[Intel-gfx] [PATCH 25/36] drm/i915: force bpp for eDP panels

[Daniel Vetter]

We'ev had our fair share of woes already which showed that we can't
rely on the bpc limits in the EDID for eDP panels without risking
black screens. So now we limit the depth by what the BIOS recommends
in the VBT:

commit 2f4f649a69a9eb51f6e98130e19dd90a260a4145
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Mon Nov 12 14:33:44 2012 +0200

    drm/i915: do not ignore eDP bpc settings from vbt

But that's not enough, since at least the panel on my ASUS Zenbook
Prime here is also unhappy if the bpc is too low. Hence just take the
firmware value and dither to get what flimsy panels want.

Like before we ensure that we don't change the bpp if the firmware
doesn't proved a value, see

commit 2f4f649a69a9eb51f6e98130e19dd90a260a4145
Author: Jani Nikula <jani.nikula at intel.com>
Date:   Mon Nov 12 14:33:44 2012 +0200

    drm/i915: do not ignore eDP bpc settings from vbt

Cc: Jani Nikula <jani.nikula at intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/intel_dp.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index dff9cc3..0b344a2 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -797,8 +797,15 @@ intel_dp_compute_config(struct intel_encoder *encoder,
 	/* Walk through all bpp values. Luckily they're all nicely spaced with 2
 	 * bpc in between. */
 	bpp = min_t(int, 8*3, pipe_config->pipe_bpp);
-	if (is_edp(intel_dp) && dev_priv->edp.bpp)
-		bpp = min_t(int, bpp, dev_priv->edp.bpp);
+
+	/* eDP panels are really fickle, try to enfore the bpp the firmware
+	 * recomments. This means we'll up-dither 16bpp framebuffers on
+	 * high-depth panels. */
+	if (is_edp(intel_dp) && dev_priv->edp.bpp) {
+		DRM_DEBUG_KMS("forcing bpp for eDP panel to BIOS-provided %i\n",
+			      dev_priv->edp.bpp);
+		bpp = dev_priv->edp.bpp;
+	}
 
 	for (; bpp >= 6*3; bpp -= 2*3) {
 		mode_rate = intel_dp_link_required(target_clock, bpp);
-- 
1.7.11.4