[PATCH v2 1/6] USB: add usb_make_subpath() for formatted device subpaths

[Shihuang Liu <shlomojune6@gmail.com>]

USB drivers often build device paths by calling usb_make_path() :and
then appending a subpath. Several callers use the deprecated strlcat()
API for this.

In preparation for removing the deprecated strlcat() API[1], add
usb_make_subpath() to format the USB device path.

Link: https://github.com/KSPP/linux/issues/370 [1]
Signed-off-by: Shihuang Liu <shlomojune6@gmail.com>
---
 include/linux/usb.h | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/include/linux/usb.h b/include/linux/usb.h
index 1da4ad1610bca..c90ccea0cdbf6 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -14,6 +14,7 @@
 #include <linux/errno.h>        /* for -ENODEV */
 #include <linux/delay.h>	/* for mdelay() */
 #include <linux/interrupt.h>	/* for in_interrupt() */
+#include <linux/stdarg.h>	/* for va_list */
 #include <linux/list.h>		/* for struct list_head */
 #include <linux/kref.h>		/* for struct kref */
 #include <linux/device.h>	/* for struct device */
@@ -987,6 +988,34 @@ static inline int usb_make_path(struct usb_device *dev, char *buf, size_t size)
 	return (actual >= (int)size) ? -1 : actual;
 }
 
+/**
+ * usb_make_subpath - returns a stable device path and formatted subpath
+ * @dev: the device whose path is being constructed
+ * @buf: where to put the string
+ * @size: how big is "buf"?
+ * @fmt: format string for the subpath, without the leading slash
+ * @...: arguments for @fmt
+ *
+ * Return: Length of the string (> 0) or negative if size was too small.
+ */
+static inline __printf(4, 5)
+int usb_make_subpath(struct usb_device *dev, char *buf, size_t size,
+		     const char *fmt, ...)
+{
+	struct va_format vaf;
+	va_list args;
+	int actual;
+
+	va_start(args, fmt);
+	vaf.fmt = fmt;
+	vaf.va = &args;
+	actual = snprintf(buf, size, "usb-%s-%s/%pV", dev->bus->bus_name,
+			  dev->devpath, &vaf);
+	va_end(args);
+
+	return (actual >= (int)size) ? -1 : actual;
+}
+
 /*-------------------------------------------------------------------------*/
 
 #define USB_DEVICE_ID_MATCH_DEVICE \
-- 
2.43.0