Reuse framebuffer after a kexec (amdgpu / efifb)

["Guilherme G. Piccoli" <gpiccoli@igalia.com>]

Hi all, I have a question about the possibility of reusing a framebuffer
after a regular (or panic) kexec - my case is with amdgpu (APU, aka, not
a separate GPU hardware), but I guess the question is kinda generic
hence I've looped most of the lists / people I think does make sense
(apologies for duplicates).


The context is: we have a hardware that has an amdgpu-controlled device
(Vangogh model) and as soon as the machine boots, efifb is providing
graphics - I understand the UEFI/GRUB outputs rely in EFI framebuffer as
well. As soon amdgpu module is available, kernel loads it and it takes
over the GPU, providing graphics. The kexec_file_load syscall allows to
pass a valid screen_info structure, so by kexec'ing a new kernel, we
have again efifb taking over on boot time, but this time I see nothing
in the screen. I've manually blacklisted amdgpu in this new kexec'ed
kernel, I'd like to rely in the simple framebuffer - the goal is to have
a tiny kernel kexec'ed. I'm using kernel version 5.16.0-rc4.

I've done some other experiments, for exemple: I've forced screen_info
model to match VLFB, so vesafb took over after the kexec, with the same
result. Also noticed that BusMaster bit was off after kexec, in the AMD
APU PCIe device, so I've set it on efifb before probe, and finally
tested the same things in qemu, with qxl, all with the same result
(blank screen).
The most interesting result I got (both with amdgpu and qemu/qxl) is
that if I blacklist these drivers and let the machine continue using
efifb since the beginning, after kexec the efifb is still able to
produce graphics.

Which then led me to think that likely there's something fundamentally
"blocking" the reuse of the simple framebuffer after kexec, like maybe
DRM stack is destroying the old framebuffer somehow? What kind of
preparation is required at firmware level to make the simple EFI VGA
framebuffer work, and could we perform this in a kexec (or "save it"
before the amdgpu/qxl drivers take over and reuse later)?

Any advice is greatly appreciated!
Thanks in advance,


Guilherme