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
