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jyoung
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PostPosted: Wed Apr 27, 2016 3:33 pm    Post subject: X won't start on the Secondary GPU Reply with quote

Hi Folks,

My machine has a hybrid graphics card comprised of a GeForce 9400M G and a GeForce 9200M GS. I'm currently running X on the more powerful 9400M G and using the 9200M GS for some CUDA code that I'm working on. I'd like to switch the two - use the less powerful 9200M GS for X and the more powerful 9400M G for CUDA. I found a really good explanation of how to do this on stackoverflow:

http://stackoverflow.com/questions/18382271/how-can-i-modify-xorg-conf-file-to-force-x-server-to-run-on-a-specific-gpu-i-a

Basically, they suggest changing the device section in xorg.conf to point to the card we'd like X to use. My 9400M G is at PCI:3:0:0 and my 9200 is at PCI:2:0:0, but when I make these changes to my xorg.conf file, X doesn't start properly. The screen flashes as if X is about to start, there's a flurry of hard drive activity, and then it goes dark. Upon reboot the /var/log/Xorg.0.log doesn't seem to record any fatal errors, although there are many references to a CRT-0 monitor which doesn't exist. lspci reports that both cards are detected - and, perhaps more importantly, I know that both cards work since I'm currently using one for X and the other for CUDA.

Does anyhow have any ideas?



The Xorg.0.log file is here:
http://pastebin.ca/3584583

and the broken xorg.conf file is here:
http://pastebin.ca/3584585

This xorg.conf file is identical to a working xorg.conf generated by nvidia-settings, with the modification that I've switched from device0 to device1.
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NeddySeagoon
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PostPosted: Wed Apr 27, 2016 6:11 pm    Post subject: Reply with quote

jyoung,

This is how Xorg put together your X server.
Code:
[   889.841] (==) Using config file: "/etc/X11/xorg.conf"
[   889.841] (==) Using config directory: "/etc/X11/xorg.conf.d"
[   889.841] (==) Using system config directory "/usr/share/X11/xorg.conf.d"
[   889.841] (==) No Layout section.  Using the first Screen section.
[   889.841] (==) No screen section available. Using defaults.
[   889.841] (**) |-->Screen "Default Screen Section" (0)
[   889.841] (**) |   |-->Monitor "<default monitor>"
[   889.841] (==) No device specified for screen "Default Screen Section".
        Using the first device section listed.
[   889.841] (**) |   |-->Device "Device0"
[   889.841] (==) No monitor specified for screen "Default Screen Section".
        Using a default monitor configuration.


The config you posted says

Code:
Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
...
Section "Screen"
    Identifier     "Screen0"
    Device         "Device1"
    Monitor        "Monitor0"


Which tells that the log you posted was not generated from the xorg.conf you posted.
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NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.
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jyoung
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PostPosted: Thu Apr 28, 2016 2:14 am    Post subject: Reply with quote

Sorry, I've been messing around with xorg.conf a lot! Here's the Xorg.0.log that was generated by the xorg.conf file mentioned above:

http://pastebin.ca/3584927
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jyoung
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PostPosted: Fri Apr 29, 2016 5:15 pm    Post subject: Reply with quote

The log file I posted earlier was the result of an experiment where I tried stripping away as many parts of the xorg.conf file as possible and seeing if X could figure it out from defaults. This seemed like a reasonable idea since my X server is extremely good at figuring out the defaults for the 9400M G card. In fact, in playing around with it, I've found that it's possible to remove nearly all of the xorg.conf file without breaking it.

Of course this didn't work -- it fails when I add the critical line to the Device Section that directs X to use the 9200M GS card:

BusID "PCI:2:0:0"
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