Index: sys/arch/arm/arm32/bus_dma.c
===================================================================
RCS file: /cvsroot/src/sys/arch/arm/arm32/bus_dma.c,v
retrieving revision 1.122
diff -u -p -r1.122 bus_dma.c
--- sys/arch/arm/arm32/bus_dma.c	20 Jun 2020 07:10:36 -0000	1.122
+++ sys/arch/arm/arm32/bus_dma.c	5 Sep 2020 07:01:09 -0000
@@ -1818,9 +1818,14 @@ _bus_dmatag_subregion(bus_dma_tag_t tag,
 	size_t nranges = 0;
 	size_t i;
 	for (i = 0, dr = tag->_ranges; i < tag->_nranges; i++, dr++) {
-		if (dr->dr_sysbase <= min_addr
-		    && max_addr <= dr->dr_sysbase + dr->dr_len - 1) {
-			subset = true;
+		/*
+		 * Does the parent tag cover a subset of the new range?
+		 * If yes, we can continue to use the parent tag.
+		 */
+		if (dr->dr_sysbase >= min_addr
+		    && dr->dr_sysbase + dr->dr_len - 1  <= max_addr) {
+		    subset = true;
+		    break;
 		}
 		if (min_addr <= dr->dr_sysbase + dr->dr_len
 		    && max_addr >= dr->dr_sysbase) {
@@ -1854,22 +1859,35 @@ _bus_dmatag_subregion(bus_dma_tag_t tag,
 		dr->dr_busbase = min_addr;
 		dr->dr_len = max_addr + 1 - min_addr;
 	} else {
-		for (i = 0; i < nranges; i++) {
-			if (min_addr > dr->dr_sysbase + dr->dr_len
-			    || max_addr < dr->dr_sysbase)
+		struct arm32_dma_range *pdr;
+
+		for (i = 0, pdr = tag->_ranges; i < tag->_nranges; i++, pdr++) {
+			KASSERT(nranges != 0);
+
+			if (min_addr > pdr->dr_sysbase + pdr->dr_len
+			    || max_addr < pdr->dr_sysbase) {
+				/*
+				 * this range doesn't overlap with new limits,
+				 * so skip.
+				 */
 				continue;
-			dr[0] = tag->_ranges[i];
+			}
+			/*
+			 * Copy the range and adjust to fit within the new
+			 * limits
+			 */
+			dr[0] = pdr[0];
 			if (dr->dr_sysbase < min_addr) {
 				psize_t diff = min_addr - dr->dr_sysbase;
 				dr->dr_busbase += diff;
 				dr->dr_len -= diff;
 				dr->dr_sysbase += diff;
 			}
-			if (max_addr != 0xffffffff
-			    && max_addr + 1 < dr->dr_sysbase + dr->dr_len) {
+			if (max_addr <= dr->dr_sysbase + dr->dr_len - 1) {
 				dr->dr_len = max_addr + 1 - dr->dr_sysbase;
 			}
 			dr++;
+			nranges--;
 		}
 	}