[PATCH 19/20] drm/i915/fbdev: Adjust fbdev stolen mem usage heuristic

[Ville Syrjala]

From: Ville Syrjälä <ville.syrjala at linux.intel.com>

Replace the "1/2 of stolen size" fbdev fb size heuristic with
something a bit more clever, that is ask the FBC code how much
stolen mem it would like to have avaialable for its CFB use.

TODO:
- This doesn't account for the fact that FBC's idea
  usable stolen size might differ from other users of stolen
- Would be nice to share the code with xe, but need to
  figure out how to abstract the stolen size and
  dgpu/lmem stuff

Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
 drivers/gpu/drm/i915/display/intel_fbdev_fb.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_fbdev_fb.c b/drivers/gpu/drm/i915/display/intel_fbdev_fb.c
index 0a6445acb100..29f3241c9270 100644
--- a/drivers/gpu/drm/i915/display/intel_fbdev_fb.c
+++ b/drivers/gpu/drm/i915/display/intel_fbdev_fb.c
@@ -16,12 +16,11 @@ bool intel_fbdev_fb_prefer_stolen(struct intel_display *display,
 {
 	struct drm_i915_private *i915 = to_i915(display->drm);
 
-	/*
-	 * If the FB is too big, just don't use it since fbdev is not very
-	 * important and we should probably use that space with FBC or other
-	 * features.
-	 */
-	return i915->dsm.usable_size >= size * 2;
+	if (size > i915->dsm.usable_size)
+		return false;
+
+	/* try to ensure FBC has enough stolen to do its job well */
+	return i915->dsm.usable_size - size >= intel_fbc_preferred_cfb_size(&i915->display);
 }
 
 struct intel_framebuffer *intel_fbdev_fb_alloc(struct drm_fb_helper *helper,
-- 
2.44.2