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

[Maxime Ripard]

Hi,

On Tue, Jun 11, 2024 at 07:48:51AM GMT, 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 at 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 don't think it's the right approach, even more so since we're so close
now to having it in every driver.

I ran the coccinelle script we started with, and here are the results:

./drivers/gpu/drm/vmwgfx/vmwgfx_drv.c:1640:25-39: ERROR: KMS driver vmw_pci_driver is missing shutdown implementation
./drivers/gpu/drm/kmb/kmb_drv.c:621:30-49: ERROR: KMS driver kmb_platform_driver is missing shutdown implementation
./drivers/gpu/drm/tiny/arcpgu.c:422:30-52: ERROR: KMS driver arcpgu_platform_driver is missing shutdown implementation

Looking at the drivers by hand, it seems consistent.

Moving forward, I think having a collection of coccinelle scripts that
we ask new driver authors to run or put them in CI somehow would be a
better path. We have other similar candidates that can't really be dealt
with any other way, like not using drmm_ memory allocations, or not
using drm_dev_enter / drm_dev_exit.

Maxime
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