ACPI: FPDT: expose FBPT and S3PT subtables via sysfs

Add sysfs files at /sys/firmware/acpi/fpdt/FBPT and
/sys/firmware/acpi/fpdt/S3PT that expose the raw contents of the
FPDT subtables.

Note that /sys/firmware/acpi/tables/FPDT only provides the top level
table, not the subtables. Adding access to the subtables enables a
usage model similar to /sys/firmware/dmi/tables/DMI, allowing
userspace tools to interpret newer record types (e.g. String Event
Records, Microcontroller Boot Performance Data Records, etc.) defined
in recent ACPI specifications [1] without requiring kernel changes.

Link: https://uefi.org/specs/ACPI/6.6/05_ACPI_Software_Programming_Model.html#performance-event-record-types [1]
Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
[ rjw: Changelog edits ]
Link: https://patch.msgid.link/20260324231456.701-2-nathaniel.l.desimone@intel.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: nate-desimone

drivers/acpi/acpi_fpdt.c

@@ -141,6 +141,9 @@ static const struct attribute_group boot_attr_group = {
 	.name = "boot",
 };
 
+static BIN_ATTR(FBPT, 0400, sysfs_bin_attr_simple_read, NULL, 0);
+static BIN_ATTR(S3PT, 0400, sysfs_bin_attr_simple_read, NULL, 0);
+
 static struct kobject *fpdt_kobj;
 
 #if defined CONFIG_X86 && defined CONFIG_PHYS_ADDR_T_64BIT
@@ -254,9 +257,34 @@ static int fpdt_process_subtable(u64 address, u32 subtable_type)
 			break;
 		}
 	}
+
+	if (subtable_type == SUBTABLE_FBPT) {
+		bin_attr_FBPT.private = subtable_header;
+		bin_attr_FBPT.size = length;
+		result = sysfs_create_bin_file(fpdt_kobj, &bin_attr_FBPT);
+		if (result)
+			pr_warn("Failed to create FBPT sysfs attribute.\n");
+	} else if (subtable_type == SUBTABLE_S3PT) {
+		bin_attr_S3PT.private = subtable_header;
+		bin_attr_S3PT.size = length;
+		result = sysfs_create_bin_file(fpdt_kobj, &bin_attr_S3PT);
+		if (result)
+			pr_warn("Failed to create S3PT sysfs attribute.\n");
+	}
+
 	return 0;
 
 err:
+	if (bin_attr_FBPT.private) {
+		sysfs_remove_bin_file(fpdt_kobj, &bin_attr_FBPT);
+		bin_attr_FBPT.private = NULL;
+	}
+
+	if (bin_attr_S3PT.private) {
+		sysfs_remove_bin_file(fpdt_kobj, &bin_attr_S3PT);
+		bin_attr_S3PT.private = NULL;
+	}
+
 	if (record_boot)
 		sysfs_remove_group(fpdt_kobj, &boot_attr_group);