[Intel-gfx] Issue after drm/i915: prefer wide & slow to fast & narrow in DP configs.

[Jesse Barnes]

On Wed, 20 Mar 2013 14:45:52 +0100
Michal Srb <msrb at suse.com> wrote:

> Hi,
> 
> I am debugging an issue with IBM POS machine (4852-570, Truman), with 
> 8086:0102 Intel Sandybridge graphic card and internal monitor connected over 
> display port.
> 
> The issue is that sporadically, after mode change, the monitor remains blank.
> There is no difference in drm.debug=0xe level log or content of <debugfs>/dri 
> between the state when monitor displays image and the state when it remains 
> blank.
> 
> (The monitor supports only 1024x768, so every mode change is into this 
> resolution. It happens when the intel driver switches resolution during boot, 
> when X starts, X ends, modetest starts or modetest ends.)
> 
> I've bisected the cause to this commit:
> 
>   commit 2514bc510d0c3aadcc5204056bb440fa36845147
>   Author: Jesse Barnes <jbarnes at virtuousgeek.org>
>   Date:   Thu Jun 21 15:13:50 2012 -0700
> 
>       drm/i915: prefer wide & slow to fast & narrow in DP configs
>     
>       High frequency link configurations have the potential to cause trouble
>       with long and/or cheap cables, so prefer slow and wide configurations
>       instead.  This patch has the potential to cause trouble for eDP
>       configurations that lie about available lanes, so if we run into that we
>       can make it conditional on eDP.
> 
> 
> The machine reports to have 2 lanes available and that seems to be correct.
> One lane was used before the commit and the screen used to show picture after 
> every mode change:
> 
>   [drm:intel_dp_mode_fixup], DP ink computation with max lane count 2 max bw 
> 0a pixel clock 65000KHz
>   [drm:intel_dp_mode_fixup], DP link bw 0a lane count 1 clock 270000 bpp 24
>   [drm:intel_dp_mode_fixup], DP link bw required 156000 available 216000
> 
> Two lanes are used after the commit and the screen occasionally remains blank 
> after mode change:
> 
>   [drm:intel_dp_mode_fixup], DP link computation with max lane count 2 max bw 
> 0a pixel clock 65000KHz
>   [drm:intel_dp_mode_fixup], DP link bw 06 lane count 2 clock 162000 bpp 24
>   [drm:intel_dp_mode_fixup], DP link bw required 156000 available 259200
> 
> 
> Could you advise me where to look further? Could this be a hardware bug? 
> Reverting the commit works as a workaround, but doesn't look like correct way 
> to fix this...

There's nothing else in the log about aux failures?  Just that the
training apparently succeeded but you ended up with a blank screen?

-- 
Jesse Barnes, Intel Open Source Technology Center