[lug] Dell Optiplex 9020 + ubuntu + 4k monitor

[D. Stimits]

> On November 10, 2019 at 4:21 PM Bear Giles <bgiles at coyotesong.com> wrote:
> 
>     I haven't worked through all of the permutations of cabling yet but
> 
>      - DP -> HDMI dongle -> KVM gives me low resolution
>      - DP -> DP cable gives me high resolution
>      - mini-DP (on second Linux system) -> HDMI dongle -> KVM gives me high resolution.
> 
>     I have a DP -> HDMI cable - no dongle required, but I haven't tried it with either the KVM or direct connection yet.
> 
>     I also haven't checked whether the system now realizes that it can use 4k with the dongle and KVM. I wouldn't be surprised if a driver doesn't recognize a monitor supports 4k via one mechanism but can use that information if it's obtained via a different mechanism.
> 
>     Maybe I'm cynical but I had a lightweight dell laptop - the niche now filled by chromebooks - that could no longer support its video for a few releases. I happened to discover that I could still bring up an external display with the VGA port... and that once I had established a connection with the external VGA port I could use the laptop's display without any problems. ?!?!
> 
>     It's not a problem at home but the entire point of these lightweight laptops is that it's not a big loss if you lose one while traveling. Needing to be able to connect to an external VGA monitor sharply limits the functionality of them. Some hotels had a panel with different types of connectors but many didn't.
> 
...
VGA is not hot plug, and thus the driver picks a resolution without knowledge of what is viewing the signal. HDMI/DisplayPort tries to use EDID, and has a detect which tries to do an i2c query upon plug-in. Now if HDMI is unable to handle the EDID data of a plug-in, then it either maintains its current mode, or else it goes to a fallback mode.

The EDID also has what it calls "extension" modes, and not every driver will see or use extensions for whatever reason. Thus an extension mode may be a valid mode, but the mode may not be visible under all circumstances.

If for some reason you are able to set a mode, e.g., through use of a VGA device not reporting EDID (meaning manual setting), and then connect a hot plug device which does support that mode, and yet only supports the device in an ignored extension mode, then the resolution will remain what it was when the VGA device was plugged in.

There is a good chance that the higher resolution modes which are supported, but not usable without workarounds, is an extension mode.

Most of the VGA-to-HDMI adapters simply drop EDID and the driver reverts to various standard modes. Other adapters will add an EDID which lists all of the more standard modes, but unless the monitor can actually use that mode (and also contains every mode the monitor can use), the adapter will cause loss of some modes. There are programmable adapters which allow you to put EDID data in the adapter to match your VGA monitor, and then you connect the VGA monitor into the HDMI port through this adapter...the surrogate EDID data substitutes for the old style non-plug-n-play driver disk of the monitor.
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