Reuse framebuffer after a kexec (amdgpu / efifb)

[Alex Deucher]

On Fri, Dec 10, 2021 at 10:24 AM Guilherme G. Piccoli
<gpiccoli at igalia.com> wrote:
>
> On 10/12/2021 12:13, Christian König wrote:
> > [...]
> > How about issuing a PCIe reset and re-initializing the ASIC with just
> > the VBIOS?
> >
> > That should be pretty straightforward I think.
> >
> > Christian.
>
>
> Thanks Christian, that'd be perfect! Is it feasible? Per Alex comment,
> we'd need to run atombios commands to reprogram the timings, display
> info, etc...like a small driver would do, a full init.
>

You need the equivalent of a GOP driver or a full GPU driver.  I think
it would be less effort to just fix up any problems amdgpu has when
trying to load after the crash than to write a new mini driver.  By
the time you add everything you'd need, you'd be pretty close to a
full GPU driver.

> Also, what kind of PCIe reset is recommended for this adapter? Like a
> hot reset, powering-off/re-power, FLR or that MODE2 reset present in
> amdgpu code? Remembering this is an APU device.

You'd need to issue the relevant device specific reset sequence.  It
would be a mode2 reset on vangogh, but varies on other asics.  It
would probably be easiest to just fix up the logic in amdgpu to detect
bad GPU state on driver load and do a GPU reset before driver init.
We already have the logic in place for some dGPUs, but APUs only
recently got full GPU reset support due to architectural limitations
and hardware bugs.

Alex