Bug#529178: video-radeon: Probably hardware limitation

[Alex Deucher]

On Thu, Jul 23, 2009 at 7:47 AM, Michal Suchanek<hramrach at centrum.cz> wrote:
> 2009/7/22 Alex Deucher <alexdeucher at gmail.com>:
>> On Wed, Jul 22, 2009 at 3:47 PM, Michal Suchanek<hramrach at centrum.cz> wrote:
>>> Package: xserver-xorg-video-radeon
>>> Followup-For: Bug #529178
>>>
>>> This is probably a hardware limitation of the card.
>>>
>>> AFAIK dual-link dvi is needed for modes above 1280x1024 to work reliably
>>> over DVI. Some card - monitor combinations might work but others would
>>> not.
>>>
>>> Better cards have higher-rate transmitters that can do modes slightly
>>> larger than 1280x1024 but a 22" screen is likely over-spec for any
>>> single link card so there is no guarantee it will work.
>>
>> 165 Mhz is the single link limit.  With reduced blanking you can run
>> 1920x1200 at 60Hz.  Most dual link monitors start out around 2560x1600.
>>
>
> Well, reduced blanking is like overclocking. It might work but it might not.
>

Not exactly.  Lots of monitors (mostly LCDs) require reduced blanking modes.

> Either way my experience with Radeon 9250 is that it cannot drive a
> 22" screen over DVI reliably, and I read in a bug report somewhere
> somebody testing about a dozen cards with similar results.
>

I can run 1920x1200 reliably on a 9250 here.  I've even run it on a
radeon 7000, so the hardware is capable.  It's more likely a issue
with the pll or watermark setup.  Lots of the higher res LCDs are VERY
picky about the pll dividers.  I would try some alternate mode lines,
e.g., :
Modeline "1680x1050R"  119.00  1680 1728 1760 1840  1050 1053 1059
1080 +hsync -vsync
Modeline "1680x1050_60.00"  147.14  1680 1784 1968 2256  1050 1051
1054 1087  -HSync +Vsync

In the earlier days of the driver, it did not use the modes from the
edid which resulting in the driver not using the specific modes he LCD
monitors required.  Changing that fixed a lot of setups.

> It might be that these older cards actually cannot do full 165 MHz.
>

The hardware definitely can.

> Radeon 9600 is a higher model than 9250 but it is about as old as the
> 9250s, perhaps even older. While it may offer better 3D performance it
> might not offer better DVI transmitter than the lower models.

The 9600 also supported up to 165 Mhz as well.

Based on the description, it really sounds like display underflow to
the display controller.  The display controller contends with the 2d
and 3d engine for memory bandwidth.  If the memory controller is not
able to fill the display controller's request fast enough you end up
with the screen blanking due to lack of incoming data.  Another thing
to try is:
Option "ColorTiling" "False"
Option "DisplayPriority" "HIGH"

The second option sets the display controller's memory controller
priority to the max, and the first option disables surface tiling.
The line buffer that feeds the display controllers is scanline based,
however, when tiling is enabled, the hw has to adjust the requests to
deal with the tiling which eats additional bandwidth.

Alex