busdma: remove parent tag tracking

Without filter functions, we do not need to keep track of tag ancestry.
All inheritance of the parent tag's parameters occurs when creating the
new child tag.

Reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D42895

13 files changed, 48 insertions, 241 deletions

diff --git a/sys/arm/arm/busdma_machdep.c b/sys/arm/arm/busdma_machdep.c
index cf1fd0209734..dd31f7779b21 100644
--- a/sys/arm/arm/busdma_machdep.c
+++ b/sys/arm/arm/busdma_machdep.c
@@ -77,7 +77,6 @@ struct bounce_page;
 struct bounce_zone;
 
 struct bus_dma_tag {
-	bus_dma_tag_t		parent;
 	bus_size_t		alignment;
 	bus_addr_t		boundary;
 	bus_addr_t		lowaddr;
@@ -86,7 +85,6 @@ struct bus_dma_tag {
 	u_int			nsegments;
 	bus_size_t		maxsegsz;
 	int			flags;
-	int			ref_count;
 	int			map_count;
 	bus_dma_lock_t		*lockfunc;
 	void			*lockfuncarg;
@@ -332,10 +330,7 @@ might_bounce(bus_dma_tag_t dmat, bus_dmamap_t map, bus_addr_t addr,
  *
  * Bouncing can be triggered by DMA that doesn't begin and end on cacheline
  * boundaries, or doesn't begin on an alignment boundary, or falls within the
- * exclusion zone of any tag in the ancestry chain.
- *
- * For exclusions, walk the chain of tags comparing paddr to the exclusion zone
- * within each tag.
+ * exclusion zone of the tag.
  */
 static int
 must_bounce(bus_dma_tag_t dmat, bus_dmamap_t map, bus_addr_t paddr,
@@ -353,17 +348,11 @@ must_bounce(bus_dma_tag_t dmat, bus_dmamap_t map, bus_addr_t paddr,
 		return (1);
 
 	/*
-	 * Even though each tag has an exclusion zone that is a superset of its
-	 * own and all its ancestors' exclusions, the exclusion zone of each tag
-	 * up the chain must be checked within the loop, because the busdma
-	 * rules say the filter function is called only when the address lies
-	 * within the low-highaddr range of the tag that filterfunc belongs to.
+	 * Check the tag's exclusion zone.
 	 */
-	while (dmat != NULL && exclusion_bounce(dmat)) {
-		if (paddr >= dmat->lowaddr && paddr <= dmat->highaddr)
-			return (1);
-		dmat = dmat->parent;
-	}
+	if (exclusion_bounce(dmat) &&
+	    paddr >= dmat->lowaddr && paddr <= dmat->highaddr)
+		return (1);
 
 	return (0);
 }
@@ -405,7 +394,6 @@ bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 		return (ENOMEM);
 	}
 
-	newtag->parent = parent;
 	newtag->alignment = alignment;
 	newtag->boundary = boundary;
 	newtag->lowaddr = trunc_page((vm_paddr_t)lowaddr) + (PAGE_SIZE - 1);
@@ -415,7 +403,6 @@ bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 	newtag->nsegments = nsegments;
 	newtag->maxsegsz = maxsegsz;
 	newtag->flags = flags;
-	newtag->ref_count = 1; /* Count ourself */
 	newtag->map_count = 0;
 	if (lockfunc != NULL) {
 		newtag->lockfunc = lockfunc;
@@ -437,14 +424,6 @@ bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 		else if (parent->boundary != 0)
 			newtag->boundary = MIN(parent->boundary,
 					       newtag->boundary);
-
-		/*
-		 * Short circuit to looking at our parent directly since we
-		 * have encapsulated all of its information.
-		 */
-		newtag->parent = parent->parent;
-		if (newtag->parent != NULL)
-			atomic_add_int(&parent->ref_count, 1);
 	}
 
 	if (exclusion_bounce_check(newtag->lowaddr, newtag->highaddr))
@@ -504,7 +483,6 @@ bus_dma_template_clone(bus_dma_template_t *t, bus_dma_tag_t dmat)
 	if (t == NULL || dmat == NULL)
 		return;
 
-	t->parent = dmat->parent;
 	t->alignment = dmat->alignment;
 	t->boundary = dmat->boundary;
 	t->lowaddr = dmat->lowaddr;
@@ -527,39 +505,17 @@ bus_dma_tag_set_domain(bus_dma_tag_t dmat, int domain)
 int
 bus_dma_tag_destroy(bus_dma_tag_t dmat)
 {
-#ifdef KTR
-	bus_dma_tag_t dmat_copy = dmat;
-#endif
-	int error;
-
-	error = 0;
+	int error = 0;
 
 	if (dmat != NULL) {
 		if (dmat->map_count != 0) {
 			error = EBUSY;
 			goto out;
 		}
-
-		while (dmat != NULL) {
-			bus_dma_tag_t parent;
-
-			parent = dmat->parent;
-			atomic_subtract_int(&dmat->ref_count, 1);
-			if (dmat->ref_count == 0) {
-				atomic_subtract_32(&tags_total, 1);
-				free(dmat, M_BUSDMA);
-				/*
-				 * Last reference count, so
-				 * release our reference
-				 * count on our parent.
-				 */
-				dmat = parent;
-			} else
-				dmat = NULL;
-		}
+		free(dmat, M_BUSDMA);
 	}
 out:
-	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat_copy, error);
+	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat, error);
 	return (error);
 }
 
diff --git a/sys/arm64/arm64/busdma_bounce.c b/sys/arm64/arm64/busdma_bounce.c
index 7585d950fcbb..3b5521a31b92 100644
--- a/sys/arm64/arm64/busdma_bounce.c
+++ b/sys/arm64/arm64/busdma_bounce.c
@@ -309,42 +309,19 @@ bounce_bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 static int
 bounce_bus_dma_tag_destroy(bus_dma_tag_t dmat)
 {
-#ifdef KTR
-	bus_dma_tag_t dmat_copy;
-#endif
-	bus_dma_tag_t parent;
-	int error;
-
-	error = 0;
-#ifdef KTR
-	dmat_copy = dmat;
-#endif
-
+	int error = 0;
 
 	if (dmat != NULL) {
 		if (dmat->map_count != 0) {
 			error = EBUSY;
 			goto out;
 		}
-		while (dmat != NULL) {
-			parent = (bus_dma_tag_t)dmat->common.parent;
-			atomic_subtract_int(&dmat->common.ref_count, 1);
-			if (dmat->common.ref_count == 0) {
-				if (dmat->segments != NULL)
-					free(dmat->segments, M_DEVBUF);
-				free(dmat, M_DEVBUF);
-				/*
-				 * Last reference count, so
-				 * release our reference
-				 * count on our parent.
-				 */
-				dmat = parent;
-			} else
-				dmat = NULL;
-		}
+		if (dmat->segments != NULL)
+			free(dmat->segments, M_DEVBUF);
+		free(dmat, M_DEVBUF);
 	}
 out:
-	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat_copy, error);
+	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat, error);
 	return (error);
 }
 
diff --git a/sys/arm64/arm64/busdma_machdep.c b/sys/arm64/arm64/busdma_machdep.c
index aa5359b4552a..c88b28aa3e22 100644
--- a/sys/arm64/arm64/busdma_machdep.c
+++ b/sys/arm64/arm64/busdma_machdep.c
@@ -61,12 +61,8 @@ int
 bus_dma_run_filter(struct bus_dma_tag_common *tc, bus_addr_t paddr)
 {
 
-	while (tc != NULL) {
-		if (paddr > tc->lowaddr && paddr <= tc->highaddr)
-			return (1);
-
-		tc = tc->parent;
-	}
+	if (paddr > tc->lowaddr && paddr <= tc->highaddr)
+		return (1);
 
 	return (0);
 }
@@ -99,7 +95,6 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 
 	common = newtag;
 	common->impl = &bus_dma_bounce_impl;
-	common->parent = parent;
 	common->alignment = alignment;
 	common->boundary = boundary;
 	common->lowaddr = trunc_page((vm_paddr_t)lowaddr) + (PAGE_SIZE - 1);
@@ -108,7 +103,6 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 	common->nsegments = nsegments;
 	common->maxsegsz = maxsegsz;
 	common->flags = flags;
-	common->ref_count = 1; /* Count ourself */
 	if (lockfunc != NULL) {
 		common->lockfunc = lockfunc;
 		common->lockfuncarg = lockfuncarg;
@@ -130,13 +124,7 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 			    common->boundary);
 		}
 
-		/*
-		 * Short circuit looking at our parent directly since we have
-		 * encapsulated all of its information.
-		 */
-		common->parent = parent->parent;
 		common->domain = parent->domain;
-		atomic_add_int(&parent->ref_count, 1);
 	}
 	common->domain = vm_phys_domain_match(common->domain, 0ul,
 	    common->lowaddr);
@@ -184,7 +172,6 @@ bus_dma_template_clone(bus_dma_template_t *t, bus_dma_tag_t dmat)
 
 	common = (struct bus_dma_tag_common *)dmat;
 
-	t->parent = (bus_dma_tag_t)common->parent;
 	t->alignment = common->alignment;
 	t->boundary = common->boundary;
 	t->lowaddr = common->lowaddr;
diff --git a/sys/arm64/include/bus_dma_impl.h b/sys/arm64/include/bus_dma_impl.h
index 1abce30b5b4c..55af1b477979 100644
--- a/sys/arm64/include/bus_dma_impl.h
+++ b/sys/arm64/include/bus_dma_impl.h
@@ -31,7 +31,6 @@
 
 struct bus_dma_tag_common {
 	struct bus_dma_impl *impl;
-	struct bus_dma_tag_common *parent;
 	bus_size_t	  alignment;
 	bus_addr_t	  boundary;
 	bus_addr_t	  lowaddr;
@@ -42,7 +41,6 @@ struct bus_dma_tag_common {
 	int		  flags;
 	bus_dma_lock_t	 *lockfunc;
 	void		 *lockfuncarg;
-	int		  ref_count;
 	int		  domain;
 };
 
diff --git a/sys/dev/iommu/busdma_iommu.c b/sys/dev/iommu/busdma_iommu.c
index f041838eac39..d870e2af3984 100644
--- a/sys/dev/iommu/busdma_iommu.c
+++ b/sys/dev/iommu/busdma_iommu.c
@@ -394,33 +394,24 @@ iommu_bus_dma_tag_set_domain(bus_dma_tag_t dmat)
 static int
 iommu_bus_dma_tag_destroy(bus_dma_tag_t dmat1)
 {
-	struct bus_dma_tag_iommu *dmat, *parent;
-	struct bus_dma_tag_iommu *dmat_copy __unused;
+	struct bus_dma_tag_iommu *dmat;
 	int error;
 
 	error = 0;
-	dmat_copy = dmat = (struct bus_dma_tag_iommu *)dmat1;
+	dmat = (struct bus_dma_tag_iommu *)dmat1;
 
 	if (dmat != NULL) {
 		if (dmat->map_count != 0) {
 			error = EBUSY;
 			goto out;
 		}
-		while (dmat != NULL) {
-			parent = (struct bus_dma_tag_iommu *)dmat->common.parent;
-			if (atomic_fetchadd_int(&dmat->common.ref_count, -1) ==
-			    1) {
-				if (dmat == dmat->ctx->tag)
-					iommu_free_ctx(dmat->ctx);
-				free(dmat->segments, M_IOMMU_DMAMAP);
-				free(dmat, M_DEVBUF);
-				dmat = parent;
-			} else
-				dmat = NULL;
-		}
+		if (dmat == dmat->ctx->tag)
+			iommu_free_ctx(dmat->ctx);
+		free(dmat->segments, M_IOMMU_DMAMAP);
+		free(dmat, M_DEVBUF);
 	}
 out:
-	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat_copy, error);
+	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat, error);
 	return (error);
 }
 
diff --git a/sys/powerpc/powerpc/busdma_machdep.c b/sys/powerpc/powerpc/busdma_machdep.c
index 3065526427e2..a4c30ee9470c 100644
--- a/sys/powerpc/powerpc/busdma_machdep.c
+++ b/sys/powerpc/powerpc/busdma_machdep.c
@@ -63,7 +63,6 @@ struct bounce_page;
 struct bounce_zone;
 
 struct bus_dma_tag {
-	bus_dma_tag_t	  parent;
 	bus_size_t	  alignment;
 	bus_addr_t	  boundary;
 	bus_addr_t	  lowaddr;
@@ -72,7 +71,6 @@ struct bus_dma_tag {
 	bus_size_t	  maxsegsz;
 	u_int		  nsegments;
 	int		  flags;
-	int		  ref_count;
 	int		  map_count;
 	bus_dma_lock_t	 *lockfunc;
 	void		 *lockfuncarg;
@@ -126,15 +124,12 @@ run_filter(bus_dma_tag_t dmat, bus_addr_t paddr)
 
 	retval = 0;
 
-	do {
-		if (dmat->iommu == NULL &&
-		    paddr > dmat->lowaddr && paddr <= dmat->highaddr)
-			retval = 1;
-		if (!vm_addr_align_ok(paddr, dmat->alignment))
-			retval = 1;
+	if (dmat->iommu == NULL &&
+	    paddr > dmat->lowaddr && paddr <= dmat->highaddr)
+		retval = 1;
+	if (!vm_addr_align_ok(paddr, dmat->alignment))
+		retval = 1;
 
-		dmat = dmat->parent;		
-	} while (retval == 0 && dmat != NULL);
 	return (retval);
 }
 
@@ -177,7 +172,6 @@ bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 		return (ENOMEM);
 	}
 
-	newtag->parent = parent;
 	newtag->alignment = alignment;
 	newtag->boundary = boundary;
 	newtag->lowaddr = trunc_page((vm_paddr_t)lowaddr) + (PAGE_SIZE - 1);
@@ -186,7 +180,6 @@ bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 	newtag->nsegments = nsegments;
 	newtag->maxsegsz = maxsegsz;
 	newtag->flags = flags;
-	newtag->ref_count = 1; /* Count ourself */
 	newtag->map_count = 0;
 	if (lockfunc != NULL) {
 		newtag->lockfunc = lockfunc;
@@ -206,13 +199,6 @@ bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 			newtag->boundary = MIN(parent->boundary,
 					       newtag->boundary);
 
-		/*
-		 * Short circuit looking at our parent directly since we have
-		 * encapsulated all of its information.
-		 */
-		newtag->parent = parent->parent;
-		if (newtag->parent != NULL)
-			atomic_add_int(&parent->ref_count, 1);
 		newtag->iommu = parent->iommu;
 		newtag->iommu_cookie = parent->iommu_cookie;
 	}
@@ -265,7 +251,6 @@ bus_dma_template_clone(bus_dma_template_t *t, bus_dma_tag_t dmat)
 	if (t == NULL || dmat == NULL)
 		return;
 
-	t->parent = dmat->parent;
 	t->alignment = dmat->alignment;
 	t->boundary = dmat->boundary;
 	t->lowaddr = dmat->lowaddr;
@@ -288,11 +273,7 @@ bus_dma_tag_set_domain(bus_dma_tag_t dmat, int domain)
 int
 bus_dma_tag_destroy(bus_dma_tag_t dmat)
 {
-	bus_dma_tag_t dmat_copy __unused;
-	int error;
-
-	error = 0;
-	dmat_copy = dmat;
+	int error = 0;
 
 	if (dmat != NULL) {
 		if (dmat->map_count != 0) {
@@ -300,25 +281,10 @@ bus_dma_tag_destroy(bus_dma_tag_t dmat)
 			goto out;
 		}
 
-		while (dmat != NULL) {
-			bus_dma_tag_t parent;
-
-			parent = dmat->parent;
-			atomic_subtract_int(&dmat->ref_count, 1);
-			if (dmat->ref_count == 0) {
-				free(dmat, M_DEVBUF);
-				/*
-				 * Last reference count, so
-				 * release our reference
-				 * count on our parent.
-				 */
-				dmat = parent;
-			} else
-				dmat = NULL;
-		}
+		free(dmat, M_DEVBUF);
 	}
 out:
-	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat_copy, error);
+	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat, error);
 	return (error);
 }
 
diff --git a/sys/riscv/include/bus_dma_impl.h b/sys/riscv/include/bus_dma_impl.h
index d6e1d4ed632e..550ba648615c 100644
--- a/sys/riscv/include/bus_dma_impl.h
+++ b/sys/riscv/include/bus_dma_impl.h
@@ -31,7 +31,6 @@
 
 struct bus_dma_tag_common {
 	struct bus_dma_impl *impl;
-	struct bus_dma_tag_common *parent;
 	bus_size_t	  alignment;
 	bus_addr_t	  boundary;
 	bus_addr_t	  lowaddr;
@@ -42,7 +41,6 @@ struct bus_dma_tag_common {
 	int		  flags;
 	bus_dma_lock_t	 *lockfunc;
 	void		 *lockfuncarg;
-	int		  ref_count;
 };
 
 struct bus_dma_impl {
diff --git a/sys/riscv/riscv/busdma_bounce.c b/sys/riscv/riscv/busdma_bounce.c
index 6ac9a9cd678a..e9801a8a732e 100644
--- a/sys/riscv/riscv/busdma_bounce.c
+++ b/sys/riscv/riscv/busdma_bounce.c
@@ -196,38 +196,19 @@ bounce_bus_dma_tag_create(bus_dma_tag_t parent, bus_size_t alignment,
 static int
 bounce_bus_dma_tag_destroy(bus_dma_tag_t dmat)
 {
-#ifdef KTR
-	bus_dma_tag_t dmat_copy = dmat;
-#endif
-	bus_dma_tag_t parent;
-	int error;
-
-	error = 0;
+	int error = 0;
 
 	if (dmat != NULL) {
 		if (dmat->map_count != 0) {
 			error = EBUSY;
 			goto out;
 		}
-		while (dmat != NULL) {
-			parent = (bus_dma_tag_t)dmat->common.parent;
-			atomic_subtract_int(&dmat->common.ref_count, 1);
-			if (dmat->common.ref_count == 0) {
-				if (dmat->segments != NULL)
-					free(dmat->segments, M_DEVBUF);
-				free(dmat, M_DEVBUF);
-				/*
-				 * Last reference count, so
-				 * release our reference
-				 * count on our parent.
-				 */
-				dmat = parent;
-			} else
-				dmat = NULL;
-		}
+		if (dmat->segments != NULL)
+			free(dmat->segments, M_DEVBUF);
+		free(dmat, M_DEVBUF);
 	}
 out:
-	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat_copy, error);
+	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat, error);
 	return (error);
 }
 
diff --git a/sys/riscv/riscv/busdma_machdep.c b/sys/riscv/riscv/busdma_machdep.c
index e992803f3ff2..630938a394e1 100644
--- a/sys/riscv/riscv/busdma_machdep.c
+++ b/sys/riscv/riscv/busdma_machdep.c
@@ -62,13 +62,10 @@ bus_dma_run_filter(struct bus_dma_tag_common *tc, bus_addr_t paddr)
 	int retval;
 
 	retval = 0;
-	do {
-		if ((paddr > tc->lowaddr && paddr <= tc->highaddr) ||
-		    !vm_addr_align_ok(paddr, tc->alignment))
-			retval = 1;
+	if ((paddr > tc->lowaddr && paddr <= tc->highaddr) ||
+	    !vm_addr_align_ok(paddr, tc->alignment))
+		retval = 1;
 
-		tc = tc->parent;		
-	} while (retval == 0 && tc != NULL);
 	return (retval);
 }
 
@@ -100,7 +97,6 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 
 	common = newtag;
 	common->impl = &bus_dma_bounce_impl;
-	common->parent = parent;
 	common->alignment = alignment;
 	common->boundary = boundary;
 	common->lowaddr = trunc_page((vm_paddr_t)lowaddr) + (PAGE_SIZE - 1);
@@ -109,7 +105,6 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 	common->nsegments = nsegments;
 	common->maxsegsz = maxsegsz;
 	common->flags = flags;
-	common->ref_count = 1; /* Count ourself */
 	if (lockfunc != NULL) {
 		common->lockfunc = lockfunc;
 		common->lockfuncarg = lockfuncarg;
@@ -129,13 +124,6 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 			common->boundary = MIN(parent->boundary,
 			    common->boundary);
 		}
-
-		/*
-		 * Short circuit looking at our parent directly since we have
-		 * encapsulated all of its information.
-		 */
-		common->parent = parent->parent;
-		atomic_add_int(&parent->ref_count, 1);
 	}
 	*dmat = common;
 	return (0);
@@ -181,7 +169,6 @@ bus_dma_template_clone(bus_dma_template_t *t, bus_dma_tag_t dmat)
 
 	common = (struct bus_dma_tag_common *)dmat;
 
-	t->parent = (bus_dma_tag_t)common->parent;
 	t->alignment = common->alignment;
 	t->boundary = common->boundary;
 	t->lowaddr = common->lowaddr;
diff --git a/sys/x86/include/busdma_impl.h b/sys/x86/include/busdma_impl.h
index 4718c67dacc1..2e4c83b04d72 100644
--- a/sys/x86/include/busdma_impl.h
+++ b/sys/x86/include/busdma_impl.h
@@ -33,7 +33,6 @@
 
 struct bus_dma_tag_common {
 	struct bus_dma_impl *impl;
-	struct bus_dma_tag_common *parent;
 	bus_size_t	  alignment;
 	bus_addr_t	  boundary;
 	bus_addr_t	  lowaddr;
@@ -44,7 +43,6 @@ struct bus_dma_tag_common {
 	int		  flags;
 	bus_dma_lock_t	 *lockfunc;
 	void		 *lockfuncarg;
-	int		  ref_count;
 	int		  domain;
 };
 
diff --git a/sys/x86/iommu/intel_ctx.c b/sys/x86/iommu/intel_ctx.c
index 30771e061ec9..76a53f6bbd1a 100644
--- a/sys/x86/iommu/intel_ctx.c
+++ b/sys/x86/iommu/intel_ctx.c
@@ -128,7 +128,6 @@ device_tag_init(struct dmar_ctx *ctx, device_t dev)
 
 	domain = CTX2DOM(ctx);
 	maxaddr = MIN(domain->iodom.end, BUS_SPACE_MAXADDR);
-	ctx->context.tag->common.ref_count = 1; /* Prevent free */
 	ctx->context.tag->common.impl = &bus_dma_iommu_impl;
 	ctx->context.tag->common.boundary = 0;
 	ctx->context.tag->common.lowaddr = maxaddr;
diff --git a/sys/x86/x86/busdma_bounce.c b/sys/x86/x86/busdma_bounce.c
index 01e799a1133d..992f455ceb96 100644
--- a/sys/x86/x86/busdma_bounce.c
+++ b/sys/x86/x86/busdma_bounce.c
@@ -227,38 +227,19 @@ bounce_bus_dma_tag_set_domain(bus_dma_tag_t dmat)
 static int
 bounce_bus_dma_tag_destroy(bus_dma_tag_t dmat)
 {
-#ifdef KTR
-	bus_dma_tag_t dmat_copy = dmat;
-#endif
-	bus_dma_tag_t parent;
-	int error;
-
-	error = 0;
+	int error = 0;
 
 	if (dmat != NULL) {
 		if (dmat->map_count != 0) {
 			error = EBUSY;
 			goto out;
 		}
-		while (dmat != NULL) {
-			parent = (bus_dma_tag_t)dmat->common.parent;
-			atomic_subtract_int(&dmat->common.ref_count, 1);
-			if (dmat->common.ref_count == 0) {
-				if (dmat->segments != NULL)
-					free(dmat->segments, M_DEVBUF);
-				free(dmat, M_DEVBUF);
-				/*
-				 * Last reference count, so
-				 * release our reference
-				 * count on our parent.
-				 */
-				dmat = parent;
-			} else
-				dmat = NULL;
-		}
+		if (dmat->segments != NULL)
+			free(dmat->segments, M_DEVBUF);
+		free(dmat, M_DEVBUF);
 	}
 out:
-	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat_copy, error);
+	CTR3(KTR_BUSDMA, "%s tag %p error %d", __func__, dmat, error);
 	return (error);
 }
 
diff --git a/sys/x86/x86/busdma_machdep.c b/sys/x86/x86/busdma_machdep.c
index bb1b6a393fb0..6fe49367f7d8 100644
--- a/sys/x86/x86/busdma_machdep.c
+++ b/sys/x86/x86/busdma_machdep.c
@@ -67,14 +67,11 @@ bus_dma_run_filter(struct bus_dma_tag_common *tc, vm_paddr_t paddr)
 	int retval;
 
 	retval = 0;
-	do {
-		if (paddr >= BUS_SPACE_MAXADDR ||
-		    (paddr > tc->lowaddr && paddr <= tc->highaddr) ||
-		    !vm_addr_align_ok(paddr, tc->alignment))
-			retval = 1;
-
-		tc = tc->parent;
-	} while (retval == 0 && tc != NULL);
+	if (paddr >= BUS_SPACE_MAXADDR ||
+	    (paddr > tc->lowaddr && paddr <= tc->highaddr) ||
+	    !vm_addr_align_ok(paddr, tc->alignment))
+		retval = 1;
+
 	return (retval);
 }
 
@@ -106,7 +103,6 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 
 	common = newtag;
 	common->impl = &bus_dma_bounce_impl;
-	common->parent = parent;
 	common->alignment = alignment;
 	common->boundary = boundary;
 	common->lowaddr = trunc_page((vm_paddr_t)lowaddr) + (PAGE_SIZE - 1);
@@ -115,7 +111,6 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 	common->nsegments = nsegments;
 	common->maxsegsz = maxsegsz;
 	common->flags = flags;
-	common->ref_count = 1; /* Count ourself */
 	if (lockfunc != NULL) {
 		common->lockfunc = lockfunc;
 		common->lockfuncarg = lockfuncarg;
@@ -136,13 +131,7 @@ common_bus_dma_tag_create(struct bus_dma_tag_common *parent,
 			    common->boundary);
 		}
 
-		/*
-		 * Short circuit looking at our parent directly since we have
-		 * encapsulated all of its information.
-		 */
-		common->parent = parent->parent;
 		common->domain = parent->domain;
-		atomic_add_int(&parent->ref_count, 1);
 	}
 	common->domain = vm_phys_domain_match(common->domain, 0ul,
 	    common->lowaddr);
@@ -204,7 +193,6 @@ bus_dma_template_clone(bus_dma_template_t *t, bus_dma_tag_t dmat)
 
 	common = (struct bus_dma_tag_common *)dmat;
 
-	t->parent = (bus_dma_tag_t)common->parent;
 	t->alignment = common->alignment;
 	t->boundary = common->boundary;
 	t->lowaddr = common->lowaddr;