Support Custom modeline

[FlyingWombat]

My display needs custom modeline timings set to work properly (my gpu can't reclock its memory). I'm just checking out Niri, but even if I don't end up using Niri long-term, this will be helpful for others.

An example config could be something like this:

output "DP-1" {
    mode "1332.75 3840 3888 3920 4000 2160 2163 2168 2314 +HSync -VSync"
}

[YaLTeR]

Hm, I'm not sure off the bat how to do that, might need Smithay support.

[FlyingWombat]

Sway/wlroots supports it with output DP-1 modeline <modeline>. That might be useful for reference.

Should I create a related issue in Smithay?

[YaLTeR]

I'll ask in the Smithay room. Maybe it can already do it.

[FlyingWombat]

FYI, I found out that linux can load custom edid firmware in KMS -- so it isn't strictly necessary to configure custom modelines from the compositor. That said, it will be much more user friendly to be able to set it from the compositor.

---

Here is some more info for anyone else who comes across this thread.

Having the kernel load custom edid firmware was easy. Just put the .bin in /usr/lib/firmware/edid/ and add drm.edid_firmware=edid/your_edid.bin to kernel cmdline.

The trouble comes in getting a proper edid binary. You can try edid-generator or building one from scratch, but most will be better off editing a copy of the existing firmware. You can grab a connection's edid with e.g. cat /sys/class/drm/card2-DP-5/edid > edid.bin.

In my case, I had to edit it by hand with a hex editor. None of the edid editors I found worked properly. wxEDID doesn't support DisplayID extension blocks -- though it may work for edid bins only containing EDID with CTA extension. CRU on wine supported all the exensions, but rewrote the entire edid assuming my display was a generic analog CRT. There is also edid-generator, but I didn't try that because I wanted to keep the other display properties from the original edid.

If one of the above tools I mentioned works for you, lucky you! Else, you can use a hex editor and follow the byte-maps described on Wikipedia's EDID and DisplayID pages. To help guide the edits, and to validate the changes use edid-decode. When you are done make sure to update the checksum bytes, if edid-decode says they don't match.

[edrex]

@FlyingWombat I went down the edid rabbit hole several years ago to add a missing half-res mode to my hidpi laptop panel. It worked but I don't recall what method I used to modify the edid file. I admire your determination :D Good job documenting the process; it's rather esoteric. Agreed that modelines in compositor are a desirable feature.