xhci: dbc: allow setting device serial number through sysfs

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>
Link: https://patch.msgid.link/20260120181148.128712-3-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd b/Documentation/ABI/testing/sysfs-bus-pci-drivers-xhci_hcd
index fc82aa4e54b005e67a6224c113337c0d4636bfcb..b0e8ded09c16e7da6be4cdfe37185b3ce25e64bc 100644 (file)
--- 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 4ba8edb37e51749b11eda1aa49ac823102a4dadb..a78d386ffdc39fe97db02ea98984d8862d8ccfef 100644 (file)
--- 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