PCI/sysfs: Expose PCI device serial number

Add a single sysfs read-only interface for reading PCI device serial
numbers from userspace in a programmatic way. This device attribute uses
the same hexadecimal 1-byte dashed formatting as lspci serial number
capability output. If a device doesn't support the serial number
capability, the serial_number sysfs attribute will not be visible.

Signed-off-by: Matthew Wood <thepacketgeek@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mario Limonciello <superm1@kernel.org>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Link: https://patch.msgid.link/20250917125815.722952-2-thepacketgeek@gmail.com

Author: thepacketgeek

Documentation/ABI/testing/sysfs-bus-pci

@@ -612,3 +612,12 @@ Description:
 
 		  # ls doe_features
 		  0001:01        0001:02        doe_discovery
+
+What:		/sys/bus/pci/devices/.../serial_number
+Date:		December 2025
+Contact:	Matthew Wood <thepacketgeek@gmail.com>
+Description:
+		This is visible only for PCI devices that support the serial
+		number extended capability. The file is read only and due to
+		the possible sensitivity of accessible serial numbers, admin
+		only.

drivers/pci/pci-sysfs.c

@@ -30,6 +30,7 @@
 #include <linux/msi.h>
 #include <linux/of.h>
 #include <linux/aperture.h>
+#include <linux/unaligned.h>
 #include "pci.h"
 
 #ifndef ARCH_PCI_DEV_GROUPS
@@ -694,6 +695,22 @@ static ssize_t boot_vga_show(struct device *dev, struct device_attribute *attr,
 }
 static DEVICE_ATTR_RO(boot_vga);
 
+static ssize_t serial_number_show(struct device *dev,
+				  struct device_attribute *attr, char *buf)
+{
+	struct pci_dev *pci_dev = to_pci_dev(dev);
+	u64 dsn;
+	u8 bytes[8];
+
+	dsn = pci_get_dsn(pci_dev);
+	if (!dsn)
+		return -EIO;
+
+	put_unaligned_be64(dsn, bytes);
+	return sysfs_emit(buf, "%8phD\n", bytes);
+}
+static DEVICE_ATTR_ADMIN_RO(serial_number);
+
 static ssize_t pci_read_config(struct file *filp, struct kobject *kobj,
 			       const struct bin_attribute *bin_attr, char *buf,
 			       loff_t off, size_t count)
@@ -1698,6 +1715,7 @@ late_initcall(pci_sysfs_init);
 
 static struct attribute *pci_dev_dev_attrs[] = {
 	&dev_attr_boot_vga.attr,
+	&dev_attr_serial_number.attr,
 	NULL,
 };
 
@@ -1710,6 +1728,9 @@ static umode_t pci_dev_attrs_are_visible(struct kobject *kobj,
 	if (a == &dev_attr_boot_vga.attr && pci_is_vga(pdev))
 		return a->mode;
 
+	if (a == &dev_attr_serial_number.attr && pci_get_dsn(pdev))
+		return a->mode;
+
 	return 0;
 }