From 0c7517f0ecd99d347821f62d4446e4a5c0d6ba00 Mon Sep 17 00:00:00 2001
From: P. J. McDermott <pj@pehjota.net>
Date: Sun, 20 Sep 2015 07:47:18 -0400
Subject: docs/hcl/gm45_lcd.html: Describe solution to LVDS channels problem

---
(limited to 'docs/hcl/gm45_lcd.html')

diff --git a/docs/hcl/gm45_lcd.html b/docs/hcl/gm45_lcd.html
index 137bc68..7ffbda4 100644
--- a/docs/hcl/gm45_lcd.html
+++ b/docs/hcl/gm45_lcd.html
@@ -64,6 +64,47 @@
 				lvds_channel_mode:Specify LVDS channel mode (0=probe BIOS [default], 1=single-channel, 2=dual-channel) (int)
 				- from /sbin/modinfo i915
 			</p>
+		<h1>
+			Proper solution to LVDS second channel enablement problem
+		</h1>
+			<p>
+				The definition of <b>register "gfx.lvds_dual_channel" = "0"</b> in
+				<b>coreboot/src/mainboard/lenovo/t400/devicetree.cb</b> in libreboot affects the
+				<b>int lvds_dual_channel</b> member of a <b>struct i915_gpu_controller_info</b>
+				(defined in <b>coreboot/src/drivers/intel/gma/i915.h</b>) named <b>gfx</b> in a
+				<b>struct northbridge_intel_gm45_config</b> (defined in
+				<b>coreboot/src/northbridge/intel/gm45/chip.h</b>).  That
+				<b>gfx.lvds_dual_channel</b> in that <b>struct northbridge_intel_gm45_config</b>
+				affects various registers set in <b>intel_gma_init()</b> in
+				<b>coreboot/src/northbridge/intel/gm45/gma.c</b>.
+			</p>
+			<p>
+				A proper solution would likely be as follows:
+			</p>
+			<ul>
+				<li>
+					Remove <b>gfx.lvds_dual_channel</b> from the <b>devicetree.cb</b> files of
+					GM45-based mainboards.
+				</li>
+				<li>
+					Remove the <b>gfx</b> member of <b>struct northbridge_intel_gm45_config</b> in
+					<b>coreboot/src/northbridge/intel/gm45/chip.h</b>.
+				</li>
+				<li>
+					In <b>intel_gma_init()</b> in <b>coreboot/src/northbridge/intel/gm45/gma.c</b>,
+					somehow detect whether both LVDS channels should be used (maybe if the
+					resolution as obtained from the decoded EDID data is greater than 1280x800
+					pixels) and set a local variable <b>lvds_dual_channel</b> accordingly.  Use that
+					variable instead of <b>info->gfx.lvds_dual_channel</b>.
+				</li>
+			</ul>
+			<p>
+				Note that this doesn't completely solve the problem for those dual-channel
+				displays and that GRUB still won't have graphics output.  More work is still
+				needed to investigate why coreboot's graphics initialization code for GM45
+				doesn't properly handle dual-channel displays (but the code for Sandy/Ivy Bridge
+				apparently does, as does Linux's i915 driver) and resolve that.
+			</p>
 	</div>
 
 	<div class="section">
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