 2nd loop of calling encoder's
+	 * ->compute_config if the first pick doesn't work out.
+	 */
+	bool bw_constrained;
+
 	/* Settings for the intel dpll used on pretty much everything but
 	 * haswell. */
 	struct dpll dpll;

drivers/gpu/drm/i915/intel_hdmi.c

@@ -784,6 +784,7 @@ bool intel_hdmi_compute_config(struct intel_encoder *encoder,
 	struct drm_device *dev = encoder->base.dev;
 	struct drm_display_mode *adjusted_mode = &pipe_config->adjusted_mode;
 	int clock_12bpc = pipe_config->requested_mode.clock * 3 / 2;
+	int desired_bpp;
 
 	if (intel_hdmi->color_range_auto) {
 		/* See CEA-861-E - 5.1 Default Encoding Parameters */
@@ -808,16 +809,21 @@ bool intel_hdmi_compute_config(struct intel_encoder *encoder,
 	 */
 	if (pipe_config->pipe_bpp > 8*3 && clock_12bpc <= 225000
 	    && HAS_PCH_SPLIT(dev)) {
-		DRM_DEBUG_KMS("forcing bpc to 12 for HDMI\n");
-		pipe_config->pipe_bpp = 12*3;
+		DRM_DEBUG_KMS("picking bpc to 12 for HDMI output\n");
+		desired_bpp = 12*3;
 
 		/* Need to adjust the port link by 1.5x for 12bpc. */
 		adjusted_mode->clock = clock_12bpc;
 		pipe_config->pixel_target_clock =
 			pipe_config->requested_mode.clock;
 	} else {
-		DRM_DEBUG_KMS("forcing bpc to 8 for HDMI\n");
-		pipe_config->pipe_bpp = 8*3;
+		DRM_DEBUG_KMS("picking bpc to 8 for HDMI output\n");
+		desired_bpp = 8*3;
+	}
+
+	if (!pipe_config->bw_constrained) {
+		DRM_DEBUG_KMS("forcing pipe bpc to %i for HDMI\n", desired_bpp);
+		pipe_config->pipe_bpp = desired_bpp;
 	}
 
 	if (adjusted_mode->clock > 225000) {

drivers/gpu/drm/i915/intel_lvds.c

@@ -248,7 +248,7 @@ static bool intel_lvds_compute_config(struct intel_encoder *intel_encoder,
 	else
 		lvds_bpp = 6*3;
 
-	if (lvds_bpp != pipe_config->pipe_bpp) {
+	if (lvds_bpp != pipe_config->pipe_bpp && !pipe_config->bw_constrained) {
 		DRM_DEBUG_KMS("forcing display bpp (was %d) to LVDS (%d)\n",
 			      pipe_config->pipe_bpp, lvds_bpp);
 		pipe_config->pipe_bpp = lvds_bpp;