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igfx: perserve frame start delay "magic" bits 27:28 in pipe conf register

it turns out, the "magic" bits 27:28 are the frame start
delay setting that need to be preserved.

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

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

@@ -592,8 +592,11 @@

 static void

 inittrans(Trans *t, Mode *m)

 {

+	/* clear all but 27:28 frame start delay (initialized by bios) */

+	t->conf.v &= 3<<27;

+

 	/* tans/pipe enable */

-	t->conf.v = 1<<31;

+	t->conf.v |= 1<<31;

 	/* trans/pipe timing */

 	t->ht.v = (m->ht - 1)<<16 | (m->x - 1);

@@ -751,14 +754,6 @@

 		error("%s: frequency %d out of range\n", ctlr->name, m->frequency);

 	initpipe(p, m);

-

-	/*

-	 * undocumented magic that makes the flickering

-	 * top bar go away on x230 on lcd. found by

-	 * comparing registers set by vesa bios.

-	 */

-	if(igfx->type == TypeIVB && islvds)

-		p->conf.v |= 3<<27;

 	ctlr->flag |= Finit;

 }

-- 

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