efifb: add a tunable to select the framebuffer cache attribute

Mapping the framebuffer with WC (Write Combined) memory type can, in
practice, cause some memory transactions to be rate-limited at a
fraction of the fb write rate.  WC allows one core to queue up many
globally visible write transactions, and in the process some unrelated
transactions may end up having to wait for all of the queued up PCI
writes to be flushed.

Add an hw.efifb.cache_attr tunable to allow mapping the framebuffer as
uncacheable instead.  We should likely be taking a more careful approach
of checking the memory map to determine which cacheability attributes
are feasible, but the knob lets us use our historically functional
behavior while offering a convenient way to switch on a stock kernel.

The only valid values for hw.efifb.cache_attr at this time are "uc" and
"wc".

Original patch by Marc De La Gueronniere <mdelagueronniere@verisign.com>
along with previous testing.

Reviewed by:	imp
MFC after:	1 week
Sponsored by:	Verisign, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D17884

1 files changed, 26 insertions, 1 deletions

diff --git a/sys/dev/vt/hw/efifb/efifb.c b/sys/dev/vt/hw/efifb/efifb.c
index b30b1d29e205..1d243599e70c 100644
--- a/sys/dev/vt/hw/efifb/efifb.c
+++ b/sys/dev/vt/hw/efifb/efifb.c
@@ -103,7 +103,32 @@ vt_efifb_init(struct vt_device *vd)
 	struct fb_info	*info;
 	struct efi_fb	*efifb;
 	caddr_t		kmdp;
+	int		memattr;
 	int		roff, goff, boff;
+	char		attr[16];
+
+	/*
+	 * XXX TODO: I think there's more nuance here than we're acknowledging,
+	 * and we should look into it.  It may be that the framebuffer lives in
+	 * a segment of memory that doesn't support one or both of these.  We
+	 * should likely be consulting the memory map for any applicable
+	 * cacheability attributes before making a final decision.
+	 */
+	memattr = VM_MEMATTR_WRITE_COMBINING;
+	if (TUNABLE_STR_FETCH("hw.efifb.cache_attr", attr, sizeof(attr))) {
+		/*
+		 * We'll allow WC but it's currently the default, UC is the only
+		 * other tested one at this time.
+		 */
+		if (strcasecmp(attr, "wc") != 0 &&
+		    strcasecmp(attr, "uc") != 0) {
+			printf("efifb: unsupported cache attr specified: %s\n",
+			    attr);
+			printf("efifb: expected \"wc\" or \"uc\"\n");
+		} else if (strcasecmp(attr, "uc") == 0) {
+			memattr = VM_MEMATTR_UNCACHEABLE;
+		}
+	}
 
 	info = vd->vd_softc;
 	if (info == NULL)
@@ -141,7 +166,7 @@ vt_efifb_init(struct vt_device *vd)
 	info->fb_size = info->fb_height * info->fb_stride;
 	info->fb_pbase = efifb->fb_addr;
 	info->fb_vbase = (intptr_t)pmap_mapdev_attr(info->fb_pbase,
-	    info->fb_size, VM_MEMATTR_WRITE_COMBINING);
+	    info->fb_size, memattr);
 
 	vt_fb_init(vd);