Re: [PATCH v3 5/5] firmware: dmi: Read additional information when decoding DMI table

[Yazen Ghannam <yazen.ghannam@amd.com>]

On Fri, Jan 16, 2026 at 04:30:32PM -0500, Yazen Ghannam wrote:
> On Sat, Jan 10, 2026 at 07:48:34AM -0600, Mario Limonciello (AMD) wrote:
> 

[...]

> ACPI tables do this a lot. So there are examples of iterating over
> variable length entries.

Here's another implementation. What do you think?

Thanks,
Yazen


From c76f27e5d55570bb25bcefc3365fb3a1852bcad3 Mon Sep 17 00:00:00 2001
From: Yazen Ghannam <yazen.ghannam@amd.com>
Date: Mon, 19 Jan 2026 18:33:09 +0000
Subject: [PATCH] x86/CPU/AMD: Print AGESA string from DMI additional
 information entry

Type 40 entries (Additional Information) are summarized in section 7.41
as part of the SMBIOS specification.  Generally, these entries aren't
interesting to save.

However on some AMD Zen systems, the AGESA version is stored here. This
is useful to save to the kernel message logs for debugging. It can be
used to cross-reference issues.

Implement an iterator for the Additional Information entries. Use this
to find and print the AGESA string. Do so in AMD code, since the use
case is AMD-specific.

Originally-by: "Mario Limonciello (AMD)" <superm1@kernel.org>
Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
---
 arch/x86/kernel/cpu/amd.c   | 53 +++++++++++++++++++++++++++++++++++++
 drivers/firmware/dmi_scan.c |  3 ++-
 include/linux/dmi.h         | 18 +++++++++++++
 3 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index bc94ff1e250a..aa04a27aeb10 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-only
 #include <linux/export.h>
 #include <linux/bitops.h>
+#include <linux/dmi.h>
 #include <linux/elf.h>
 #include <linux/mm.h>
 #include <linux/kvm_types.h>
@@ -1404,3 +1405,55 @@ static __init int print_s5_reset_status_mmio(void)
 	return 0;
 }
 late_initcall(print_s5_reset_status_mmio);
+
+static void __init amd_dmi_scan_additional(const struct dmi_header *d, void *p)
+{
+	struct dmi_a_info *info = (struct dmi_a_info *)d;
+	void *next, *end;
+
+	/*
+	 * DMI Additional Info table has a 'count' field. But it's not very
+	 * helpful since the entries are variable length. So don't use it.
+	 */
+	if (info->header.type != DMI_ENTRY_ADDITIONAL ||
+	    info->header.length < DMI_A_INFO_MIN_SIZE)
+		return;
+
+	/*
+	 * Get the first entry.
+	 * The minimum table size guarantees at least one entry is present.
+	 */
+	next = (void *)(info + 1);
+	end  = (void *)info + info->header.length;
+
+	do {
+		struct dmi_a_info_entry *entry;
+		const char *string_ptr;
+
+		entry = (struct dmi_a_info_entry *)next;
+
+		/*
+		 * Not much can be done to validate data. At least the entry
+		 * length shouldn't be 0.
+		 */
+		if (!entry->length)
+			return;
+
+		string_ptr = dmi_string_nosave(&info->header, entry->str_num);
+
+		/* Only one AGESA string is expected. */
+		if (!strncmp(string_ptr, "AGESA", 5)) {
+			pr_info("%s\n", string_ptr);
+			break;
+		}
+
+		next += entry->length;
+	} while (end - next >= DMI_A_INFO_ENT_MIN_SIZE);
+}
+
+static __init int print_dmi_agesa(void)
+{
+	dmi_walk(amd_dmi_scan_additional, NULL);
+	return 0;
+}
+late_initcall(print_dmi_agesa);
diff --git a/drivers/firmware/dmi_scan.c b/drivers/firmware/dmi_scan.c
index caa09ddf0dfa..8b196992618d 100644
--- a/drivers/firmware/dmi_scan.c
+++ b/drivers/firmware/dmi_scan.c
@@ -47,7 +47,7 @@ static struct dmi_memdev_info {
 static int dmi_memdev_nr;
 static int dmi_memdev_populated_nr __initdata;
 
-static const char * __init dmi_string_nosave(const struct dmi_header *dm, u8 s)
+const char *dmi_string_nosave(const struct dmi_header *dm, u8 s)
 {
 	const u8 *bp = ((u8 *) dm) + dm->length;
 	const u8 *nsp;
@@ -66,6 +66,7 @@ static const char * __init dmi_string_nosave(const struct dmi_header *dm, u8 s)
 
 	return dmi_empty_string;
 }
+EXPORT_SYMBOL_GPL(dmi_string_nosave);
 
 static const char * __init dmi_string(const struct dmi_header *dm, u8 s)
 {
diff --git a/include/linux/dmi.h b/include/linux/dmi.h
index 2eedf44e6801..48691d589b88 100644
--- a/include/linux/dmi.h
+++ b/include/linux/dmi.h
@@ -91,6 +91,21 @@ struct dmi_device {
 	void *device_data;	/* Type specific data */
 };
 
+#define DMI_A_INFO_ENT_MIN_SIZE 0x6
+struct dmi_a_info_entry {
+	u8 length;
+	u16 handle;
+	u8 offset;
+	u8 str_num;
+	u8 value[];
+} __packed;
+
+#define DMI_A_INFO_MIN_SIZE	0xB
+struct dmi_a_info {
+	struct dmi_header header;
+	u8 count;
+} __packed;
+
 #ifdef CONFIG_DMI
 
 struct dmi_dev_onboard {
@@ -120,6 +135,7 @@ extern void dmi_memdev_name(u16 handle, const char **bank, const char **device);
 extern u64 dmi_memdev_size(u16 handle);
 extern u8 dmi_memdev_type(u16 handle);
 extern u16 dmi_memdev_handle(int slot);
+const char *dmi_string_nosave(const struct dmi_header *dm, u8 s);
 
 #else
 
@@ -153,6 +169,8 @@ static inline u8 dmi_memdev_type(u16 handle) { return 0x0; }
 static inline u16 dmi_memdev_handle(int slot) { return 0xffff; }
 static inline const struct dmi_system_id *
 	dmi_first_match(const struct dmi_system_id *list) { return NULL; }
+static inline const char *
+	dmi_string_nosave(const struct dmi_header *dm, u8 s) { return NULL; }
 
 #endif
 
-- 
2.52.0