[PATCH 2/4] xhci: dbc: allow setting device serial number through sysfs

[Mathias Nyman <mathias.nyman@linux.intel.com>]

From: Łukasz Bartosik <ukaszb@chromium.org>

Add dbc_serial sysfs attribute to allow changing the serial number
string descriptor presented by the debug device when a host requests a
string descriptor with iSerialNumber index.

Value can only be changed while debug capability (DbC) is in disabled
state to prevent USB device descriptor change while connected to a USB
host.

The default value is "0001".
The field length can be from 1 to 126 characters.
String is terminated at null or newline, driver does not support empty
string.

[ mn: Improve commit message and sysfs entry documentation ]
Signed-off-by: Łukasz Bartosik <ukaszb@chromium.org>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
---
 .../testing/sysfs-bus-pci-drivers-xhci_hcd    | 15 ++++++++
 drivers/usb/host/xhci-dbgcap.c                | 36 +++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd b/Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd
index fc82aa4e54b0..b0e8ded09c16 100644
--- a/Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd
+++ b/Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd
@@ -85,3 +85,18 @@ Description:
 		up to 5000. The default value is 64 ms.
 		This polling interval is used while DbC is enabled but has no
 		active data transfers.
+
+What:		/sys/bus/pci/drivers/xhci_hcd/.../dbc_serial
+Date:		January 2026
+Contact:	Łukasz Bartosik <ukaszb@chromium.org>
+Description:
+		The dbc_serial attribute allows to change the serial number
+		string descriptor presented by the debug device when a host
+		requests a string descriptor with iSerialNumber index.
+		Index is found in the iSerialNumber field in the device
+		descriptor.
+		Value can only be changed while debug capability (DbC) is in
+		disabled state to prevent USB device descriptor change while
+		connected to a USB host.
+		The default value is "0001".
+		The field length can be from 1 to 126 characters.
diff --git a/drivers/usb/host/xhci-dbgcap.c b/drivers/usb/host/xhci-dbgcap.c
index 4ba8edb37e51..a78d386ffdc3 100644
--- a/drivers/usb/host/xhci-dbgcap.c
+++ b/drivers/usb/host/xhci-dbgcap.c
@@ -1210,6 +1210,40 @@ static ssize_t dbc_bcdDevice_store(struct device *dev,
 	return size;
 }
 
+static ssize_t dbc_serial_show(struct device *dev,
+			    struct device_attribute *attr,
+			    char *buf)
+{
+	struct xhci_hcd	*xhci = hcd_to_xhci(dev_get_drvdata(dev));
+	struct xhci_dbc	*dbc = xhci->dbc;
+
+	return sysfs_emit(buf, "%s\n", dbc->str.serial);
+}
+
+static ssize_t dbc_serial_store(struct device *dev,
+			     struct device_attribute *attr,
+			     const char *buf, size_t size)
+{
+	struct xhci_hcd	*xhci = hcd_to_xhci(dev_get_drvdata(dev));
+	struct xhci_dbc	*dbc = xhci->dbc;
+	size_t len;
+
+	if (dbc->state != DS_DISABLED)
+		return -EBUSY;
+
+	len = strcspn(buf, "\n");
+	if (!len)
+		return -EINVAL;
+
+	if (len > USB_MAX_STRING_LEN)
+		return -E2BIG;
+
+	memcpy(dbc->str.serial, buf, len);
+	dbc->str.serial[len] = '\0';
+
+	return size;
+}
+
 static ssize_t dbc_bInterfaceProtocol_show(struct device *dev,
 				 struct device_attribute *attr,
 				 char *buf)
@@ -1297,6 +1331,7 @@ static DEVICE_ATTR_RW(dbc);
 static DEVICE_ATTR_RW(dbc_idVendor);
 static DEVICE_ATTR_RW(dbc_idProduct);
 static DEVICE_ATTR_RW(dbc_bcdDevice);
+static DEVICE_ATTR_RW(dbc_serial);
 static DEVICE_ATTR_RW(dbc_bInterfaceProtocol);
 static DEVICE_ATTR_RW(dbc_poll_interval_ms);
 
@@ -1305,6 +1340,7 @@ static struct attribute *dbc_dev_attrs[] = {
 	&dev_attr_dbc_idVendor.attr,
 	&dev_attr_dbc_idProduct.attr,
 	&dev_attr_dbc_bcdDevice.attr,
+	&dev_attr_dbc_serial.attr,
 	&dev_attr_dbc_bInterfaceProtocol.attr,
 	&dev_attr_dbc_poll_interval_ms.attr,
 	NULL
-- 
2.43.0