[RFC 2/7] drm/fb-helper: Ensure driver module is pinned in fb_open()

[Noralf Trønnes]

If struct fb_ops is defined in a library like cma, fb_open() and fbcon
takes a ref on the library instead of the driver module. Use
fb_ops.fb_open/fb_release to ensure that the driver module is pinned.

Add drm_fb_helper_fb_open() and drm_fb_helper_fb_release() to
DRM_FB_HELPER_DEFAULT_OPS().

Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
---
 drivers/gpu/drm/drm_fb_helper.c | 40 ++++++++++++++++++++++++++++++++++++++++
 include/drm/drm_fb_helper.h     | 15 +++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 035784ddd133..2c6adf1d80c2 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -1207,6 +1207,46 @@ void drm_fb_helper_cfb_imageblit(struct fb_info *info,
 }
 EXPORT_SYMBOL(drm_fb_helper_cfb_imageblit);
 
+/**
+ * drm_fb_helper_fb_open - implementation for &fb_ops.fb_open
+ * @info: fbdev registered by the helper
+ * @user: 1=userspace, 0=fbcon
+ *
+ * If &fb_ops is wrapped in a library, pin the driver module.
+ */
+int drm_fb_helper_fb_open(struct fb_info *info, int user)
+{
+	struct drm_fb_helper *fb_helper = info->par;
+	struct drm_device *dev = fb_helper->dev;
+
+	if (info->fbops->owner != dev->driver->fops->owner) {
+		if (!try_module_get(dev->driver->fops->owner))
+			return -ENODEV;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_fb_helper_fb_open);
+
+/**
+ * drm_fb_helper_fb_release - implementation for &fb_ops.fb_release
+ * @info: fbdev registered by the helper
+ * @user: 1=userspace, 0=fbcon
+ *
+ * If &fb_ops is wrapped in a library, unpin the driver module.
+ */
+int drm_fb_helper_fb_release(struct fb_info *info, int user)
+{
+	struct drm_fb_helper *fb_helper = info->par;
+	struct drm_device *dev = fb_helper->dev;
+
+	if (info->fbops->owner != dev->driver->fops->owner)
+		module_put(dev->driver->fops->owner);
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_fb_helper_fb_release);
+
 /**
  * drm_fb_helper_set_suspend - wrapper around fb_set_suspend
  * @fb_helper: driver-allocated fbdev helper, can be NULL
diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index b069433e7fc1..6f546ca3a6a1 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -241,6 +241,8 @@ struct drm_fb_helper {
  * functions. To be used in struct fb_ops of drm drivers.
  */
 #define DRM_FB_HELPER_DEFAULT_OPS \
+	.fb_open	= drm_fb_helper_fb_open, \
+	.fb_release	= drm_fb_helper_fb_release, \
 	.fb_check_var	= drm_fb_helper_check_var, \
 	.fb_set_par	= drm_fb_helper_set_par, \
 	.fb_setcmap	= drm_fb_helper_setcmap, \
@@ -297,6 +299,9 @@ void drm_fb_helper_cfb_copyarea(struct fb_info *info,
 void drm_fb_helper_cfb_imageblit(struct fb_info *info,
 				 const struct fb_image *image);
 
+int drm_fb_helper_fb_open(struct fb_info *info, int user);
+int drm_fb_helper_fb_release(struct fb_info *info, int user);
+
 void drm_fb_helper_set_suspend(struct drm_fb_helper *fb_helper, bool suspend);
 void drm_fb_helper_set_suspend_unlocked(struct drm_fb_helper *fb_helper,
 					bool suspend);
@@ -473,6 +478,16 @@ static inline void drm_fb_helper_cfb_imageblit(struct fb_info *info,
 {
 }
 
+static inline int drm_fb_helper_fb_open(struct fb_info *info, int user)
+{
+	return -ENODEV;
+}
+
+static inline int drm_fb_helper_fb_release(struct fb_info *info, int user)
+{
+	return -ENODEV;
+}
+
 static inline void drm_fb_helper_set_suspend(struct drm_fb_helper *fb_helper,
 					     bool suspend)
 {
-- 
2.14.2