[PATCH v2] drm/fb-helper/generic: Only restore when in use

[Noralf Trønnes]

On drm_driver->last_close the generic fbdev emulation will restore fbdev
regardless of it being used or not. This is a problem for e-ink displays
which don't want to be overwritten with zeroes when DRM userspace closes.

Amend this by adding an open counter to track use in order to know when
to restore.

v1: Avoid special-casing and move the check to drm_fbdev_client_restore()
    (Daniel Vetter)

Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
---

I was tied up in my previous attempt to solve this which predated the 
generic emulation, so I failed to see that I could solve this within
the scope of the generic fbdev.


 drivers/gpu/drm/drm_fb_helper.c | 24 +++++++++++++++++++++++-
 include/drm/drm_fb_helper.h     | 10 ++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 9a69ad7e9f3b..be93e7393704 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -2942,16 +2942,32 @@ EXPORT_SYMBOL(drm_fb_helper_output_poll_changed);
 static int drm_fbdev_fb_open(struct fb_info *info, int user)
 {
 	struct drm_fb_helper *fb_helper = info->par;
+	unsigned int fb_open_count;
 
 	if (!try_module_get(fb_helper->dev->driver->fops->owner))
 		return -ENODEV;
 
+	mutex_lock(&fb_helper->lock);
+	fb_open_count = fb_helper->fb_open_count++;
+	mutex_unlock(&fb_helper->lock);
+
+	if (!fb_open_count)
+		drm_fb_helper_blank(FB_BLANK_UNBLANK, info);
+
 	return 0;
 }
 
 static int drm_fbdev_fb_release(struct fb_info *info, int user)
 {
 	struct drm_fb_helper *fb_helper = info->par;
+	unsigned int fb_open_count;
+
+	mutex_lock(&fb_helper->lock);
+	fb_open_count = --fb_helper->fb_open_count;
+	mutex_unlock(&fb_helper->lock);
+
+	if (!fb_open_count)
+		drm_fb_helper_blank(FB_BLANK_POWERDOWN, info);
 
 	module_put(fb_helper->dev->driver->fops->owner);
 
@@ -3143,8 +3159,14 @@ static void drm_fbdev_client_unregister(struct drm_client_dev *client)
 static int drm_fbdev_client_restore(struct drm_client_dev *client)
 {
 	struct drm_fb_helper *fb_helper = drm_fb_helper_from_client(client);
+	unsigned int fb_open_count;
 
-	drm_fb_helper_restore_fbdev_mode_unlocked(fb_helper);
+	mutex_lock(&fb_helper->lock);
+	fb_open_count = fb_helper->fb_open_count;
+	mutex_unlock(&fb_helper->lock);
+
+	if (fb_open_count)
+		drm_fb_helper_restore_fbdev_mode_unlocked(fb_helper);
 
 	return 0;
 }
diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index bb9acea61369..e6ca1119d524 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -247,6 +247,16 @@ struct drm_fb_helper {
 	 * See also: @deferred_setup
 	 */
 	int preferred_bpp;
+
+	/**
+	 * @fb_open_count:
+	 *
+	 * The generic fbdev emulation uses this to track userspace/fbcon opens
+	 * to know when to restore.
+	 *
+	 * Protected by @lock.
+	 */
+	unsigned int fb_open_count;
 };
 
 static inline struct drm_fb_helper *
-- 
2.15.1