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

[Maxime Ripard <mripard@kernel.org>]

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On Wed, Jun 12, 2024 at 07:49:31AM GMT, Doug Anderson wrote:
> Hi,
> 
> On Wed, Jun 12, 2024 at 1:58 AM Daniel Vetter <daniel@ffwll.ch> wrote:
> >
> > On Tue, Jun 11, 2024 at 07:48:51AM -0700, Douglas Anderson wrote:
> > > At shutdown if you've got a _properly_ coded DRM modeset driver then
> > > you'll get these two warnings at shutdown time:
> > >
> > >   Skipping disable of already disabled panel
> > >   Skipping unprepare of already unprepared panel
> > >
> > > These warnings are ugly and sound concerning, but they're actually a
> > > sign of a properly working system. That's not great.
> > >
> > > It's not easy to get rid of these warnings. Until we know that all DRM
> > > modeset drivers used with panel-simple and panel-edp are properly
> > > calling drm_atomic_helper_shutdown() or drm_helper_force_disable_all()
> > > then the panel drivers _need_ to disable/unprepare themselves in order
> > > to power off the panel cleanly. However, there are lots of DRM modeset
> > > drivers used with panel-edp and panel-simple and it's hard to know
> > > when we've got them all. Since the warning happens only on the drivers
> > > that _are_ updated there's nothing to encourage broken DRM modeset
> > > drivers to get fixed.
> > >
> > > In order to flip the warning to the proper place, we need to know
> > > which modeset drivers are going to shutdown properly. Though ugly, do
> > > this by creating a list of everyone that shuts down properly. This
> > > allows us to generate a warning for the correct case and also lets us
> > > get rid of the warning for drivers that are shutting down properly.
> > >
> > > Maintaining this list is ugly, but the idea is that it's only short
> > > term. Once everyone is converted we can delete the list and call it
> > > done. The list is ugly enough and adding to it is annoying enough that
> > > people should push to make this happen.
> > >
> > > Implement this all in a shared "header" file included by the two panel
> > > drivers that need it. This avoids us adding an new exports while still
> > > allowing the panel drivers to be modules. The code waste should be
> > > small and, as per above, the whole solution is temporary.
> > >
> > > Signed-off-by: Douglas Anderson <dianders@chromium.org>
> > > ---
> > > I came up with this idea to help us move forward since otherwise I
> > > couldn't see how we were ever going to fix panel-simple and panel-edp
> > > since they're used by so many DRM Modeset drivers. It's a bit ugly but
> > > I don't hate it. What do others think?
> >
> > I think it's terrible :-)
> 
> Well, we're in agreement. It is terrible. However, in my opinion it's
> still a reasonable way to move forward.
> 
> 
> > 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.

Maxime

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