A hotplug bug in AMDGPU

[Mikulas Patocka]

On Mon, 3 May 2021, Alex Deucher wrote:

> On Mon, May 3, 2021 at 11:40 AM Mikulas Patocka <mpatocka at redhat.com> wrote:
> >
> > Hi
> >
> > There's a bug with monitor hotplug starting with the kernel 5.7.
> >
> > I have Radeon RX 570. If I boot the system with the monitor unplugged and
> > then plug the monitor via DVI, the kernel 5.6 and below will properly
> > initialized graphics; the kernels 5.7+ will not initialize it - and the
> > monitor reports no signal.
> >
> > I bisected the issue and it is caused by the patch
> > 4fdda2e66de0b7d37aa27af3c1bbe25ecb2d5408 ("drm/amdgpu/runpm: enable runpm
> > on baco capable VI+ asics")
> >
> > When I remove the code that sets adev->runpm on the kernel 5.12, monitor
> > hotplug works correctly.
> 
> This isn't really a hotplug bug per se.  That patch enabled runtime
> power management which powered down the GPU completely to save power.
> Unfortunately when it's powered down, hotplug interrupts won't work
> because the entire GPU is powered off.  Disabling runtime pm will
> allow hotplug interrupts to work, but will cause the GPU to burn a lot
> more power.

I measured it and it saves 15W. Hard to say if it's worth to pay this for 
the hotplug capability or not.

I can re-activate the card by logging in and typing "rmmod amdgpu;modprobe 
amdgpu". But what should less technically savvy users do?

> I'm not sure what the best solution is.  You can manually
> wake the card via sysfs (either via the runtime pm controls in
> /sys/class/drm/card0/device/power or by reading a sensor on the board
> like temperature) then hotplut the monitor or via a direct request to
> probe the displays via the display server.
> 
> Alex

Mikulas