firmware: dmi_scan: Look for SMBIOS 3 entry point first

Since version 3.0.0 of the SMBIOS specification, there can be
multiple entry points in memory, pointing to one or two DMI tables.
If both a 32-bit ("_SM_") entry point and a 64-bit ("_SM3_") entry
point are present, the specification requires that the latter points
to a table which is a super-set of the table pointed to by the
former. Therefore we should give preference to the 64-bit ("_SM3_")
entry point.

However, currently the code is picking the first valid entry point
it finds. Per specification, we should look for a 64-bit ("_SM3_")
entry point first, and if we can't find any, look for a 32-bit
("_SM_" or "_DMI_") entry point. Modify the code to do that.
Signed-off-by: Jean Delvare <jdelvare@suse.de>

drivers/firmware/dmi_scan.c

@@ -649,6 +649,21 @@ void __init dmi_scan_machine(void)
 		if (p == NULL)
 			goto error;
 
+		/*
+		 * Same logic as above, look for a 64-bit entry point
+		 * first, and if not found, fall back to 32-bit entry point.
+		 */
+		memcpy_fromio(buf, p, 16);
+		for (q = p + 16; q < p + 0x10000; q += 16) {
+			memcpy_fromio(buf + 16, q, 16);
+			if (!dmi_smbios3_present(buf)) {
+				dmi_available = 1;
+				dmi_early_unmap(p, 0x10000);
+				goto out;
+			}
+			memcpy(buf, buf + 16, 16);
+		}
+
 		/*
 		 * Iterate over all possible DMI header addresses q.
 		 * Maintain the 32 bytes around q in buf.  On the
@@ -659,7 +674,7 @@ void __init dmi_scan_machine(void)
 		memset(buf, 0, 16);
 		for (q = p; q < p + 0x10000; q += 16) {
 			memcpy_fromio(buf + 16, q, 16);
-			if (!dmi_smbios3_present(buf) || !dmi_present(buf)) {
+			if (!dmi_present(buf)) {
 				dmi_available = 1;
 				dmi_early_unmap(p, 0x10000);
 				goto out;