stand/fdt: Fallback to name + ".dtbo" if we fail to load name

This behavior also matches a Linux-ism by allowing fdt_overlays to specify
names of overlays without an extension, e.g. fdt-overlays="sunxi-h3-h5-emac"

If we fail to load the file given by a name in fdt_overlays, try again with
".dtbo" appended to it. This still allows overlays to lack .dtbo extension
if user prefers it and just adds a fallback cushion.

Future work could move this from a hard-coded ".dtbo" to a loader.conf(5)
configuration option.

Reviewed by:	gonzo
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D13968

stand/fdt/fdt_loader_cmd.c

@@ -73,6 +73,7 @@ static size_t fdtp_size = 0;
 static vm_offset_t fdtp_va = 0;
 
 static int fdt_load_dtb(vm_offset_t va);
+static void fdt_print_overlay_load_error(int err, const char *filename);
 
 static int fdt_cmd_nyi(int argc, char *argv[]);
 
@@ -286,36 +287,57 @@ fdt_load_dtb_overlay(const char * filename)
 
 	debugf("fdt_load_dtb_overlay(%s)\n", filename);
 
-	/* Attempt to load and validate a new dtb from a file. */
+	/* Attempt to load and validate a new dtb from a file. FDT_ERR_NOTFOUND
-	if ((bfp = file_loadraw(filename, "dtbo", 1)) == NULL) {
+	 * is normally a libfdt error code, but libfdt would actually return
-		printf("failed to load file '%s'\n", filename);
+	 * -FDT_ERR_NOTFOUND. We re-purpose the error code here to convey a
-		return (1);
+	 * similar meaning: the file itself was not found, which can still be
-	}
+	 * considered an error dealing with FDT pieces.
+	 */
+	if ((bfp = file_loadraw(filename, "dtbo", 1)) == NULL)
+		return (FDT_ERR_NOTFOUND);
 
 	COPYOUT(bfp->f_addr, &header, sizeof(header));
 	err = fdt_check_header(&header);
 
 	if (err < 0) {
 		file_discard(bfp);
-		if (err == -FDT_ERR_BADVERSION)
+		return (err);
-			printf("incompatible blob version: %d, should be: %d\n",
-			    fdt_version(fdtp), FDT_LAST_SUPPORTED_VERSION);
-
-		else
-			printf("error validating blob: %s\n",
-			    fdt_strerror(err));
-		return (1);
 	}
 
 	return (0);
 }
 
+static void
+fdt_print_overlay_load_error(int err, const char *filename)
+{
+
+	switch (err) {
+		case FDT_ERR_NOTFOUND:
+			printf("%s: failed to load file\n", filename);
+			break;
+		case -FDT_ERR_BADVERSION:
+			printf("%s: incompatible blob version: %d, should be: %d\n",
+			    filename, fdt_version(fdtp),
+			    FDT_LAST_SUPPORTED_VERSION);
+			break;
+		default:
+			/* libfdt errs are negative */
+			if (err < 0)
+				printf("%s: error validating blob: %s\n",
+				    filename, fdt_strerror(err));
+			else
+				printf("%s: unknown load error\n", filename);
+			break;
+	}
+}
+
 int
 fdt_load_dtb_overlays(const char * filenames)
 {
 	char *names;
-	char *name;
+	char *name, *name_ext;
 	char *comaptr;
+	int err, namesz;
 
 	debugf("fdt_load_dtb_overlay(%s)\n", filenames);
 
@@ -327,7 +349,23 @@ fdt_load_dtb_overlays(const char * filenames)
 		comaptr = strchr(name, ',');
 		if (comaptr)
 			*comaptr = '\0';
-		fdt_load_dtb_overlay(name);
+		err = fdt_load_dtb_overlay(name);
+		if (err == FDT_ERR_NOTFOUND) {
+			/* Allocate enough to append ".dtbo" */
+			namesz = strlen(name) + 6;
+			name_ext = malloc(namesz);
+			if (name_ext == NULL) {
+				fdt_print_overlay_load_error(err, name);
+				name = comaptr + 1;
+				continue;
+			}
+			snprintf(name_ext, namesz, "%s.dtbo", name);
+			err = fdt_load_dtb_overlay(name_ext);
+			free(name_ext);
+		}
+		/* Catch error with either initial load or fallback load */
+		if (err != 0)
+			fdt_print_overlay_load_error(err, name);
 		name = comaptr + 1;
 	} while(comaptr);