smbios: add parsing API

Add a very simple API to be able to access SMBIOS strings
like vendor, model and bios version.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

lib/smbios-parser.c

@@ -0,0 +1,96 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2020, Bachmann electronic GmbH
+ */
+
+#include <common.h>
+#include <smbios.h>
+
+static inline int verify_checksum(const struct smbios_entry *e)
+{
+	/*
+	 * Checksums for SMBIOS tables are calculated to have a value, so that
+	 * the sum over all bytes yields zero (using unsigned 8 bit arithmetic).
+	 */
+	u8 *byte = (u8 *)e;
+	u8 sum = 0;
+
+	for (int i = 0; i < e->length; i++)
+		sum += byte[i];
+
+	return sum;
+}
+
+const struct smbios_entry *smbios_entry(u64 address, u32 size)
+{
+	const struct smbios_entry *entry = (struct smbios_entry *)(uintptr_t)address;
+
+	if (!address | !size)
+		return NULL;
+
+	if (memcmp(entry->anchor, "_SM_", 4))
+		return NULL;
+
+	if (verify_checksum(entry))
+		return NULL;
+
+	return entry;
+}
+
+static const struct smbios_header *next_header(const struct smbios_header *curr)
+{
+	u8 *pos = ((u8 *)curr) + curr->length;
+
+	/* search for _double_ NULL bytes */
+	while (!((*pos == 0) && (*(pos + 1) == 0)))
+		pos++;
+
+	/* step behind the double NULL bytes */
+	pos += 2;
+
+	return (struct smbios_header *)pos;
+}
+
+const struct smbios_header *smbios_header(const struct smbios_entry *entry, int type)
+{
+	const unsigned int num_header = entry->struct_count;
+	const struct smbios_header *header = (struct smbios_header *)entry->struct_table_address;
+
+	for (unsigned int i = 0; i < num_header; i++) {
+		if (header->type == type)
+			return header;
+
+		header = next_header(header);
+	}
+
+	return NULL;
+}
+
+static const char *string_from_smbios_table(const struct smbios_header *header,
+                                           int idx)
+{
+	unsigned int i = 1;
+	u8 *pos;
+
+	if (!header)
+		return NULL;
+
+	pos = ((u8 *)header) + header->length;
+
+	while (i < idx) {
+		if (*pos == 0x0)
+			i++;
+
+		pos++;
+	}
+
+	return (const char *)pos;
+}
+
+const char *smbios_string(const struct smbios_header *header, int index)
+{
+	if (!header)
+		return NULL;
+
+	return string_from_smbios_table(header, index);
+}