While trying to prepare my system for a third GPU I ran in into a bit of a conundrum.
The current setup consists of a an Aorus Master with an RTX 3000 card in PCIe slot 1 (the 16x one, next to the CPU) and an older AMD something from a used Dell for the host.
Now I have an AMD W7100 incoming and thought, how neat, now I can use the x4-slot (No 3) for a NVME adapter (SSD is x4 only, anyway) and use the x8 slot for the second (host) GPU.
That way I may even be able to follow Wendells guide on how to mod the W7100 for SR-IOV and have a little more bandwidth on the x8-slot.
But, it didn’t work: after booting with the AMD card (it’s actually an ATI, I think) in slot 2 and the 3090 in slot 1 I couldn’t log into Ubuntu, it would always yank be back to login screen, like in a loop.
After digging around, I found these:
Jan 05 13:11:17 zenmasterl /usr/lib/gdm3/gdm-x-session[2776]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:11:26 zenmasterl /usr/lib/gdm3/gdm-x-session[2809]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:11:53 zenmasterl /usr/lib/gdm3/gdm-x-session[3034]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:18:05 zenmasterl /usr/lib/gdm3/gdm-x-session[2701]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:26:16 zenmasterl /usr/lib/gdm3/gdm-x-session[2758]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:26:44 zenmasterl /usr/lib/gdm3/gdm-x-session[2968]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:29:13 zenmasterl /usr/lib/gdm3/gdm-x-session[3768]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:36:24 zenmasterl /usr/lib/gdm3/gdm-x-session[3035]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Jan 05 13:40:43 zenmasterl /usr/lib/gdm3/gdm-x-session[2721]: (WW) VGA arbiter: cannot open kernel arbiter, no multi-card support
Now why are these there? Shouldn’t the x8-slot hang off the PCH and the two cards should not interfere? What am I missing?
Sure enough, putting the AMD card back in slot 3 works.
w/ best regards
Gerd