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

[Shihuang Liu]

On Wed, Oct 21, 2026 at 03:17:56PM +0200, Alejandro Colomar wrote:
> Hi Shihuang,
> 
> > Date: 2026-10-21 20:31:41+0800
> > From: Shihuang Liu <shlomojune6@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > ---
> >  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;
> 
> Why are you open-coding usb_make_path() here instead of calling it?
> 
> 
> Have a lovely day!
> Alex
> 
Hi, Alex

With open-coding, a single snprintf() is enough, so we don't need to
call usb_make_path() and then another function to append the suffix.

Thanks,
Shihuang
> > +}
> > +
> >  /*-------------------------------------------------------------------------*/
> >  
> >  #define USB_DEVICE_ID_MATCH_DEVICE \
> > -- 
> > 2.43.0
> > 
> > 
> 
> -- 
> <https://www.alejandro-colomar.es>