[PATCH RFC] drm/bridge: panel: Add module_get/but calls to attached panel driver

[Jyri Sarha]

Currently there is no way for a master drm driver to protect against an
attached panel driver from being unloaded while it is in use. The
least we can do is to indicate the usage by incrementing the module
reference count.

Signed-off-by: Jyri Sarha <jsarha at ti.com>
cc: eric at anholt.net
cc: laurent.pinchart at ideasonboard.com
---
I do not see any module_get/put code in drm core. Is there is a reason
for that?

There is two more alternative places for adding the module_get/put
code. One is puting it directly to drm_panel_attach() and
drm_panel_detach(). However, if the same module implements both the
master drm driver and the panel (like tilcdc does with its
tilcdc_panel.c), then attaching the panel will lock the module in for
no good reason. Still, this solution should work with drm bridges as I
do not see any reason why anybody would implement bridge drivers in
the same module with the master drm driver.

The other place to put the code would in the master drm driver. But
for handling the situation with bridges would need the device pointer
in struct drm_bridge.

 drivers/gpu/drm/bridge/panel.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/bridge/panel.c b/drivers/gpu/drm/bridge/panel.c
index 6d99d4a..0a10be6 100644
--- a/drivers/gpu/drm/bridge/panel.c
+++ b/drivers/gpu/drm/bridge/panel.c
@@ -161,6 +161,10 @@ struct drm_bridge *drm_panel_bridge_add(struct drm_panel *panel,
 	if (!panel)
 		return ERR_PTR(-EINVAL);
 
+	if (WARN_ON(!panel->dev->driver) ||
+	    !try_module_get(panel->dev->driver->owner))
+		return ERR_PTR(-ENODEV);
+
 	panel_bridge = devm_kzalloc(panel->dev, sizeof(*panel_bridge),
 				    GFP_KERNEL);
 	if (!panel_bridge)
@@ -199,6 +203,9 @@ void drm_panel_bridge_remove(struct drm_bridge *bridge)
 	panel_bridge = drm_bridge_to_panel_bridge(bridge);
 
 	drm_bridge_remove(bridge);
+
+	module_put(panel_bridge->panel->dev->driver->owner);
+
 	devm_kfree(panel_bridge->panel->dev, bridge);
 }
 EXPORT_SYMBOL(drm_panel_bridge_remove);
-- 
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