diff --git a/diff.c b/diff.c
index d50355e842b..dabd7f50ec2 100644
--- a/diff.c
+++ b/diff.c
@@ -1268,46 +1268,37 @@ static void emit_binary_diff(FILE *file, mmfile_t *one, mmfile_t *two)
 	emit_binary_diff_body(file, two, one);
 }
 
-static void diff_filespec_check_attr(struct diff_filespec *one)
+void diff_filespec_load_driver(struct diff_filespec *one)
 {
-	struct userdiff_driver *drv;
-	int check_from_data = 0;
-
-	if (one->checked_attr)
-		return;
-
-	drv = userdiff_find_by_path(one->path);
-	one->is_binary = 0;
-
-	/* binaryness */
-	if (drv == USERDIFF_ATTR_TRUE)
-		;
-	else if (drv == USERDIFF_ATTR_FALSE)
-		one->is_binary = 1;
-	else
-		check_from_data = 1;
-
-	if (check_from_data) {
-		if (!one->data && DIFF_FILE_VALID(one))
-			diff_populate_filespec(one, 0);
-
-		if (one->data)
-			one->is_binary = buffer_is_binary(one->data, one->size);
-	}
+	if (!one->driver)
+		one->driver = userdiff_find_by_path(one->path);
+	if (!one->driver)
+		one->driver = userdiff_find_by_name("default");
 }
 
 int diff_filespec_is_binary(struct diff_filespec *one)
 {
-	diff_filespec_check_attr(one);
+	if (one->is_binary == -1) {
+		diff_filespec_load_driver(one);
+		if (one->driver->binary != -1)
+			one->is_binary = one->driver->binary;
+		else {
+			if (!one->data && DIFF_FILE_VALID(one))
+				diff_populate_filespec(one, 0);
+			if (one->data)
+				one->is_binary = buffer_is_binary(one->data,
+						one->size);
+			if (one->is_binary == -1)
+				one->is_binary = 0;
+		}
+	}
 	return one->is_binary;
 }
 
 static const struct userdiff_funcname *diff_funcname_pattern(struct diff_filespec *one)
 {
-	struct userdiff_driver *drv = userdiff_find_by_path(one->path);
-	if (!drv)
-		drv = userdiff_find_by_name("default");
-	return drv && drv->funcname.pattern ? &drv->funcname : NULL;
+	diff_filespec_load_driver(one);
+	return one->driver->funcname.pattern ? &one->driver->funcname : NULL;
 }
 
 void diff_set_mnemonic_prefix(struct diff_options *options, const char *a, const char *b)
@@ -1559,6 +1550,7 @@ struct diff_filespec *alloc_filespec(const char *path)
 	spec->path = (char *)(spec + 1);
 	memcpy(spec->path, path, namelen+1);
 	spec->count = 1;
+	spec->is_binary = -1;
 	return spec;
 }
 
diff --git a/diffcore.h b/diffcore.h
index 8ae35785fd4..713cca785c4 100644
--- a/diffcore.h
+++ b/diffcore.h
@@ -22,6 +22,8 @@
 
 #define MINIMUM_BREAK_SIZE     400 /* do not break a file smaller than this */
 
+struct userdiff_driver;
+
 struct diff_filespec {
 	unsigned char sha1[20];
 	char *path;
@@ -40,8 +42,10 @@ struct diff_filespec {
 #define DIFF_FILE_VALID(spec) (((spec)->mode) != 0)
 	unsigned should_free : 1; /* data should be free()'ed */
 	unsigned should_munmap : 1; /* data should be munmap()'ed */
-	unsigned checked_attr : 1;
-	unsigned is_binary : 1; /* data should be considered "binary" */
+
+	struct userdiff_driver *driver;
+	/* data should be considered "binary"; -1 means "don't know yet" */
+	int is_binary;
 };
 
 extern struct diff_filespec *alloc_filespec(const char *);
diff --git a/userdiff.c b/userdiff.c
index 80e2857abb8..58478a69122 100644
--- a/userdiff.c
+++ b/userdiff.c
@@ -7,7 +7,7 @@ static int ndrivers;
 static int drivers_alloc;
 
 #define FUNCNAME(name, pattern) \
-	{ name, NULL, { pattern, REG_EXTENDED } }
+	{ name, NULL, -1, { pattern, REG_EXTENDED } }
 static struct userdiff_driver builtin_drivers[] = {
 FUNCNAME("html", "^[ \t]*(<[Hh][1-6][ \t].*>.*)$"),
 FUNCNAME("java",
@@ -32,22 +32,23 @@ FUNCNAME("python", "^[ \t]*((class|def)[ \t].*)$"),
 FUNCNAME("ruby", "^[ \t]*((class|module|def)[ \t].*)$"),
 FUNCNAME("bibtex", "(@[a-zA-Z]{1,}[ \t]*\\{{0,1}[ \t]*[^ \t\"@',\\#}{~%]*).*$"),
 FUNCNAME("tex", "^(\\\\((sub)*section|chapter|part)\\*{0,1}\\{.*)$"),
+{ "default", NULL, -1, { NULL, 0 } },
 };
 #undef FUNCNAME
 
 static struct userdiff_driver driver_true = {
 	"diff=true",
 	NULL,
+	0,
 	{ NULL, 0 }
 };
-struct userdiff_driver *USERDIFF_ATTR_TRUE = &driver_true;
 
 static struct userdiff_driver driver_false = {
 	"!diff",
 	NULL,
+	1,
 	{ NULL, 0 }
 };
-struct userdiff_driver *USERDIFF_ATTR_FALSE = &driver_false;
 
 static struct userdiff_driver *userdiff_find_by_namelen(const char *k, int len)
 {
@@ -89,6 +90,7 @@ static struct userdiff_driver *parse_driver(const char *var,
 		drv = &drivers[ndrivers++];
 		memset(drv, 0, sizeof(*drv));
 		drv->name = xmemdupz(name, namelen);
+		drv->binary = -1;
 	}
 	return drv;
 }
@@ -109,6 +111,15 @@ static int parse_string(const char **d, const char *k, const char *v)
 	return 1;
 }
 
+static int parse_tristate(int *b, const char *k, const char *v)
+{
+	if (v && !strcasecmp(v, "auto"))
+		*b = -1;
+	else
+		*b = git_config_bool(k, v);
+	return 1;
+}
+
 int userdiff_config_basic(const char *k, const char *v)
 {
 	struct userdiff_driver *drv;
@@ -117,6 +128,8 @@ int userdiff_config_basic(const char *k, const char *v)
 		return parse_funcname(&drv->funcname, k, v, 0);
 	if ((drv = parse_driver(k, v, "xfuncname")))
 		return parse_funcname(&drv->funcname, k, v, REG_EXTENDED);
+	if ((drv = parse_driver(k, v, "binary")))
+		return parse_tristate(&drv->binary, k, v);
 
 	return 0;
 }
diff --git a/userdiff.h b/userdiff.h
index c64c5f56697..1c1eb042b44 100644
--- a/userdiff.h
+++ b/userdiff.h
@@ -9,12 +9,10 @@ struct userdiff_funcname {
 struct userdiff_driver {
 	const char *name;
 	const char *external;
+	int binary;
 	struct userdiff_funcname funcname;
 };
 
-extern struct userdiff_driver *USERDIFF_ATTR_TRUE;
-extern struct userdiff_driver *USERDIFF_ATTR_FALSE;
-
 int userdiff_config_basic(const char *k, const char *v);
 int userdiff_config_porcelain(const char *k, const char *v);
 struct userdiff_driver *userdiff_find_by_name(const char *name);