[Bug 82781] Option to disable mclk reclocking with AMD R9 280X (TAHITI) to avoid screen flickering on VGA/CRT

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https://bugzilla.kernel.org/show_bug.cgi?id=82781

--- Comment #9 from Alex Deucher <alexdeucher@gmail.com> ---
(In reply to Christian Birchinger from comment #7)
> Yes, that patch would indeed fix it in my case.
> 
> I was pretty sure i took standard VESA modes and the fact that Windows
> Generic Monitor (no overrides or "hacks" there) values also using the same
> timings tells
> me that they re rather "normal" modes.
> 
> i'm going to take your fix and live with it because i prefer when all my
> screen
> modes remain exactly the same between OS switches.
> 
> But for curiosity reasons. Do you have a suggestion for mode sources with
> higher
> vblank pauses which are still considered "standard" and not totaly uniq and
> custom? Or might it be possible my 85 Hz value simply has this 450 as
> standard.
>

There's nothing wrong with the modes or non-standard about a mode with a 450 us
vblank time.  The issue is it takes 450 us for the mclk to change.  It just so
happens that when the vblank period is the same as the switch limit so there is
very little margin for error.

As for tweaking your modeline, you can try adjusting the htotal or vblank_end
parameters.  CRT multi-sync monitors are pretty flexible when it comes to
timing.

You can use gtf of cvt to generate vesa compatible modelines.  e.g.,
$ gtf 1600 1200 85

  # 1600x1200 @ 85.00 Hz (GTF) hsync: 107.10 kHz; pclk: 234.76 MHz
  Modeline "1600x1200_85.00"  234.76  1600 1720 1896 2192  1200 1201 1204 1260 
-HSync +Vsync

$ cvt 1600 1200 85
# 1600x1200 84.95 Hz (CVT 1.92M3) hsync: 107.21 kHz; pclk: 235.00 MHz
Modeline "1600x1200_85.00"  235.00  1600 1728 1896 2192  1200 1203 1207 1262
-hsync +vsync

your mode:
[    30.543] (II) RADEON(0): Modeline "1600x1200"x85.0  229.50  1600 1664 1856
2160  1200 1201 1204 1250 +hsync +vsync (106.2 kHz eP)

gtf was the vesa formula used for CRTs and cvt is the newer formula designed
for DFPs.  I'm not sure how your modes were generated, but they don't seem to
follow either gtf or cvt.  Both the gtf (540 us vblank period) and cvt (558 us
vblank period) modes should work fine as there is pleny of margin.

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