Reuse framebuffer after a kexec (amdgpu / efifb)

[Christian König]

Hi Guilherme,

Am 09.12.21 um 17:00 schrieb Guilherme G. Piccoli:
> 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)?

unfortunately what you try here will most likely not work easily.

During bootup the ASIC is initialized in a VGA compatibility mode by the 
VBIOS which also allows efifb to display something. And among the first 
things amdgpu does is to disable this compatibility mode :)

What you need to do to get this working again is to issue a PCIe reset 
of the GPU and then re-init the ASIC with the VBIOS tables.

Alex should know more details about how to do this.

Regards,
Christian.

>
> Any advice is greatly appreciated!
> Thanks in advance,
>
>
> Guilherme