PROBLEM: AST2500 BMC video output disabled by reboot (regression)

[Nick Bowler]

Hi Doug,

On Mon, Apr 28, 2025 at 01:40:25PM -0700, Doug Anderson wrote:
> On Sun, Apr 20, 2025 at 9:26■PM Nick Bowler <nbowler at draconx.ca> wrote:
> > I recently noticed that on current kernels I lose video output from
> > my Blackbird's AST2500 BMC after a reboot
[...]
> >   ce3d99c8349584bc0fbe1e21918a3ea1155343aa is the first bad commit
> >   commit ce3d99c8349584bc0fbe1e21918a3ea1155343aa
> >   Author: Douglas Anderson <dianders at chromium.org>
> >   Date:   Fri Sep 1 16:39:53 2023 -0700
> >
> >       drm: Call drm_atomic_helper_shutdown() at shutdown time for misc drivers
[...]
> Bleh. That's not good. If I had to guess there's some subtle bug /
> missing timing constraint that's being triggered here. A few things to
> try:
> 
> 1. Add a several second delay after the call to
> "drm_atomic_helper_shutdown()", like msleep(5000) or something like
> that. That's kind of a shot in the dark, but it's fairly common for
> panels to get upset if you turn them off and then turn them on again
> too quickly. This would be my blind guess of what is happening.

Adding msleep(5000) does nothing except that once the video turns off
it now takes 5 seconds longer to reboot.

> 2. Could you give more details about what panel you're using?

According to the documentation I have for the machine, the video output
of the AST2500 BMC is connected to an IT66121 HDMI transmitter.

Then in turn I have that connected to some generic HDMI->VGA adapter
(PrimeCables branded).  I also tried with another much more expensive
device (Extron DVI-RGB 200) and observe no difference in behaviour.

i think these devices are working and there's just no output signal
on the hdmi port.

> Ideally it'd be great if you could say which device tree you're using too.

Not sure how to answer this.  Do you want me to look at something
specific in /proc/device-tree?  Or dump it somehow?

> 3. Any chance you can gather the `dmesg` from a failing boot and
> provide it somehow? Are there any errors in the logs from the failing
> boot?

To clarify, there is no boot failure.  There is just no video output
after rebooting.  I can then boot Linux again by any method that works
without being able to see the screen, and then everything is fine once
I do that.

I've attached the dmesg output (gzipped) from after such a reboot.
Except for the order and the timestamps, the messages are identical to
when I boot after rebooting a kernel which does not disable the video.

Thanks,
  Nick
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