I have a fresh install of Kubuntu 15.04 and I am using the nouveau driver. The system came up to a black screen from ram sleep with the following message. "[15395.859064] nouveau EI PBUS][0000:01:00.0] MMIO read of 0x00000000 FAULT at 0x002140 [ !ENGINE ] I had to cold boot my system. Reproducible: Always
Please report this issue directly to nouveau developers via https://bugs.freedesktop.org/