[PATCH v1] drm: Do not allow to set NOFB for active primary plane

[Stanislav Lisovskiy]

There was an issue reported from end users, confirmed
also locally that when user executes xrandr --output <some DP>
--rotate left/right, the other eDP screen gets blank after rotation.

Investigation showed that reason was that primary plane
was that screen was for some reason disabled, while cursor
plane was still enabled.
After some effort it turns out that userspace might wrongly
assign NOFB to active primary plane for some reason.

Then this gets detected in drm_atomic_helper_check_plane_state,
called from ->plane_check and plane gets deactivated, leaving
the screen blank and cursor visible. This can be cured by reboot
or xrandr off/on sequence for that crtc.

This patch is proposal to fix the issue by forbiding fb removal
from active primary plane. If one needs to remove fb plane must be
disabled first.

Not sure if this is correct, however it fixes the issue at least.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110375
Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy at intel.com>
---
 drivers/gpu/drm/drm_atomic_uapi.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
index ea797d4c82ee..e2f078b302f3 100644
--- a/drivers/gpu/drm/drm_atomic_uapi.c
+++ b/drivers/gpu/drm/drm_atomic_uapi.c
@@ -229,6 +229,15 @@ drm_atomic_set_fb_for_plane(struct drm_plane_state *plane_state,
 {
 	struct drm_plane *plane = plane_state->plane;
 
+	if (!fb) {
+		if (plane->state->visible && plane->type == DRM_PLANE_TYPE_PRIMARY) {
+			DRM_DEBUG_ATOMIC("Not allowed to set [NOFB] for active"
+					" primary [PLANE:%d:%s] - disable first",
+					plane->base.id, plane->name);
+			return;
+		}
+	}
+
 	if (fb)
 		DRM_DEBUG_ATOMIC("Set [FB:%d] for [PLANE:%d:%s] state %p\n",
 				 fb->base.id, plane->base.id, plane->name,
-- 
2.17.1