[PATCH] drm/vblank: Require a driver register vblank support for 0 or all CRTCs

[Lyude Paul <lyude@redhat.com>]

Currently, there's nothing actually stopping a driver from only registering
vblank support for some of it's CRTCs and not for others. As far as I can
tell, this isn't really defined behavior on the C side of things - as the
documentation explicitly mentions to not use drm_vblank_init() if you don't
have vblank support - since DRM then steps in and adds its own vblank
emulation implementation.

So, let's fix this edge case and check to make sure it's all or none.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 3ed4351a83ca ("drm: Extract drm_vblank.[hc]")
Cc: Stefan Agner <stefan@agner.ch>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.13+
---
 drivers/gpu/drm/drm_vblank.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
index 94e45ed6869d0..4d00937e8ca2e 100644
--- a/drivers/gpu/drm/drm_vblank.c
+++ b/drivers/gpu/drm/drm_vblank.c
@@ -525,9 +525,19 @@ static void drm_vblank_init_release(struct drm_device *dev, void *ptr)
  */
 int drm_vblank_init(struct drm_device *dev, unsigned int num_crtcs)
 {
+	struct drm_crtc *crtc;
 	int ret;
 	unsigned int i;
 
+	// Confirm that the required vblank functions have been filled out for all CRTCS
+	drm_for_each_crtc(crtc, dev) {
+		if (!crtc->funcs->enable_vblank || !crtc->funcs->disable_vblank) {
+			drm_err(dev, "CRTC vblank functions not initialized for %s, abort\n",
+				crtc->name);
+			return -EINVAL;
+		}
+	}
+
 	spin_lock_init(&dev->vbl_lock);
 	spin_lock_init(&dev->vblank_time_lock);
 

base-commit: 22512c3ee0f47faab5def71c4453638923c62522
-- 
2.46.1