Ubuntu 18.04 屏幕分辨率

Question

我刚刚从 16.04 升级到 18.04。现在我的显示器分辨率卡在：

xrandr
xrandr: Failed to get size of gamma for output default Screen 0: minimum 640 x 480, current 640 x 480, maximum 640 x 480 default connected primary 640x480+0+0 0mm x 0mm 640x480 73.00*

xrandr --verbose
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 640 x 480, maximum 640 x 480
default connected primary 640x480+0+0 (0x2aa) normal (normal) 0mm x 0mm
    Identifier: 0x2a9
    Timestamp:  66608
    Subpixel:   unknown
    Clones:    
    CRTC:       0
    CRTCs:      0
    Transform:  1.000000 0.000000 0.000000
                0.000000 1.000000 0.000000
                0.000000 0.000000 1.000000
               filter:
    _MUTTER_PRESENTATION_OUTPUT: 0
  640x480 (0x2aa) 22.426MHz *current
        h: width   640 start    0 end    0 total  640 skew    0 clock  35.04KHz
        v: height  480 start    0 end    0 total  480           clock  73.00Hz

xrandr -s 6
Size index 6 is too large, there are only 1 sizes

另外我的 grub 也没有提到 nomodeset。

请问有什么帮助吗？