[PATCH RFC] drm: introduce DRM_MODE_FB_PERSIST

[Simon Ser]

This new ADDFB2 flag allows callers to mark a framebuffer as
"persistent", and no longer have RMFB semantics when the DRM
file is closed.

[1]: https://lore.kernel.org/dri-devel/YTJypepF1Hpc2YYT@reader/

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>
---

I'm not sure this is enough, but posting this to get initial
feedback and allow to start e.g. Plymouth experiments. I'll
follow up with an IGT test soon.

 drivers/gpu/drm/drm_framebuffer.c |  6 ++++--
 include/uapi/drm/drm_mode.h       | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_framebuffer.c b/drivers/gpu/drm/drm_framebuffer.c
index 07f5abc875e9..9b398838e1f4 100644
--- a/drivers/gpu/drm/drm_framebuffer.c
+++ b/drivers/gpu/drm/drm_framebuffer.c
@@ -292,7 +292,8 @@ drm_internal_framebuffer_create(struct drm_device *dev,
 	struct drm_framebuffer *fb;
 	int ret;
 
-	if (r->flags & ~(DRM_MODE_FB_INTERLACED | DRM_MODE_FB_MODIFIERS)) {
+	if (r->flags & ~(DRM_MODE_FB_INTERLACED | DRM_MODE_FB_MODIFIERS |
+			 DRM_MODE_FB_PERSIST)) {
 		DRM_DEBUG_KMS("bad framebuffer flags 0x%08x\n", r->flags);
 		return ERR_PTR(-EINVAL);
 	}
@@ -789,7 +790,8 @@ void drm_fb_release(struct drm_file *priv)
 	 * at it any more.
 	 */
 	list_for_each_entry_safe(fb, tfb, &priv->fbs, filp_head) {
-		if (drm_framebuffer_read_refcount(fb) > 1) {
+		if (drm_framebuffer_read_refcount(fb) > 1 &&
+		    !(fb->flags & DRM_MODE_FB_PERSIST)) {
 			list_move_tail(&fb->filp_head, &arg.fbs);
 		} else {
 			list_del_init(&fb->filp_head);
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index e1e351682872..c7a7089ec31e 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -662,6 +662,21 @@ struct drm_mode_fb_cmd {
 
 #define DRM_MODE_FB_INTERLACED	(1<<0) /* for interlaced framebuffers */
 #define DRM_MODE_FB_MODIFIERS	(1<<1) /* enables ->modifer[] */
+/**
+ * DRM_MODE_FB_PERSIST
+ *
+ * DRM framebuffers are normally implicitly removed when their owner closes the
+ * DRM FD. Passing this flag will make the framebuffer persistent: it will not
+ * be implicitly removed. This is useful to implement flicker-free transitions
+ * between two processes.
+ *
+ * This flag doesn't change the behavior of &DRM_IOCTL_MODE_RMFB.
+ *
+ * User-space should ensure the framebuffer doesn't expose any sensitive user
+ * information: persistent framebuffers can be read back by the next DRM
+ * master.
+ */
+#define DRM_MODE_FB_PERSIST (1 << 2)
 
 struct drm_mode_fb_cmd2 {
 	__u32 fb_id;
-- 
2.33.0