linux-kernel - lvds downclocking breaks on G45/thinkpad T500

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Date:	Sat, 26 Dec 2009 17:49:29 +0100
From:	Andres Freund <andres@...razel.de>
To:	Zhao Yakui <yakui.zhao@...el.com>, David Airlie <airlied@...ux.ie>,
	linux-kernel@...r.kernel.org, dri-devel@...ts.sourceforge.net
Cc:	Adam Jackson <ajax@...hat.com>
Subject: lvds downclocking breaks on G45/thinkpad T500

Hi Zhao,

On 2.6.33-rc1 I noticed that the display of my T500 with a G45 reacts very 
slowly (you can see the text appearing) and colors take a time to "solidify". 
I.e. making a white terminal black takes some seconds.
 
After some bisecting and guessing I recognized that the problem showed up 
after:

commit 18f9ed12f8c977e25d65a16af8e8d73f72417ba1
Author: Zhao Yakui <yakui.zhao@...el.com>
Date:   Fri Nov 20 03:24:16 2009 +0000

    drm/i915: Enable LVDS downclock feature through EDID.
    
    If more than one mode with the same resolution defined in EDID has different
    refresh rate, it is thought that the downclock is found for LVDS.
    We will program the different FPx0/1 register so that we can select 
dynamically
    between the low and high frequency.
    
    On the g4x platform we will use the CxSR feature to switch the different
    refresh rate if the LVDS downclock feature is supported.
    
    Signed-off-by: Zhao Yakui <yakui.zhao@...el.com>
    Reviewed-by: Jesse Barnes <jbarnes@...tuousgeek.org>
    Signed-off-by: Eric Anholt <eric@...olt.net>

Setting dev_priv->lvds_downclock_avail = 0 in intel_find_lvds_downclock() 
unsurprisingly avoids the issue.

Anything I can do to help analyzing the issue?

Andres

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