I regularly use an external monitor, connected through USB-C/thunderbolt to my laptop. It usually works flawlessly, but every now and then (between a few weeks or a few days), the monitor starts blinking (turns black for 1~2 seconds, and this repeats every 10 to 30 seconds). Technically what is apparently happening is that the monitor losses video signal sync on the Displayport connection through the USB-C cable.
I don't think this problem is specific to my devices, as I have read in many places about this behavior on completely different USB-C monitors and laptops. But for the record, I'm having this problem with a viewsonic vp2771, and a System76 Lemur Pro laptop (lemp9). It has an Intel CometLake chipset, with i915 video interface:
~$ sudo lspci -v -s 00:02.0
00:02.0 VGA compatible controller: Intel Corporation CometLake-U GT2 [UHD Graphics] (rev 02) (prog-if 00 [VGA controller])
DeviceName: VGA compatible controller
Subsystem: CLEVO/KAPOK Computer UHD Graphics
Flags: bus master, fast devsel, latency 0, IRQ 137
Memory at b0000000 (64-bit, non-prefetchable) [size=16M]
Memory at a0000000 (64-bit, prefetchable) [size=256M]
I/O ports at 1000 [size=64]
Expansion ROM at 000c0000 [virtual] [disabled] [size=128K]
Capabilities: [40] Vendor Specific Information: Len=0c <?>
Capabilities: [70] Express Root Complex Integrated Endpoint, MSI 00
Capabilities: [ac] MSI: Enable+ Count=1/1 Maskable- 64bit-
Capabilities: [d0] Power Management version 2
Capabilities: [100] Process Address Space ID (PASID)
Capabilities: [200] Address Translation Service (ATS)
Capabilities: [300] Page Request Interface (PRI)
Kernel driver in use: i915
Kernel modules: i915
I don't see any changes in the logs when these blinks happen. I have checked with journalctl --follow and with sudo dmesg --follow, and nothing new is shown during the blackouts. I am not sure where else to look for logs or any sort of feedback.
Things I have tried:
Swapping the cable for other USB-C cables with thunderbolt, shorter and longer, but the blinks keep ocurring.
Dissabling the laptop's battery charging (thus lowering the current draw from 2A, to just 0,5A), but the problem persists.
I have some things connected to the monitor's USB hub (like keyboard and mouse, printer, ethernet adapter), and I have tried disconnecting everything to check if this would solve the problem, but the problem persists.
I usually use the monitor's audio, and on my laptop I select "HDMI / DisplayPort" as sound output. When the blinks started, I have tried using the internal speakers, but the external monitor still flickers.
If I connect the monitor via HDMI, video works correctly. If I reconnect USB-C, blinking resumes.
If I lower the resolution, the flickering becomes much less frequent, although it still happens every couple of minutes.
I can't figure out what triggers this behavior, but after rebooting the laptop, the problem seems to go away for a couple of days.
