Fix E6400 display issue with 1440 x 900 panel

The E6400 uses a 100 MHz reference clock on DPLL_REF_SSCLK, whereas
libgfxinit assumed that the reference was always 96 MHz. The frequency
difference caused by a 100 MHz reference with PLL config values
calculated assuming a 96 MHz reference were not significant enough to
cause noticable issues with the more common 1280 x 800 panels, but are
enough to matter for the 1440 x 900 panels which use a higher pixel
clock. This only affected the pre-OS graphics environment provided by
libgfxinit, as Linux drivers would determine the reference clock
frequency based on data in the VBT.

Fix this by making the reference clock frequency in libgfxinit
configurable for GM45 based on a new coreboot Kconfig, which is set to
100 MHz for the E6400.

Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>

1 files changed, 36 insertions, 0 deletions

diff --git a/config/coreboot/dell/patches/0010-mb-dell-e6400-Use-100-MHz-reference-clock-for-displa.patch b/config/coreboot/dell/patches/0010-mb-dell-e6400-Use-100-MHz-reference-clock-for-displa.patch
new file mode 100644
index 00000000..7f85d0ed
--- /dev/null
+++ b/config/coreboot/dell/patches/0010-mb-dell-e6400-Use-100-MHz-reference-clock-for-displa.patch
@@ -0,0 +1,36 @@
+From c35d431a0f1b326b18cd5bbb7d1d49e67d3309bd Mon Sep 17 00:00:00 2001
+From: Nicholas Chin <nic.c3.14@gmail.com>
+Date: Mon, 20 May 2024 10:24:16 -0600
+Subject: [PATCH] mb/dell/e6400: Use 100 MHz reference clock for display
+
+The E6400 uses a 100 MHz reference clock for spread spectrum support on
+LVDS, whereas libgfxinit previously assumed a 96 MHz input clock. For
+the more common 1280 x 800 display panels, the numerical error was not
+large enough to cause noticable issues, but the actual pixel clock
+frequency derived from a 100 MHz reference using PLL configs calculated
+assuming a 96 MHz reference was not close enough for 1440 x 900 panels,
+which require a much higher pixel clock. This resulted in a garbled
+display in the pre-OS graphics environment provided by libgfxinit.
+
+Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
+---
+ src/mainboard/dell/e6400/Kconfig | 3 +++
+ 1 file changed, 3 insertions(+)
+
+diff --git a/src/mainboard/dell/e6400/Kconfig b/src/mainboard/dell/e6400/Kconfig
+index 034de4be2b..4cb16af697 100644
+--- a/src/mainboard/dell/e6400/Kconfig
++++ b/src/mainboard/dell/e6400/Kconfig
+@@ -17,6 +17,9 @@ config BOARD_SPECIFIC_OPTIONS
+ 	select INTEL_GMA_HAVE_VBT
+ 	select EC_DELL_MEC5035
+ 
++config INTEL_GMA_DPLL_REF_FREQ
++	default 100000000
++
+ config MAINBOARD_DIR
+ 	default "dell/e6400"
+ 
+-- 
+2.45.1
+