From 3062870897fe65fe202bc30fbe9864985df2e115 Mon Sep 17 00:00:00 2001
From: Conrad Meyer <cem@FreeBSD.org>
Date: Wed, 18 Nov 2015 22:20:13 +0000
Subject: [PATCH] ntb_hw: Add tunable to disable write-combining

The tunable 'hw.ntb.enable_writecombine' may be set to zero to
administratively disable write combining the mapped NTB region.

Sponsored by:	EMC / Isilon Storage Division
---
 sys/dev/ntb/ntb_hw/ntb_hw.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/sys/dev/ntb/ntb_hw/ntb_hw.c b/sys/dev/ntb/ntb_hw/ntb_hw.c
index 740cb2b0b071..d0702f80e8ed 100644
--- a/sys/dev/ntb/ntb_hw/ntb_hw.c
+++ b/sys/dev/ntb/ntb_hw/ntb_hw.c
@@ -326,6 +326,10 @@ SYSCTL_UINT(_hw_ntb, OID_AUTO, debug_level, CTLFLAG_RWTUN,
 	}							\
 } while (0)
 
+static unsigned g_ntb_enable_wc = 1;
+SYSCTL_UINT(_hw_ntb, OID_AUTO, enable_writecombine, CTLFLAG_RDTUN,
+    &g_ntb_enable_wc, 0, "Set to 1 to map memory windows write combining");
+
 static struct ntb_hw_info pci_ids[] = {
 	/* XXX: PS/SS IDs left out until they are supported. */
 	{ 0x0C4E8086, "BWD Atom Processor S1200 Non-Transparent Bridge B2B",
@@ -766,10 +770,13 @@ map_memory_window_bar(struct ntb_softc *ntb, struct ntb_pci_bar_info *bar)
 		save_bar_parameters(bar);
 	}
 
+	print_map_success(ntb, bar, "mw");
+	if (g_ntb_enable_wc == 0)
+		return (0);
+
 	/* Mark bar region as write combining to improve performance. */
 	rc = pmap_change_attr((vm_offset_t)bar->vbase, bar->size,
 	    VM_MEMATTR_WRITE_COMBINING);
-	print_map_success(ntb, bar, "mw");
 	if (rc == 0)
 		device_printf(ntb->device,
 		    "Marked BAR%d v:[%p-%p] p:[%p-%p] as "