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igfx: maintain single/double-channel LVDS mode from BIOS

--- a/sys/src/cmd/aux/vga/igfx.c

+++ b/sys/src/cmd/aux/vga/igfx.c

@@ -850,7 +850,12 @@

 	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);

 		}