[PATCH] 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 fbdev use to know when to
restore. Make use of it in the generic fbdev emulation.

It wasn't possible to add this funtionality to say all the drivers that
use the DRM_FB_HELPER_DEFAULT_OPS macro, because some of its users set
.fb_open and .fb_release.

If some driver wants this functionality it can export drm_fbdev_fb_open()
and drm_fbdev_fb_release() and use them.

Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
---
 drivers/gpu/drm/drm_fb_helper.c | 25 +++++++++++++++++++++++--
 include/drm/drm_fb_helper.h     | 14 ++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index 9a69ad7e9f3b..24ffaee11f5a 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c
@@ -530,7 +530,7 @@ static int restore_fbdev_mode(struct drm_fb_helper *fb_helper)
 int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
 {
 	bool do_delayed;
-	int ret;
+	int ret = 0;
 
 	if (!drm_fbdev_emulation || !fb_helper)
 		return -ENODEV;
@@ -539,7 +539,8 @@ int drm_fb_helper_restore_fbdev_mode_unlocked(struct drm_fb_helper *fb_helper)
 		return 0;
 
 	mutex_lock(&fb_helper->lock);
-	ret = restore_fbdev_mode(fb_helper);
+	if (fb_helper->fb_open_count)
+		ret = restore_fbdev_mode(fb_helper);
 
 	do_delayed = fb_helper->delayed_hotplug;
 	if (do_delayed)
@@ -888,6 +889,8 @@ int drm_fb_helper_init(struct drm_device *dev,
 		i++;
 	}
 
+	fb_helper->fb_open_count = 1;
+
 	dev->fb_helper = fb_helper;
 
 	return 0;
@@ -2942,16 +2945,32 @@ 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);
 
@@ -3050,6 +3069,8 @@ int drm_fb_helper_generic_probe(struct drm_fb_helper *fb_helper,
 		      sizes->surface_width, sizes->surface_height,
 		      sizes->surface_bpp);
 
+	fb_helper->fb_open_count = 0;
+
 	format = drm_mode_legacy_fb_format(sizes->surface_bpp, sizes->surface_depth);
 	buffer = drm_client_framebuffer_create(client, sizes->surface_width,
 					       sizes->surface_height, format);
diff --git a/include/drm/drm_fb_helper.h b/include/drm/drm_fb_helper.h
index bb9acea61369..846ca8509427 100644
--- a/include/drm/drm_fb_helper.h
+++ b/include/drm/drm_fb_helper.h
@@ -247,6 +247,20 @@ struct drm_fb_helper {
 	 * See also: @deferred_setup
 	 */
 	int preferred_bpp;
+
+	/**
+	 * @fb_open_count:
+	 *
+	 * Used to track userspace/fbcon opens to know when to restore fbdev in
+	 * drm_fb_helper_restore_fbdev_mode_unlocked(). The value is set to 1 by
+	 * default which means it will always restore. Drivers that want to
+	 * track this can set this to zero in their
+	 * &drm_fb_helper_funcs.fb_probe function. Additionally the value must
+	 * be increased in &fb_ops.fb_open and decreased in &fb_ops.fb_release.
+	 *
+	 * The value is protected by @lock.
+	 */
+	unsigned int fb_open_count;
 };
 
 static inline struct drm_fb_helper *
-- 
2.15.1