I just partitioned my main 200GB (gparted is nice...) to set up Ubuntu. I have three monitors on two video cards:

• NEC 1530V and Polyview V17D on PCI-E ATI X300 (primary display in BIOS)

• ViewSonic VG800 on PCI ATI Radeon 9200

I installed the ATI drivers via sudo apt-get install xorg-driver-fglrx, with fglrxinfo showing:

The two monitors on the X300 are currently enabled, but both displaying the same thing. Ideally I would want all three screens enabled independently of each other (as in XP), but I could live with two of them working.

This is the default xorg.conf file, which I assume needs to be edited with magic words:

Any suggestions on what I need to do to get it to work?

According to this thread, you need to choose one out of four drivers (one exclusive for nVidia cards): Xinerama (open source and binary driver), MergedFB (open source driver) and BigDesktop (binary driver). Apparently, BigDesktop is the easiest one you can try, as the others require some tweaks.

BigDesktop got the 2nd monitor working, but the 3rd monitor on the 2nd card still won't work. I'll have to mess around with it later.

Adding the second card is going to need a second "Monitor"/"Screen" and "Device" and an extra "Screen" line in the "ServerLayout" section.

I can't wait till Xorg gets its auto config stuff done. No more need for that lame config file.

Apparently there are no instructions for two ATI video cards. There was an example of using two screens with two NVidia cards, but none for two ATI cards.

I tried doing something like that, but I wasn't sure where I get the names, etc.

First, this auto-detected section seems wrong:

The NEC LCD1530V monitor is not on the 9200, but the X300.

With the "BigDesktop" taking care of the two monitors on the first card, should it be like this?:

• 1 ServerLayout, referencing two Screens

• 2 Screens, one for each Device

• 2 Devices,

• 3 Monitors

Only two Monitors were listed in the config file, both of the ones connected to the PCI-E card (Radeon X300). Do I need to add the third monitor by hand? And if so, whence does the name (Identifier) come? If it's just arbitrary, how do I know which Monitor section relates with which physical monitor?

See here to see how some dude set two NVidia cards, maybe you can do something similar with two ATI cards?

Yes, identifiers are arbitrary, you would name them like "left monitor", "main monitor" and "right monitor", or something like that.

Actually, the BigDesktop mode will likely not need to reference two screens, it should appear to X as one monitor (and is rather annoying that way too, I prefer Xinerama since apps and gnome/kde will know that theres two monitors, instead of one big one, unless ati fixed that...)

The Monitor sections get applied with the card they are attached to, its not arbitrary, you just need to match up the proper Monitor to the "Display". The Monitor section provides the timings, and other settings, the device section configures the driver, and the Screen section links them together, along with default resolutions.

Should only "need" two Device sections, (ati drivers use DDC to grab the display info, the Monitor sections are sorta superfluous these days), the Monitor section for the second monitor on the main card need not exist, since ATIs driver handles it for you, X never really sees it, at least through the config.

So your list is mostly right. minus one monitor from list, and thats about it. Course it wont hurt to keep it. theres just no reason to specify it anywhere.

edit:

I'm not sure what it applies to, but I use the:

   Option       "DesktopSetup" "horizontal"

Mode. I also let it use DDC/EDID to guess all the settings for both monitors since it does a better job than I do.

I got the second card to work using lspci to detect its BusID. I think that was the main thing causing it not to work.

The primary card has two screens attached to it, sharing one wide desktop just like XP. That is, there is only one start menu, but I can drag windows from one to the other. Maximizing a window only maximizes on the monitor it is at.

The secondary card is like it's own thing. The mouse moves over to it, but it cannot share windows, etc.

And in Windows all three make a single big desktop too? By the way, you could post your xorg.conf to have it as future reference if someone else ever asks this question.

Windows XP makes it all one big desktop, with windows maximizing to the size of the monitor.

I am comparing it with this xorg.conf, but my head hurts just at checking all those resolutions... my xorg.conf has only 8, 16 and 32bpp at 800x600 and 1024x768...

Apparently you are missing one of the screens in the ServerLayout?

I'm not sure if its possible, but if you can enable Xinerama for the combination of the BigDesktop screen and the other card, that might work. If it doesn't you'll have to use that funky X proxy thing that handles moving windows from card to card. The reason it works so easily on a "BigDesktop" mode is because the two screens look into the same framebuffer, so its one wide surface, which is kindof hard, if not impossible, to do with two separate cards.

With ATIs drivers, theres no need to add the second monitor in the dual display.. They handle everything, hence the "DesktopSetup" "horizontal".

Hmm... the guy in that forum post has two ATI cards, but he defines three screens. Maybe (just a big maybe here) if you use the DesktopSetup horizontal, it does not allow you to use three monitors as one desktop?

It may make it more difficult. Though I'm still not sure if you'll be able to drag stuff between desktops if all three use a separate framebuffer. If you can, thats the way to go.

Why can't you just hook up your second monitor and reboot? Oh right, Linux makes you do things the hard way. This is fun.