我有一台新的 4k 显示器，能够以 60hz 的频率显示 3840x2160 的分辨率。不幸的是，我只有 HDMI，这意味着我所能期望的最好结果就是以 30hz 的频率运行（我的理解）。如果我尝试使用显示管理器将分辨率切换为 3840x2160，外部显示器就会变成空白。
我用 xrandr 尝试了几种方法。第一种方法是
% xrandr --output DP-1 --mode 3840x2160 -r 30.00
（我也尝试了 xrandr 显示的所有其他可用显示速率---见下文。）
这会导致屏幕闪烁，但随后返回到 1920x1080（默认在 60hz 下可以正常工作）。
我也尝试过创建新的模式：
% gtf 3840 2160 30
# 3840x2160 @ 30.00 Hz (GTF) hsync: 65.91 kHz; pclk: 339.57 MHz
Modeline "3840x2160_30.00" 339.57 3840 4080 4496 5152 2160 2161 2164 2197 -HSync +Vsync
% xrandr --newmode testmode 339.57 3840 4080 4496 5152 2160 2161 2164 2197 -HSync +Vsync
% xrandr --addmode DP-1 testmode
% xrandr --output DP-1 --mode testmode
这实际上会导致一些事情发生。笔记本电脑屏幕和外接显示器都会变成块状、橙色和条纹状，持续约五秒钟，然后恢复到之前的设置。
如果我随后立即查看 /var/log/syslog 的末尾，它会显示：
Apr 1 18:50:40 hal-blade kernel: [ 2682.254996] [drm:intel_dp_link_training_clock_recovery [i915_bpo]] *ERROR* failed to enable link training
Apr 1 18:50:40 hal-blade kernel: [ 2682.263677] [drm:intel_dp_start_link_train [i915_bpo]] *ERROR* failed to start channel equalization
Apr 1 18:50:40 hal-blade kernel: [ 2682.355692] [drm:intel_cpu_fifo_underrun_irq_handler [i915_bpo]] *ERROR* CPU pipe B FIFO underrun
Apr 1 18:50:40 hal-blade kernel: [ 2682.360121] [drm:intel_dp_link_training_clock_recovery [i915_bpo]] *ERROR* too many voltage retries, give up
Apr 1 18:50:40 hal-blade kernel: [ 2682.367507] [drm:intel_dp_link_training_clock_recovery [i915_bpo]] *ERROR* too many voltage retries, give up
Apr 1 18:50:40 hal-blade kernel: [ 2682.374924] [drm:intel_dp_link_training_clock_recovery [i915_bpo]] *ERROR* too many voltage retries, give up
Apr 1 18:50:40 hal-blade kernel: [ 2682.382413] [drm:intel_dp_link_training_clock_recovery [i915_bpo]] *ERROR* too many voltage retries, give up
最后，这里有一些输出，希望它们有用：
% xrandr --verbose
... (skipping part about built in display) ...
DP-1 connected primary 1920x1080+0+0 (0x79) normal (normal left inverted right x axis y axis) 600mm x 340mm
Identifier: 0x43
Timestamp: 22921
Subpixel: unknown
Gamma: 1.0:1.0:1.0
Brightness: 1.0
Clones: HDMI-1
CRTC: 0
CRTCs: 0 1 2
Transform: 1.000000 0.000000 0.000000
0.000000 1.000000 0.000000
0.000000 0.000000 1.000000
filter:
EDID:
00ffffffffffff001e6d085b766d0000
021b0103803c2278ea3035a7554ea326
0f50542108007140818081c0a9c0d1c0
81000101010104740030f2705a80b058
8a0058542100001e565e00a0a0a02950
3020350058542100001a000000fd0038
3d1e871e000a202020202020000000fc
004c4720556c7472612048440a2001a7
02031d7146902205040301230907076d
030c001000b83c200060010203023a80
1871382d40582c450058542100001e00
0000ff003730324e54524c3055303232
0a000000000000000000000000000000
00000000000000000000000000000000
00000000000000000000000000000000
0000000000000000000000000000008f
Broadcast RGB: Automatic
supported: Automatic, Full, Limited 16:235
audio: auto
supported: force-dvi, off, auto, on
3840x2160 (0x73) 297.000MHz +HSync +VSync +preferred
h: width 3840 start 4016 end 4104 total 4400 skew 0 clock 67.50KHz
v: height 2160 start 2168 end 2178 total 2250 clock 30.00Hz
3840x2160 (0x74) 297.000MHz +HSync +VSync
h: width 3840 start 4896 end 4984 total 5280 skew 0 clock 56.25KHz
v: height 2160 start 2168 end 2178 total 2250 clock 25.00Hz
3840x2160 (0x75) 297.000MHz +HSync +VSync
h: width 3840 start 5116 end 5204 total 5500 skew 0 clock 54.00KHz
v: height 2160 start 2168 end 2178 total 2250 clock 24.00Hz
3840x2160 (0x76) 296.703MHz +HSync +VSync
h: width 3840 start 4016 end 4104 total 4400 skew 0 clock 67.43KHz
v: height 2160 start 2168 end 2178 total 2250 clock 29.97Hz
3840x2160 (0x77) 296.703MHz +HSync +VSync
h: width 3840 start 5116 end 5204 total 5500 skew 0 clock 53.95KHz
v: height 2160 start 2168 end 2178 total 2250 clock 23.98Hz
... (skipping lower resolution items) ...
HDMI-1 disconnected (normal left inverted right x axis y axis)
Identifier: 0x44
Timestamp: 22921
Subpixel: unknown
Clones: DP-1
CRTCs: 0 1 2
Transform: 1.000000 0.000000 0.000000
0.000000 1.000000 0.000000
0.000000 0.000000 1.000000
filter:
aspect ratio: Automatic
supported: Automatic, 4:3, 16:9
Broadcast RGB: Automatic
supported: Automatic, Full, Limited 16:235
audio: auto
supported: force-dvi, off, auto, on
（注意：我不完全确定 DP-1/HDMI-1 是怎么回事。机器上只有一个 HDMI 端口。还有 DP-2 和 HDMI-2。）
% lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation Device 5916 (rev 02)
我不确定还有什么有用...这是在 Ubuntu 16.04.2 LTS xenial 上，内核为 4.4.0-71-generic。
谢谢！