drm/nouveau/kms/nv50-: Check vbl count after CRC context flip

While I haven't seen us take too long in nv50_crc_ctx_flip_work() outside
of users with kernel logging on a serial port, it probably would be a good
idea to check how long we take just in case we need to go faster in the
future.

Cc: Martin Peres <martin.peres@free.fr>
Cc: Jeremy Cline <jcline@redhat.com>
Cc: Simon Ser <contact@emersion.fr>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://gitlab.freedesktop.org/drm/nouveau/-/merge_requests/10

drivers/gpu/drm/nouveau/dispnv50/crc.c

@@ -86,6 +86,8 @@ static void nv50_crc_ctx_flip_work(struct kthread_work *base)
 	struct drm_crtc *crtc = &head->base.base;
 	struct drm_device *dev = crtc->dev;
 	struct nv50_disp *disp = nv50_disp(dev);
+	const uint64_t start_vbl = drm_crtc_vblank_count(crtc);
+	uint64_t end_vbl;
 	u8 new_idx = crc->ctx_idx ^ 1;
 
 	/*
@@ -94,9 +96,7 @@ static void nv50_crc_ctx_flip_work(struct kthread_work *base)
 	 */
 	if (!mutex_trylock(&disp->mutex)) {
 		drm_dbg_kms(dev, "Lock contended, delaying CRC ctx flip for %s\n", crtc->name);
-		drm_vblank_work_schedule(work,
-					 drm_crtc_vblank_count(crtc) + 1,
-					 true);
+		drm_vblank_work_schedule(work, start_vbl + 1, true);
 		return;
 	}
 
@@ -107,6 +107,12 @@ static void nv50_crc_ctx_flip_work(struct kthread_work *base)
 	nv50_crc_program_ctx(head, &crc->ctx[new_idx]);
 	mutex_unlock(&disp->mutex);
 
+	end_vbl = drm_crtc_vblank_count(crtc);
+	if (unlikely(end_vbl != start_vbl))
+		NV_ERROR(nouveau_drm(dev),
+			 "Failed to flip CRC context on %s on time (%llu > %llu)\n",
+			 crtc->name, end_vbl, start_vbl);
+
 	spin_lock_irq(&crc->lock);
 	crc->ctx_changed = true;
 	spin_unlock_irq(&crc->lock);