Clarify ttm state transitions tt_unpopulated<->tt_unbound<->tt_bound.

Untested.  Hastily `proven' by eyeballing all callers of ttm_tt_wire,
ttm_agp_tt_populate, and ttm_bus_dma_populate.

diff --git a/sys/external/bsd/drm2/dist/drm/ttm/ttm_tt.c b/sys/external/bsd/drm2/dist/drm/ttm/ttm_tt.c
index c331cc5032df..cbaa5092e0e0 100644
--- a/sys/external/bsd/drm2/dist/drm/ttm/ttm_tt.c
+++ b/sys/external/bsd/drm2/dist/drm/ttm/ttm_tt.c
@@ -360,9 +360,9 @@ EXPORT_SYMBOL(ttm_tt_bind);
  * ttm_tt_wire(ttm)
  *
  *	Wire the uvm pages of ttm and fill the ttm page array.  ttm
- *	must be unpopulated or unbound, and must be marked swapped.
- *	This does not change either state -- the caller is expected to
- *	include it among other operations for such a state transition.
+ *	must be unpopulated, and must be marked swapped.  This does not
+ *	change either state -- the caller is expected to include it
+ *	among other operations for such a state transition.
  */
 int
 ttm_tt_wire(struct ttm_tt *ttm)
@@ -372,9 +372,8 @@ ttm_tt_wire(struct ttm_tt *ttm)
 	unsigned i;
 	int error;
 
-	KASSERTMSG((ttm->state == tt_unpopulated || ttm->state == tt_unbound),
-	    "ttm_tt %p must be unpopulated or unbound for wiring,"
-	    " but state=%d",
+	KASSERTMSG((ttm->state == tt_unpopulated),
+	    "ttm_tt %p must be unpopulated for wiring, but state=%d",
 	    ttm, (int)ttm->state);
 	KASSERT(ISSET(ttm->page_flags, TTM_PAGE_FLAG_SWAPPED));
 	KASSERT(uobj != NULL);
diff --git a/sys/external/bsd/drm2/ttm/ttm_agp_backend.c b/sys/external/bsd/drm2/ttm/ttm_agp_backend.c
index d4ef4620b15b..57ac74938f63 100644
--- a/sys/external/bsd/drm2/ttm/ttm_agp_backend.c
+++ b/sys/external/bsd/drm2/ttm/ttm_agp_backend.c
@@ -78,9 +78,9 @@ int
 ttm_agp_tt_populate(struct ttm_tt *ttm)
 {
 
-	if (ttm->state != tt_unpopulated)
-		return 0;
-
+	KASSERT((ttm->state == tt_unpopulated),
+	    "ttm_agp_tt_populate: ttm %p state is not tt_unpopulated: %d",
+	    ttm, (int)ttm->state);
 	return ttm_bus_dma_populate(container_of(ttm, struct ttm_dma_tt, ttm));
 }
 
@@ -88,6 +88,9 @@ void
 ttm_agp_tt_unpopulate(struct ttm_tt *ttm)
 {
 
+	KASSERT((ttm->state == tt_unbound),
+	    "ttm_agp_tt_unpopulate: ttm %p state is not tt_unbound: %d",
+	    ttm, (int)ttm->state);
 	ttm_bus_dma_unpopulate(container_of(ttm, struct ttm_dma_tt, ttm));
 }
 
diff --git a/sys/external/bsd/drm2/ttm/ttm_bus_dma.c b/sys/external/bsd/drm2/ttm/ttm_bus_dma.c
index 3717cdd34d9b..e12e578ad34e 100644
--- a/sys/external/bsd/drm2/ttm/ttm_bus_dma.c
+++ b/sys/external/bsd/drm2/ttm/ttm_bus_dma.c
@@ -47,37 +47,29 @@ __KERNEL_RCSID(0, "$NetBSD: ttm_bus_dma.c,v 1.2 2016/04/24 04:26:12 riastradh Ex
  *	its DMA map.  The wiring and loading are stable as long as the
  *	associated bo is reserved.
  *
- *	Transitions from tt_unpopulated or tt_unbound to tt_unbound.
- *	Marks as wired, a.k.a. !swapped.
+ *	Transitions from tt_unpopulated to tt_unbound.  Marks as wired,
+ *	a.k.a. !swapped.
  */
 int
 ttm_bus_dma_populate(struct ttm_dma_tt *ttm_dma)
 {
 	int ret;
 
-	KASSERT(ttm_dma->ttm.state != tt_bound);
-
-	/* Check the current state.  */
-	if (ttm_dma->ttm.state == tt_unbound) {
-		/*
-		 * If it's populated, then if the pages are wired and
-		 * loaded already, nothing to do.
-		 */
-		if (!ISSET(ttm_dma->ttm.page_flags, TTM_PAGE_FLAG_SWAPPED))
-			return 0;
-	} else if (ttm_dma->ttm.state == tt_unpopulated) {
-		/* If it's unpopulated, it can't be swapped.  */
-		KASSERT(!ISSET(ttm_dma->ttm.page_flags,
-			TTM_PAGE_FLAG_SWAPPED));
-		/* Pretend it is now, for the sake of ttm_tt_wire.  */
-		ttm_dma->ttm.page_flags |= TTM_PAGE_FLAG_SWAPPED;
-	}
+	KASSERT(ttm_dma->ttm.state == tt_unpopulated);
+
+	/* If it's unpopulated, it can't be swapped.  */
+	KASSERT(!ISSET(ttm_dma->ttm.page_flags, TTM_PAGE_FLAG_SWAPPED));
+	/* Pretend it is now, for the sake of ttm_tt_wire.  */
+	ttm_dma->ttm.page_flags |= TTM_PAGE_FLAG_SWAPPED;
 
 	/* Wire the uvm pages and fill the ttm page array.  */
 	ret = ttm_tt_wire(&ttm_dma->ttm);
 	if (ret)
 		goto fail0;
 
+	/* Mark it populated but unbound.  */
+	ttm_dma->ttm.state = tt_unbound;
+
 	/* Load the DMA map.  */
 	/* XXX errno NetBSD->Linux */
 	ret = -bus_dmamap_load_pglist(ttm_dma->ttm.bdev->dmat,
@@ -89,15 +81,14 @@ ttm_bus_dma_populate(struct ttm_dma_tt *ttm_dma)
 	/* Mark it wired.  */
 	ttm_dma->ttm.page_flags &= ~TTM_PAGE_FLAG_SWAPPED;
 
-	/* Mark it populated but unbound.  */
-	ttm_dma->ttm.state = tt_unbound;
-
 	/* Success!  */
 	return 0;
 
 fail2: __unused
 	bus_dmamap_unload(ttm_dma->ttm.bdev->dmat, ttm_dma->dma_address);
-fail1:	ttm_tt_unwire(&ttm_dma->ttm);
+fail1:	KASSERT(ttm->state == tt_unbound);
+	ttm_tt_unwire(&ttm_dma->ttm);
+	ttm->state = tt_unpopulated;
 fail0:	KASSERT(ret);
 	return ret;
 }