drm/i915: fix FBC buffer size checks

According to my experiments (and later confirmation from the hardware
developers), the maximum sizes mentioned in the specification delimit
how far in the buffer the hardware tracking can go. And the hardware
calculates the size based on the plane address we provide - and the
provided plane address might not be the real x:0,y:0 point due to the
compute_page_offset() function.

On platforms that do the x/y offset adjustment trick it will be really
hard to reproduce a bug, but on the current SKL we can reproduce the
bug with igt/kms_frontbuffer_tracking/fbc-farfromfence. With this
patch, we'll go from "CRC assertion failure" to "FBC unexpectedly
disabled", which is still a failure on the test suite but is not a
perceived user bug - you will just not save as much power as you could
if FBC is disabled.

v2, rewrite patch after clarification from the Hadware guys:
  - Rename function so it's clear what the check is for.
  - Use the new intel_fbc_get_plane_source_sizes() function in order
    to get the proper sizes as seen by FBC.
v3:
  - Rebase after the s/sizes/size/ on the previous patch.
  - Adjust comment wording (Ville).
  - s/used_/effective_/ (Ville).

Testcase: igt/kms_frontbuffer_tracking/fbc-farfromfence (SKL)
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_fbc.c

@@ -799,10 +799,16 @@ static bool pixel_format_is_valid(struct drm_framebuffer *fb)
 	}
 }
 
-static bool pipe_size_is_valid(struct intel_crtc *crtc)
+/*
+ * For some reason, the hardware tracking starts looking at whatever we
+ * programmed as the display plane base address register. It does not look at
+ * the X and Y offset registers. That's why we look at the crtc->adjusted{x,y}
+ * variables instead of just looking at the pipe/plane size.
+ */
+static bool intel_fbc_hw_tracking_covers_screen(struct intel_crtc *crtc)
 {
 	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
-	unsigned int max_w, max_h;
+	unsigned int effective_w, effective_h, max_w, max_h;
 
 	if (INTEL_INFO(dev_priv)->gen >= 8 || IS_HASWELL(dev_priv)) {
 		max_w = 4096;
@@ -815,8 +821,11 @@ static bool pipe_size_is_valid(struct intel_crtc *crtc)
 		max_h = 1536;
 	}
 
-	return crtc->config->pipe_src_w <= max_w &&
-	       crtc->config->pipe_src_h <= max_h;
+	intel_fbc_get_plane_source_size(crtc, &effective_w, &effective_h);
+	effective_w += crtc->adjusted_x;
+	effective_h += crtc->adjusted_y;
+
+	return effective_w <= max_w && effective_h <= max_h;
 }
 
 /**
@@ -893,7 +902,7 @@ static void __intel_fbc_update(struct drm_i915_private *dev_priv)
 		goto out_disable;
 	}
 
-	if (!pipe_size_is_valid(intel_crtc)) {
+	if (!intel_fbc_hw_tracking_covers_screen(intel_crtc)) {
 		set_no_fbc_reason(dev_priv, FBC_MODE_TOO_LARGE);
 		goto out_disable;
 	}