From patchwork Thu Aug  6 15:12:39 2009
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Date: Thu, 06 Aug 2009 17:12:39 +0200
From: =?ISO-8859-1?Q?Ren=E9_Gabri=EBls?= <renegabriels@gmail.com>
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To: intel-gfx@lists.freedesktop.org
Subject: [Intel-gfx] Patch to fix fdo bug #21064
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The attached patch fixes my garbled screen issue on my Intel 855GM notebook. For
some reason the VGA_2X_MODE bit in the VGACNTRL register is set to 1 on my
machine, causing the video chip to scale up the framebuffer, which results in a
funky screen.

In the patch I forced this bit to zero and the problem is gone.  I'm not sure if
there's anybody who's actually using this 2X_MODE on an 855GM.  However, before
this bug was introduced (git: db9f5915ce812144ffd9d2aa42e8ba856129c35e), this
bit was forced to zero as well.

- René

diff --git a/src/i830_display.c b/src/i830_display.c
index 59ededc..d180b2b 100644
--- a/src/i830_display.c
+++ b/src/i830_display.c
@@ -1096,6 +1096,10 @@ i830_disable_vga_plane (xf86CrtcPtr crtc)
 
     vgacntrl |= VGA_DISP_DISABLE;
 
+    /* disable "VGA/Pop-up 2X Centered Mode Scaling" on 855GM platform */
+    if (IS_I855(pI830))
+        vgacntrl &= ~VGA_2X_MODE;
+    
     OUTREG(VGACNTRL, vgacntrl);
     i830WaitForVblank(pScrn);