[PATCH v2 8/8] drm/amdgpu: Call drm_atomic_helper_shutdown() at shutdown time

[Doug Anderson]

Hi,


On Mon, Jun 17, 2024 at 8:01 AM Alex Deucher <alexdeucher at gmail.com> wrote:
>
> On Wed, Jun 12, 2024 at 6:37 PM Douglas Anderson <dianders at chromium.org> wrote:
> >
> > Based on grepping through the source code this driver appears to be
> > missing a call to drm_atomic_helper_shutdown() at system shutdown
> > time. Among other things, this means that if a panel is in use that it
> > won't be cleanly powered off at system shutdown time.
> >
> > The fact that we should call drm_atomic_helper_shutdown() in the case
> > of OS shutdown/restart comes straight out of the kernel doc "driver
> > instance overview" in drm_drv.c.
> >
> > Suggested-by: Maxime Ripard <mripard at kernel.org>
> > Cc: Alex Deucher <alexander.deucher at amd.com>
> > Cc: Christian König <christian.koenig at amd.com>
> > Cc: Xinhui Pan <Xinhui.Pan at amd.com>
> > Signed-off-by: Douglas Anderson <dianders at chromium.org>
> > ---
> > This commit is only compile-time tested.
> >
> > ...and further, I'd say that this patch is more of a plea for help
> > than a patch I think is actually right. I'm _fairly_ certain that
> > drm/amdgpu needs this call at shutdown time but the logic is a bit
> > hard for me to follow. I'd appreciate if anyone who actually knows
> > what this should look like could illuminate me, or perhaps even just
> > post a patch themselves!
>
> I'm not sure this patch makes sense or not.  The driver doesn't really
> do a formal tear down in its shutdown routine, it just quiesces the
> hardware.  What are the actual requirements of the shutdown function?
> In the past when we did a full driver tear down in shutdown, it
> delayed the shutdown sequence and users complained.

The "inspiration" for this patch is to handle panels properly.
Specifically, panels often have several power/enable signals going to
them and often have requirements that these signals are powered off in
the proper order with the proper delays between them. While we can't
always do so when the system crashes / reboots in an uncontrolled way,
panel manufacturers / HW Engineers get upset if we don't power things
off properly during an orderly shutdown/reboot. When panels are
powered off badly it can cause garbage on the screen and, so I've been
told, can even cause long term damage to the panels over time.

In Linux, some panel drivers have tried to ensure a proper poweroff of
the panel by handling the shutdown() call themselves. However, this is
ugly and panel maintainers want panel drivers to stop doing it. We
have removed the code doing this from most panels now [1]. Instead the
assumption is that the DRM modeset drivers should be calling
drm_atomic_helper_shutdown() which will make sure panels get an
orderly shutdown.

For a lot more details, see the cover letter [2] which then contains
links to even more discussions about the topic.

[1] https://lore.kernel.org/r/20240605002401.2848541-1-dianders@chromium.org
[2] https://lore.kernel.org/r/20240612222435.3188234-1-dianders@chromium.org