sunxi: video: Force h/vsync active high when using ext. vga dac on some boards

On both my A13-OLinuxIno and my A13-OLinuxIno-Micro, the vga output gives an
unstable image when active low v or hsync is used.

The problem seems to be specific to the OLinuxIno A13 (normal & micro)
boards. I've just looked up the schematics and they use an opendrain driver
for the vga sync lines, and with sync pulses it is the logical high->low
edge of the pulse which counts for the timing, which with an active low
sync is being driven by the pull-up, and that simply seems to not drive
it hard enough to get a stable image.

So force v and hsync active high on these boards. independent of what the
modeline says. This fixes the unstable image.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>

1 files changed, 10 insertions, 0 deletions

diff --git a/board/sunxi/Kconfig b/board/sunxi/Kconfig
index 4b31dcca20..adbd2b5716 100644
--- a/board/sunxi/Kconfig
+++ b/board/sunxi/Kconfig
@@ -185,6 +185,16 @@ config VIDEO_VGA_VIA_LCD
 	LCD interface driving a VGA connector, such as found on the
 	Olimex A13 boards.
 
+config VIDEO_VGA_VIA_LCD_FORCE_SYNC_ACTIVE_HIGH
+	boolean "Force sync active high for VGA via LCD controller support"
+	depends on VIDEO_VGA_VIA_LCD
+	default n
+	---help---
+	Say Y here if you've a board which uses opendrain drivers for the vga
+	hsync and vsync signals. Opendrain drivers cannot generate steep enough
+	positive edges for a stable video output, so on boards with opendrain
+	drivers the sync signals must always be active high.
+
 config VIDEO_VGA_EXTERNAL_DAC_EN
 	string "LCD panel power enable pin"
 	depends on VIDEO_VGA_VIA_LCD