[PATCH RFC 2/2] drm: introduce CLOSEFB IOCTL

[Simon Ser]

This new IOCTL allows callers to close a framebuffer without
disabling planes or CRTCs. This takes inspiration from Rob Clark's
unref_fb IOCTL [1] and DRM_MODE_FB_PERSIST [2].

[1]: https://lore.kernel.org/dri-devel/20170509153654.23464-1-robdclark@gmail.com/
[2]: https://lore.kernel.org/dri-devel/20211006151921.312714-1-contact@emersion.fr/

Signed-off-by: Simon Ser <contact at emersion.fr>
Cc: Hans de Goede <hdegoede at redhat.com>
Cc: Dennis Filder <d.filder at web.de>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Pekka Paalanen <ppaalanen at gmail.com>
Cc: Rob Clark <robdclark at gmail.com>
Cc: Sean Paul <seanpaul at chromium.org>
---
 drivers/gpu/drm/drm_crtc_internal.h |  2 ++
 drivers/gpu/drm/drm_framebuffer.c   | 19 +++++++++++++++++++
 drivers/gpu/drm/drm_ioctl.c         |  1 +
 include/uapi/drm/drm.h              | 19 +++++++++++++++++++
 4 files changed, 41 insertions(+)

diff --git a/drivers/gpu/drm/drm_crtc_internal.h b/drivers/gpu/drm/drm_crtc_internal.h
index 63279e984342..c6377b9ba981 100644
--- a/drivers/gpu/drm/drm_crtc_internal.h
+++ b/drivers/gpu/drm/drm_crtc_internal.h
@@ -221,6 +221,8 @@ int drm_mode_addfb2_ioctl(struct drm_device *dev,
 			  void *data, struct drm_file *file_priv);
 int drm_mode_rmfb_ioctl(struct drm_device *dev,
 			void *data, struct drm_file *file_priv);
+int drm_mode_closefb_ioctl(struct drm_device *dev,
+			   void *data, struct drm_file *file_priv);
 int drm_mode_getfb(struct drm_device *dev,
 		   void *data, struct drm_file *file_priv);
 int drm_mode_getfb2_ioctl(struct drm_device *dev,
diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
index 2352972ba6ac..15f7c22dbd4a 100644
--- a/drivers/gpu/drm/drm_framebuffer.c
+++ b/drivers/gpu/drm/drm_framebuffer.c
@@ -500,6 +500,25 @@ int drm_mode_rmfb_ioctl(struct drm_device *dev,
 	return drm_mode_rmfb(dev, *fb_id, file_priv);
 }
 
+int drm_mode_closefb_ioctl(struct drm_device *dev,
+			   void *data, struct drm_file *file_priv)
+{
+	uint32_t *fb_id = data;
+	struct drm_framebuffer *fb;
+	int ret;
+
+	if (!drm_core_check_feature(dev, DRIVER_MODESET))
+		return -EOPNOTSUPP;
+
+	fb = drm_framebuffer_lookup(dev, file_priv, *fb_id);
+	if (!fb)
+		return -ENOENT;
+
+	ret = drm_mode_closefb(fb, file_priv);
+	drm_framebuffer_put(fb);
+	return ret;
+}
+
 /**
  * drm_mode_getfb - get FB info
  * @dev: drm device for the ioctl
diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 8b8744dcf691..545762bc16d0 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c
@@ -670,6 +670,7 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_ADDFB, drm_mode_addfb_ioctl, 0),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_ADDFB2, drm_mode_addfb2_ioctl, 0),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_RMFB, drm_mode_rmfb_ioctl, 0),
+	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CLOSEFB, drm_mode_closefb_ioctl, 0),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_PAGE_FLIP, drm_mode_page_flip_ioctl, DRM_MASTER),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_DIRTYFB, drm_mode_dirtyfb_ioctl, DRM_MASTER),
 	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_DUMB, drm_mode_create_dumb_ioctl, 0),
diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
index 3b810b53ba8b..8726f003f382 100644
--- a/include/uapi/drm/drm.h
+++ b/include/uapi/drm/drm.h
@@ -1097,6 +1097,25 @@ extern "C" {
 #define DRM_IOCTL_SYNCOBJ_TIMELINE_SIGNAL	DRM_IOWR(0xCD, struct drm_syncobj_timeline_array)
 
 #define DRM_IOCTL_MODE_GETFB2		DRM_IOWR(0xCE, struct drm_mode_fb_cmd2)
+/**
+ * DRM_IOCTL_MODE_CLOSEFB - Close a framebuffer.
+ *
+ * This closes a framebuffer previously added via ADDFB/ADDFB2. The IOCTL
+ * argument is a framebuffer object ID.
+ *
+ * This IOCTL is similar to &DRM_IOCTL_MODE_RMFB, except it doesn't disable
+ * planes and CRTCs. As long as the framebuffer is used by a plane, it's kept
+ * alive. When the plane no longer uses the framebuffer (because the
+ * framebuffer is replaced with another one, or the plane is disabled), the
+ * framebuffer is cleaned up.
+ *
+ * This is useful to implement flicker-free transitions between two processes.
+ *
+ * Depending on the threat model, user-space may want to ensure that the
+ * framebuffer doesn't expose any sensitive user information: closed
+ * framebuffers attached to a plane can be read back by the next DRM master.
+ */
+#define DRM_IOCTL_MODE_CLOSEFB		DRM_IOWR(0xCF, unsigned int)
 
 /*
  * Device specific ioctls should only be in their respective headers
-- 
2.33.0