Forcing fkms over fbturbo for RPi4

Is there a combination of settings in config.txt and/or cmdline.txt that can force kms/fkms over fbturbo?

I know removing the /etc/X11/xorg.conf.d/99-fbturbo.conf will do it, but is there a way from startup files? I would like to leave xf86-video-fbturbo-git installed, and be able to switch between software and hardware rendering via the startup files.

I have never seen any one do it. It appears when Xorg loads up it looks first to see if fbturbo is loaded.

I switch back and forth all of the time by renaming xorg.conf.d/99-fbturbo.conf back and forth.

sudo mv /etc/X11/xorg.conf.d/99-fbturbo.conf etc/X11/xorg.conf.d/99-fbturbo~

I figured as much, but was hoping there might be some kernel command line parameter that could do it.

Since the need for this has to do with a read-only nfsroot with a systemd.overlayfs, I created a nofbturbo.service which executes a bash script that greps /proc/cmdline for a nofbturbo kernel command line parameter. If nofbturbo is present, it “deletes” the 99-fbturbo.conf file from the overlayfs. Now all nfsroot clients use the same image, and the tftp boot config files alone, dictates the rendering. So some clients are hardware and some software… pretty spiffy if I may say so myself.

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