Re: [gentoo-user] KVM problems - anyone know _why_ it happens?

[Mark Knecht <markknecht@gmail.com>]

On Sun, Jan 22, 2012 at 8:06 AM, Michael Mol <mikemol@gmail.com> wrote:
> On Sun, Jan 22, 2012 at 10:54 AM, Mark Knecht <markknecht@gmail.com> wrote:
>> I've seen reports for years about folks having problems with some KVMs
>> under Linux. I've never personally had one myself. However I've been
>> helping a Windows friend break his Redmond addiction over the last few
>> months using Gentoo. He has a nice 3 monitor KDE-based system that's
>> been working fine but there was one monitor that refused to set up
>> with the right resolution. We left it alone for a long time as it was
>> usable but finally yesterday got together to figure out what was
>> happening. From the title it should be clear that the problem was a
>> KVM hooked to that one monitor. Removing the KVM completely solved the
>> problem.
>>
>> Now, what I'm wondering is why this same video card/KVM/monitor
>> combination which apparently worked in Windows should have any
>> problems in Linux? Anyone know why?
>>
>> In the spirit of full discloser I don't really know that this
>> _specific_ video card was tested in Windows, but he owns multiple
>> NVidia 8400GS cards and it's my understanding that other 8400GS cards
>> did work with this KVM & monitor, so unless it's this specific card
>> having a defect, or even being just a bit weak in some way, it would
>> seem to be the insertion of the KVM itself that upset things.
>>
>> Looking at the monitor's specs/requirements for running the higher
>> resolutions it uses, as should not be a surprise, higher frequencies
>> to do higher resolutions. If the KVM was filtering those a bit then
>> it's possible things wouldn't work, but that doesn't explain why it
>> did work in Windows.
>>
>> Basically, I looked around in Google for anyone that had real info
>> about why this problem occurs, couldn't find any that made sense, and
>> am wondering how to choose a KVM that's going to work out of the box
>> short of asking for model numbers, etc.
>
> I assume these are VGA displays?
>
> I've noticed that the CRTs attached to my Win7 box at work don't get
> configured for the highest refresh rate unless I force it. Also, I've
> noticed it decide that '1280x1024' is the 'recommended' resolution for
> my displays, though they'll do 1600x1200@60Hz.
>
> It could just be a matter of Windows using 75Hz instead of 85Hz, or
> 60Hz instead of 75Hz.
>
>
> --
> :wq
>

That could certainly somehow be part of it, although in the manual for
the monitor (Acer 2216W) said the resolution of interest (1680x1050)
only runs at one horizontal/vertical set of dot clock so it isn't like
there was a choice there of down shifting and X just chose the lower
rate. According to the manual, if Windows set up 1680x1050 then it
must have been using the only rates, etc. (I think!)

And yes, the interface on that monitor is the old-style VGA.

It's fairly clear that X kept saying there wasn't a resolution
available from the monitor to support what I had requested in the
xorg.conf file. I was asking for 1680x1050, being told the monitor
didn't support it, and then given 1280x1024 instead. When he removed
the KVM all those messages went away, X & KDE said the monitor was
running 1680x1050, and the OSD on the monitor itself said it was doing
the requested setup.

I played a bit with get-edid | parse-edid. Logically that stuff even
working says the VGA monitor cable is bidirectional. I started
wondering if the KVM messes up the data coming back, or what else
might be going on.

Thanks for the ideas,
Mark