[alin23]

Most of the delay is I/O bound. Handshaking happens over the Display Data Channel (DDC) which is a protocol over I²C (a two-wire comunication bus). HDMI/DP/DVI have 2 dedicated wires inside the cable for this, USB-C has to switch over to use differential signaling on pins A8 and B8, and then switch back after communication is done.

The bandwidth of that bus is small indeed, but most of the latency comes from artificial delays and error recovery in code: monitor is sending a message to host then sleeping 30ms to make sure the reply is ready, host doing the same thing, and this happens multiple times to negotiate pixel clock, signal timing, colors, resolution, additional features like suspend/reset/HDR/overscan etc.)

You can see how in the MCCS specification [1] both host and display entrypoints for communication start with a delay: https://shots.panaitiu.com/e8D7tQ

You can also see how that looks in Lunar's DDC code [2] [3].

For now, it's incredible that we actually have such a standard as DDC and most monitors work with most devices. A coordinated effort between monitor vendors and laptop/PC/GPU manufacturers into creating another faster standard with modern technology seems out of the question. But maybe with the advent of Thunderbolt this may change in the future.

[1] https://files.lunar.fyi/mccs.pdf

[2] https://github.com/alin23/Lunar/blob/master/Lunar/DDC/DDC.c#... : Delay on requesting data from monitor

[3] https://github.com/alin23/Lunar/blob/master/Lunar/DDC/DDC.c#... : Delay on communication error before retry

[wzdd]

Naively it seems like you could optimise this at least for the second and subsequent switches: you could detect if the cable corresponding to the input you've previously switched to has been reconnected since last switch. If it hasn't, then you could re-use the previous EDID values for the device.

You'd obviously want this to be optional for special cases, but would that work, if implemented in a monitor or KVM?

It's not so much the delay that's annoying, but the renegotiation with something which is so obviously the same device.

[alin23]

I believe that is already being done. As I said in my comment here (https://news.ycombinator.com/item?id=34049171) switching between two active inputs takes about 1 second for me.

The problem that OP is facing is switching to an inactive input that went into power saving mode. In that case, the link between the inactive device and the monitor may have been broken and it's impossible to know if the device is the same as the one before after reconnection (with the same exact video settings etc.).

This doesn't have to be the case though. Before I found out how to use I²C on Apple Silicon Macs, I was using a Raspberry Pi connected to an HDMI input of the monitor that could listen to DDC commands through an HTTP server: https://alinpanaitiu.com/blog/journey-to-ddc-on-m1-macs/#the...

And because I didn't want that Pi Zero to get hot, I managed to only keep the DDC lines open and stopped the video signal from being sent using `vcgencmd display_power 0`.

With those settings, the monitor still switched between inputs in 1 second, with the added small latency of having to run `vcgencmd display_power 1` (which was fast enough once it was automated to run on input switching)