X hangs at console on start with modesetting driver and Iris mesa driver on Broadwell
I have a Broadwell laptop with an Intel HD 5500 GPU. It is currently running Linux 5.5-rc1 with Mesa 19.3-rc6 and Xorg 1.20.6, but the issue occurs with Linux 5.4.x and Mesa 19.2 as well. If I set MESA_LOADER_DRIVER_OVERRIDE=iris and execute "startx", it hangs at the console and nothing else is ever displayed besides the mouse cursor. If I switch to another VT and back, X quits (crashes?) and I get the bash prompt again.
If I use the "intel" Xorg driver instead of "modesetting", the issue does not occur. Also, if I do the same thing on a Skylake system, the issue does not occur. (Needless to say, the issue does not occur with the mesa "i965" driver either.)
This is related to mesa/mesa#2107 (closed). I initially reported that issue after getting tearing with kwin when using iris, but later discovered that when using iris for modesetting/glamor itself, this issue occurs. https://gitlab.freedesktop.org/mesa/mesa/uploads/4485ccd8a955b522feb14336ec2c6bbb/Xorg.0.log is an Xorg log recorded when reproducing the issue. There is also some additional relevant info attached to that bug report, but I am of course willing to provide anything else that would help. It was recommended there that I report the secondary issue with the hang on start here.