Index: head/sys/dev/pci/pci.c
===================================================================
--- head/sys/dev/pci/pci.c
+++ head/sys/dev/pci/pci.c
@@ -2902,13 +2902,21 @@
 	 * Determine the BAR's length by writing all 1's.  The bottom
 	 * log_2(size) bits of the BAR will stick as 0 when we read
 	 * the value back.
+	 *
+	 * NB: according to the PCI Local Bus Specification, rev. 3.0:
+	 * "Software writes 0FFFFFFFFh to both registers, reads them back,
+	 * and combines the result into a 64-bit value." (section 6.2.5.1)
+	 *
+	 * Writes to both registers must be performed before attempting to
+	 * read back the size value.
 	 */
+	testval = 0;
 	pci_write_config(dev, reg, 0xffffffff, 4);
-	testval = pci_read_config(dev, reg, 4);
 	if (ln2range == 64) {
 		pci_write_config(dev, reg + 4, 0xffffffff, 4);
 		testval |= (pci_addr_t)pci_read_config(dev, reg + 4, 4) << 32;
 	}
+	testval |= pci_read_config(dev, reg, 4);
 
 	/*
 	 * Restore the original value of the BAR.  We may have reprogrammed