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

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

[Mark Knecht]

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.

Cheers,
Mark

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

[Michael Mol]

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

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

[Mark Knecht]

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

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

[Stroller]

On 22 January 2012, at 15:54, Mark Knecht wrote:
> ...
> 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.

For all you've written, I'm afraid the best answer you're likely to get it "buy a KVM you can return" (or buy secondhand at a price you can resell without loss). 

Video resolutions / refresh rates / timing / &c is a bit of a black art. What the heck is a modeline? 

The manufacturer of a KVM isn't going to be able to test it with all the 10,000 monitors available on the market today - not even 1% of them. They're happy if it works 99% of the time, and if you complain to them they'll say "oh, it must be a bug with that monitor" and maybe, eventually, give you a refund for the sake of a peaceful life. 

There are probably a handful of different KVM chips that are made by big manufacturers in Taiwan, and then sold by the 10,000 to other manufacturers, this time manufacturers of actual KVM units. These second tier manufacturers probably all wrap the KVM integrated circuits up in slightly different ways, using slightly different resistors and capacitors to interface with input and output. 

So in terms of your specific questions, I'm not able to answer very helpfully. I think you could waste a lot of time trying to get these answers, whereas you could swap in a specific model and try it in a matter of minutes - and that way you'd know for sure.

Stroller.

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

[G.Wolfe Woodbury]

On 01/22/2012 12:42 PM, Mark Knecht wrote:
> 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 
Many of the cheap KVM models do, indeed, mess up the EDID data coming 
from the monitor.  I suspect that this is from old design specs that 
have too much pull-up/pull-down on the EDID lead since the boxen haven 
been re-engineered for newer, higher resolution and higher speed monitors.

I have had problems specifically with the BELKIN KVMs.

It may also be that the video drivers for Linux are just enough 
different (necessarily) from the MSFT drivers to not reliably sense the 
EDID return signals.

I did as others suggested and tried several until I found one that 
worked.  Sometimes a slightly different model/serial/part no KVM from 
the same manufacturer would/wouldn't work.

-- 
G.Wolfe Woodbury
redwolfe@gmail.com

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

[Mark Knecht]

On Sun, Jan 22, 2012 at 10:07 AM, G.Wolfe Woodbury <redwolfe@gmail.com> wrote:
> On 01/22/2012 12:42 PM, Mark Knecht wrote:
>>
>> 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
>
> Many of the cheap KVM models do, indeed, mess up the EDID data coming from
> the monitor.  I suspect that this is from old design specs that have too
> much pull-up/pull-down on the EDID lead since the boxen haven been
> re-engineered for newer, higher resolution and higher speed monitors.
>
> I have had problems specifically with the BELKIN KVMs.
>
> It may also be that the video drivers for Linux are just enough different
> (necessarily) from the MSFT drivers to not reliably sense the EDID return
> signals.
>
> I did as others suggested and tried several until I found one that worked.
>  Sometimes a slightly different model/serial/part no KVM from the same
> manufacturer would/wouldn't work.
>
> --
> G.Wolfe Woodbury
> redwolfe@gmail.com
>
>

Thanks.  Sounds about like what was going on at his place and it's a
reasonable evaluation potentially.

I guess I could have made it a bit more clear early on - I'm not
looking to solve anything here and I'm not buying KVMs. I was just
curious as to the root cause of the problem. I've seen it discussed
periodically for over a decade but never any strong analysis of why it
happens. It's not a huge problem to the community.

Cheers,
Mark

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

[Michael Mol]

On Sun, Jan 22, 2012 at 12:47 PM, Stroller
<stroller@stellar.eclipse.co.uk> wrote:
>
> On 22 January 2012, at 15:54, Mark Knecht wrote:
>> ...
>> 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.
>
> For all you've written, I'm afraid the best answer you're likely to get it "buy a KVM you can return" (or buy secondhand at a price you can resell without loss).
>
> Video resolutions / refresh rates / timing / &c is a bit of a black art. What the heck is a modeline?

A modeline is a one-liner way of representing a display driver mode. I
always preferred the block form; it's far more readable. It's been
almost a decade since I've written one, though. Haven't really needed
to since XFree86 4 came out.

I don't remember all of the bits of information in it, but here are
some of the highlights:
* Vertical refresh rate (in kHz): how rapidly the CRT's electron beam
crosses the screen horizontally.
* Horizontal refresh rate (in Hz): how rapidly the CRT's electron beam
crosses the screen vertically.
* Some horizontal and vertical overdraw values I don't remember the
specifics of, but it amounts to realizing that the electron beam is
active past the bezel of your monitor.

"xvidtune", by the way, was an awesome way to do live testing and
configuration of these values, and it would emit a modeline for you
when you asked it.

-- 
:wq

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

[Michael Mol]

On Sun, Jan 22, 2012 at 1:07 PM, G.Wolfe Woodbury <redwolfe@gmail.com> wrote:
> On 01/22/2012 12:42 PM, Mark Knecht wrote:
>>
>> 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
>
> Many of the cheap KVM models do, indeed, mess up the EDID data coming from
> the monitor.  I suspect that this is from old design specs that have too
> much pull-up/pull-down on the EDID lead since the boxen haven been
> re-engineered for newer, higher resolution and higher speed monitors.
>
> I have had problems specifically with the BELKIN KVMs.
>
> It may also be that the video drivers for Linux are just enough different
> (necessarily) from the MSFT drivers to not reliably sense the EDID return
> signals.

Concur. It sounds like the EDID block isn't making it back or is
somehow messed up. x11-misc/read-edid would help in investigating that
kind of issue.


-- 
:wq

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

[Mark Knecht]

On Sun, Jan 22, 2012 at 10:58 AM, Michael Mol <mikemol@gmail.com> wrote:
> On Sun, Jan 22, 2012 at 1:07 PM, G.Wolfe Woodbury <redwolfe@gmail.com> wrote:
>> On 01/22/2012 12:42 PM, Mark Knecht wrote:
>>>
>>> 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
>>
>> Many of the cheap KVM models do, indeed, mess up the EDID data coming from
>> the monitor.  I suspect that this is from old design specs that have too
>> much pull-up/pull-down on the EDID lead since the boxen haven been
>> re-engineered for newer, higher resolution and higher speed monitors.
>>
>> I have had problems specifically with the BELKIN KVMs.
>>
>> It may also be that the video drivers for Linux are just enough different
>> (necessarily) from the MSFT drivers to not reliably sense the EDID return
>> signals.
>
> Concur. It sounds like the EDID block isn't making it back or is
> somehow messed up. x11-misc/read-edid would help in investigating that
> kind of issue.
>

Yeah, I did that sort of stuff already. It didn't tell me anything
specific as best I could tell. However I haven't done it with the KVM
out. Maybe comparing the two responses would give some clues.

Also, thanks for the pointer to xvidtune. Interesting little app.
QUESTION: Do you know how to get the data for each monitor/video card
combo? So far I haven't figured out how to tell it Screen2. Seems to
only give me Screen0 as best I can tell.

I'm a bit nervous to just start trying things.

Cheers,
Mark

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

[Michael Mol]

On Sun, Jan 22, 2012 at 2:30 PM, Mark Knecht <markknecht@gmail.com> wrote:
> On Sun, Jan 22, 2012 at 10:58 AM, Michael Mol <mikemol@gmail.com> wrote:
>> On Sun, Jan 22, 2012 at 1:07 PM, G.Wolfe Woodbury <redwolfe@gmail.com> wrote:
>>> On 01/22/2012 12:42 PM, Mark Knecht wrote:
>>>>
>>>> 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
>>>
>>> Many of the cheap KVM models do, indeed, mess up the EDID data coming from
>>> the monitor.  I suspect that this is from old design specs that have too
>>> much pull-up/pull-down on the EDID lead since the boxen haven been
>>> re-engineered for newer, higher resolution and higher speed monitors.
>>>
>>> I have had problems specifically with the BELKIN KVMs.
>>>
>>> It may also be that the video drivers for Linux are just enough different
>>> (necessarily) from the MSFT drivers to not reliably sense the EDID return
>>> signals.
>>
>> Concur. It sounds like the EDID block isn't making it back or is
>> somehow messed up. x11-misc/read-edid would help in investigating that
>> kind of issue.
>>
>
> Yeah, I did that sort of stuff already. It didn't tell me anything
> specific as best I could tell. However I haven't done it with the KVM
> out. Maybe comparing the two responses would give some clues.
>
> Also, thanks for the pointer to xvidtune. Interesting little app.
> QUESTION: Do you know how to get the data for each monitor/video card
> combo? So far I haven't figured out how to tell it Screen2. Seems to
> only give me Screen0 as best I can tell.
>
> I'm a bit nervous to just start trying things.

If you're not using Xinerama, you can specify screens by changing
$DISPLAY. e.g. :0.0 is screen 0, :0.1 is screen 1, :0.2 is screen 2...

-- 
:wq

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

[Mark Knecht]

On Sun, Jan 22, 2012 at 12:07 PM, Michael Mol <mikemol@gmail.com> wrote:
> On Sun, Jan 22, 2012 at 2:30 PM, Mark Knecht <markknecht@gmail.com> wrote:
<SNIP>
>>
>> Also, thanks for the pointer to xvidtune. Interesting little app.
>> QUESTION: Do you know how to get the data for each monitor/video card
>> combo? So far I haven't figured out how to tell it Screen2. Seems to
>> only give me Screen0 as best I can tell.
>>
>> I'm a bit nervous to just start trying things.
>
> If you're not using Xinerama, you can specify screens by changing
> $DISPLAY. e.g. :0.0 is screen 0, :0.1 is screen 1, :0.2 is screen 2...
>

I am using xinerama. 3 monitors across 2 NVidia adpaters. Same on my
friends system.

One interesting thing about this configuration. On his machine he uses
two identical 8400GS adapters. He can run glxgears and is able to move
the gears to any screen and see the gears. He can also use the KDE
features to look at all desktops and application using (Ctrl-F8 I
think) or by dragging his mouse to the upper left corner.

On my setup I use a GTX465 for screen 0 & 1 but have an NVidia 8400GS
for screen 2. On this system I only see the gears on screen 0 & 1. On
screen 2 I see just a black box and the FPS count skyrockets as the
CPU/GPU aren't really doing anything so it goes through the loop very
fast. Also, on this system the Ctrl-F8 thing mouse to the upper left
corner don't work at all.

I've assumed that's because I'm using two different NVidia cards.

- Mark

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

[Alan McKinnon]

On Sun, 22 Jan 2012 12:18:17 -0800
Mark Knecht <markknecht@gmail.com> wrote:

> On Sun, Jan 22, 2012 at 12:07 PM, Michael Mol <mikemol@gmail.com>
> wrote:
> > On Sun, Jan 22, 2012 at 2:30 PM, Mark Knecht <markknecht@gmail.com>
> > wrote:
> <SNIP>
> >>
> >> Also, thanks for the pointer to xvidtune. Interesting little app.
> >> QUESTION: Do you know how to get the data for each monitor/video
> >> card combo? So far I haven't figured out how to tell it Screen2.
> >> Seems to only give me Screen0 as best I can tell.
> >>
> >> I'm a bit nervous to just start trying things.
> >
> > If you're not using Xinerama, you can specify screens by changing
> > $DISPLAY. e.g. :0.0 is screen 0, :0.1 is screen 1, :0.2 is screen
> > 2...
> >
> 
> I am using xinerama. 3 monitors across 2 NVidia adpaters. Same on my
> friends system.
> 
> One interesting thing about this configuration. On his machine he uses
> two identical 8400GS adapters. He can run glxgears and is able to move
> the gears to any screen and see the gears. He can also use the KDE
> features to look at all desktops and application using (Ctrl-F8 I
> think) or by dragging his mouse to the upper left corner.
> 
> On my setup I use a GTX465 for screen 0 & 1 but have an NVidia 8400GS
> for screen 2. On this system I only see the gears on screen 0 & 1. On
> screen 2 I see just a black box and the FPS count skyrockets as the
> CPU/GPU aren't really doing anything so it goes through the loop very
> fast. Also, on this system the Ctrl-F8 thing mouse to the upper left
> corner don't work at all.
> 
> I've assumed that's because I'm using two different NVidia cards.
> 
> - Mark
> 

The docs for the nVidia driver go into this in quite some detail, and
it's related to how the nVidia hardware is built plus how the nVidia
software works plus how Xinerama works. Basically, not all combinations
are supported, especially when you have different "screens" (that's the
nVidia concept of a screen, not the X concept) with different
resolutions on different GPUs. Or something like that, but I don;t have
the docs to have anymore since my last 8600M went to visit the silicon
daddy in the sky

-- 
Alan McKinnnon
alan.mckinnon@gmail.com

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

[Mark Knecht]

On Sun, Jan 22, 2012 at 12:52 PM, Alan McKinnon <alan.mckinnon@gmail.com> wrote:
> On Sun, 22 Jan 2012 12:18:17 -0800
> Mark Knecht <markknecht@gmail.com> wrote:
<SNIP>
>> On my setup I use a GTX465 for screen 0 & 1 but have an NVidia 8400GS
>> for screen 2. On this system I only see the gears on screen 0 & 1. On
>> screen 2 I see just a black box and the FPS count skyrockets as the
>> CPU/GPU aren't really doing anything so it goes through the loop very
>> fast. Also, on this system the Ctrl-F8 thing mouse to the upper left
>> corner don't work at all.
>>
>> I've assumed that's because I'm using two different NVidia cards.
>>
>> - Mark
>>
>
> The docs for the nVidia driver go into this in quite some detail, and
> it's related to how the nVidia hardware is built plus how the nVidia
> software works plus how Xinerama works. Basically, not all combinations
> are supported, especially when you have different "screens" (that's the
> nVidia concept of a screen, not the X concept) with different
> resolutions on different GPUs. Or something like that, but I don;t have
> the docs to have anymore since my last 8600M went to visit the silicon
> daddy in the sky
>
> --
> Alan McKinnnon
> alan.mckinnon@gmail.com
>
>

Thanks Alan. I've looked at those docs in days long past myself, but
never really needed this sort of info. I'll go back and check them out
again.

Cheers,
Mark