[RFC v4 21/25] drm/fb-helper: Add drm_fb_helper_fb_open/release()

[Noralf Trønnes]

These helpers keep track of fbdev users and drm_driver.last_close will
only restore fbdev when actually in use. Additionally the display is
turned off when the last user is closing. fbcon is a user in this context.

If struct fb_ops is defined in a library, fb_open() takes a ref on the
library (fb_ops.owner) instead of the driver module. Fix that by ensuring
that the driver module is pinned.

The functions are not added to the DRM_FB_HELPER_DEFAULT_OPS() macro,
because some of its users do set fb_open/release themselves.

Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
---
 drivers/gpu/drm/drm_fb_helper.c | 54 ++++++++++++++++++++++++++++++++++++++++-
 include/drm/drm_fb_helper.h     | 29 ++++++++++++++++++++++
 2 files changed, 82 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 98e5bc92c9f2..b1124c08b1ed 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -177,7 +177,7 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
 	if (!drm_fbdev_emulation || !fb_helper)
 		return -ENODEV;
 
-	if (READ_ONCE(fb_helper->deferred_setup))
+	if (READ_ONCE(fb_helper->deferred_setup) || !READ_ONCE(fb_helper->open_count))
 		return 0;
 
 	mutex_lock(&fb_helper->lock);
@@ -368,6 +368,7 @@ void drm_fb_helper_prepare(struct drm_device *dev, struct drm_fb_helper *helper,
 	INIT_WORK(&helper->dirty_work, drm_fb_helper_dirty_work);
 	helper->dirty_clip.x1 = helper->dirty_clip.y1 = ~0;
 	mutex_init(&helper->lock);
+	helper->open_count = 1;
 	helper->funcs = funcs;
 	helper->dev = dev;
 }
@@ -620,6 +621,53 @@ int drm_fb_helper_defio_init(struct drm_fb_helper *fb_helper)
 }
 EXPORT_SYMBOL(drm_fb_helper_defio_init);
 
+/**
+ * drm_fb_helper_fb_open - implementation for &fb_ops.fb_open
+ * @info: fbdev registered by the helper
+ * @user: 1=userspace, 0=fbcon
+ *
+ * Increase fbdev use count.
+ * 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;
+	}
+
+	fb_helper->open_count++;
+
+	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
+ *
+ * Decrease fbdev use count and turn off if there are no users left.
+ * 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 (!(--fb_helper->open_count))
+		drm_fb_helper_dpms(info, DRM_MODE_DPMS_OFF);
+
+	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_sys_read - wrapper around fb_sys_read
  * @info: fb_info struct pointer
@@ -1436,6 +1484,10 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 	if (ret < 0)
 		return ret;
 
+	/* Block restore without users if we do track it */
+	if (fb_helper->fbdev->fbops->fb_open == drm_fb_helper_fb_open)
+		fb_helper->open_count = 0;
+
 	strcpy(fb_helper->fb->comm, "[fbcon]");
 	return 0;
 }
diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index 5f66f253a97b..330983975d5e 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -184,6 +184,22 @@ struct drm_fb_helper {
 	 * See also: @deferred_setup
 	 */
 	int preferred_bpp;
+
+	/**
+	 * @open_count:
+	 *
+	 * Keeps track of fbdev use to know when to not restore fbdev and to
+	 * disable the pipeline when the last user is gone.
+	 *
+	 * Drivers that use drm_fb_helper_fb_open() as their \.fb_open
+	 * callback will get an initial value of 0 and get restore based on
+	 * actual use. Others will get an initial value of 1 which means that
+	 * fbdev will always be restored. Drivers that call
+	 * drm_fb_helper_fb_open() in their \.fb_open, thus needs to set the
+	 * initial value to 0 themselves in their &drm_fb_helper_funcs->fb_probe
+	 * callback.
+	 */
+	unsigned int open_count;
 };
 
 /**
@@ -230,6 +246,9 @@ void drm_fb_helper_deferred_io(struct fb_info *info,
 			       struct list_head *pagelist);
 int drm_fb_helper_defio_init(struct drm_fb_helper *fb_helper);
 
+int drm_fb_helper_fb_open(struct fb_info *info, int user);
+int drm_fb_helper_fb_release(struct fb_info *info, int user);
+
 ssize_t drm_fb_helper_sys_read(struct fb_info *info, char __user *buf,
 			       size_t count, loff_t *ppos);
 ssize_t drm_fb_helper_sys_write(struct fb_info *info, const char __user *buf,
@@ -376,6 +395,16 @@ static inline int drm_fb_helper_defio_init(struct drm_fb_helper *fb_helper)
 	return -ENODEV;
 }
 
+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 ssize_t drm_fb_helper_sys_read(struct fb_info *info,
 					     char __user *buf, size_t count,
 					     loff_t *ppos)
-- 
2.15.1