Getting Guix System to use llvmpipe

[Philip McGrath <philip@philipmcgrath.com>]

Hi folks,

I'm trying to install Guix System on real hardware for the first time, 
and I'm having some graphics trouble.

The machine has an older Asus M5A78L-M/USB3 motherboard with an onboard 
Radeon HD 3000 GPU---the R600 family, IIUC, so covered by the `radeon` 
driver rather than `amdgpu`. I understand that functionality will be 
limited since I'm not using nonfree firmware, and I can live with that.

I've tried Debian 11 on this board (without "nonfree"), and, after 
noting the missing firmware, it falls back to work acceptably with 
`llvmpipe`.

Unfortunately, Guix System is working less well. When it starts the 
display manager (I've tried both GDM and SDDM, with both X11 and 
Wayland), the screen goes blank permanently, and the logs show that 
starting the X server fails: initially with an error about an undefined 
symbol `exaGetPixmapDriverPrivate`, though through various incantations 
on the kernel command line or in the X configuration I can produce other 
errors, like "X: ../include/privates.h:121: dixGetPrivateAddr: Assertion 
`key->initialized' failed."

Can I configure Guix to give up on the missing firmware and just use 
`llvmpipe`, as Debian does?

Or, of course, if there's something even better I should do, I'd be glad 
to hear about that!

Thanks,
Philip