[PATCH] drm/panel: Avoid warnings w/ panel-simple/panel-edp at shutdown

[Doug Anderson]

Hi,

On Wed, Jun 12, 2024 at 8:03 AM Maxime Ripard <mripard at kernel.org> wrote:
>
> > > Why does something like this now work?
> > >
> > > drm_panel_shutdown_fixup(panel)
> > > {
> > >         /* if you get warnings here, fix your main drm driver to call
> > >          * drm_atomic_helper_shutdown()
> > >          */
> > >         if (WARN_ON(panel->enabled))
> > >                 drm_panel_disable(panel);
> > >
> > >         if (WARN_ON(panel->prepared))
> > >                 drm_panel_unprepare(panel);
> > > }
> > >
> > > And then call that little helper in the relevant panel drivers? Also feel
> > > free to bikeshed the name and maybe put a more lengthly explainer into the
> > > kerneldoc for that ...
> > >
> > > Or am I completely missing the point here?
> >
> > The problem is that the ordering is wrong, I think. Even if the OS was
> > calling driver shutdown functions in the perfect order (which I'm not
> > convinced about since panels aren't always child "struct device"s of
> > the DRM device), the OS should be calling panel shutdown _before_
> > shutting down the DRM device. That means that with your suggestion:
> >
> > 1. Shutdown starts and panel is on.
> >
> > 2. OS calls panel shutdown call, which prints warnings because panel
> > is still on.
> >
> > 3. OS calls DRM driver shutdown call, which prints warnings because
> > someone else turned the panel off.
> >
> > :-P
> >
> > Certainly if I goofed and the above is wrong then let me know--I did
> > my experiments on this many months ago and didn't try repeating them
> > again now.
> >
> > In any case, the only way I could figure out around this was some sort
> > of list. As mentioned in the commit message, it's super ugly and
> > intended to be temporary. Once we solve all the current in-tree
> > drivers, I'd imagine that long term for new DRM drivers this kind of
> > thing would be discovered during bringup of new boards. Usually during
> > bringup of new boards EEs measure timing signals and complain if
> > they're not right. If some EE cared and said we weren't disabling the
> > panel correctly at shutdown time then we'd know there was a problem.
>
> Based on a discussion we had today With Sima on IRC, I think there's
> another way forward.
>
> We were actually discussing refcount'ing the panels to avoid lifetime
> issues. It would require some API overhaul to have a function to
> allocate the drm_panel structure and init'ing the refcount, plus some to
> get / put the references.
>
> Having this refcount would mean that we also get a release function now,
> called when the panel is free'd.
>
> Could we warn if the panel is still prepared/enabled and is about to be
> freed?
>
> It would require to switch panel-simple and panel-edp to that new API,
> but it should be easy enough.

I think there are two problems here:

1. The problem is at shutdown here. Memory isn't freed at shutdown
time. This isn't a lifetime issue. No release functions are involved
in shutdown and we don't free memory then.

2. As I tried to point out, even if we were guaranteed the correct
order it still doesn't help us. In other words: if all device links
were perfect and all ordering was proper then the panel should get
shutdown _before_ the DRM device. That means we can't put a check in
the panel code to see if the DRM device has been shutdown.


-Doug