drm/panel: simple: Add a comment about unprepare+disable at shutdown/remove

It's the responsibility of a correctly written DRM modeset driver to
call drm_atomic_helper_shutdown() at shutdown time and that should be
disabling / unpreparing the panel if needed. Panel drivers shouldn't
be calling these functions themselves.

A recent effort was made to fix as many DRM modeset drivers as
possible [1] [2] [3] and most drivers are fixed now.

Unfortunately, it's very difficult to know exactly which DRM modeset
drivers are using panel-simple due to the sheer number of panels it
handles. For now, we'll leave the calls and just add a comment to keep
people from copying this code.

[1] https://lore.kernel.org/r/[e-mail]
[2] https://lore.kernel.org/r/[e-mail]
[3] https://lore.kernel.org/r/[e-mail]

Acked-by: Linus Walleij <[e-mail]>
Acked-by: Maxime Ripard <[e-mail]>
Signed-off-by: Douglas Anderson <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/20240503143327.RFT.v2.27.I639183ac987e139092491a94e22d46a5d857580c@changeid

drivers/gpu/drm/panel/panel-simple.c

···
 	return err;
 }
 
+static void panel_simple_shutdown(struct device *dev)
+{
+	struct panel_simple *panel = dev_get_drvdata(dev);
+
+	/*
+	 * NOTE: the following two calls don't really belong here. It is the
+	 * responsibility of a correctly written DRM modeset driver to call
+	 * drm_atomic_helper_shutdown() at shutdown time and that should
+	 * cause the panel to be disabled / unprepared if needed. For now,
+	 * however, we'll keep these calls due to the sheer number of
+	 * different DRM modeset drivers used with panel-simple. The fact that
+	 * we're calling these and _also_ the drm_atomic_helper_shutdown()
+	 * will try to disable/unprepare means that we can get a warning about
+	 * trying to disable/unprepare an already disabled/unprepared panel,
+	 * but that's something we'll have to live with until we've confirmed
+	 * that all DRM modeset drivers are properly calling
+	 * drm_atomic_helper_shutdown().
+	 */
+	drm_panel_disable(&panel->base);
+	drm_panel_unprepare(&panel->base);
+}
+
 static void panel_simple_remove(struct device *dev)
 {
 	struct panel_simple *panel = dev_get_drvdata(dev);
 
 	drm_panel_remove(&panel->base);
-	drm_panel_disable(&panel->base);
-	drm_panel_unprepare(&panel->base);
+	panel_simple_shutdown(dev);
 
 	pm_runtime_dont_use_autosuspend(dev);
 	pm_runtime_disable(dev);
 	if (panel->ddc)
 		put_device(&panel->ddc->dev);
-}
-
-static void panel_simple_shutdown(struct device *dev)
-{
-	struct panel_simple *panel = dev_get_drvdata(dev);
-
-	drm_panel_disable(&panel->base);
-	drm_panel_unprepare(&panel->base);
 }
 
 static const struct drm_display_mode ampire_am_1280800n3tzqw_t00h_mode = {