[9front] [PATCH 1/3] igfx: maintain single/double-channel LVDS mode from BIOS

[Michael Forney <mforney@mforney.org>]

---
Not quite sure why single-channel mode doesn't work on my laptop;
I just get a backlit black screen. The linux driver seems to use
whatever the BIOS set, so it seems like we should do the same.

The linux driver also has a parameter i915.lvds_channel_mode to
force single/double channel. I confirmed that when I set
i915.lvds_channel_mode=1, I get a black screen in linux as well.
So I think it may be a hardware limitation rather than something
igfx is doing wrong.

 sys/src/cmd/aux/vga/igfx.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/sys/src/cmd/aux/vga/igfx.c b/sys/src/cmd/aux/vga/igfx.c
index a2537329b..db283587b 100644
--- a/sys/src/cmd/aux/vga/igfx.c
+++ b/sys/src/cmd/aux/vga/igfx.c
@@ -850,7 +850,12 @@ initdpll(Igfx *igfx, int x, int freq, int port)
 	dpll->ctrl.v &= ~(3<<24);
 	if(port == PortLCD){
 		p2 = 14;
-		if(freq > 112*MHz){
+		/*
+		 * Use dual-channel LVDS if the display clock is
+		 * outside the range of single-channel, or it was
+		 * preconfigured by the BIOS.
+		 */
+		if(freq > 112*MHz || (igfx->lvds.v>>4 & 3) == 3){
 			p2 >>= 1;
 			dpll->ctrl.v |= (1<<24);
 		}
-- 
2.34.1