linux-kernel - Re: [PATCH] usb/serial: Add compat

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Date:	Thu, 27 Nov 2008 10:27:04 -0800
From:	Keith Packard <keithp@...thp.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	keithp@...thp.com, linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...e.de>
Subject: Re: [PATCH] usb/serial: Add compat_ioctl pass-through

On Thu, 2008-11-27 at 15:31 +0100, Arnd Bergmann wrote:
> On Thursday 27 November 2008, Keith Packard wrote:
> > +static long serial_compat_ioctl(struct tty_struct *tty, struct file *file,
> > +                              unsigned int cmd, unsigned long arg)
> > +{
> > +       struct usb_serial_port *port = tty->driver_data;
> > +       long retval = -ENODEV;
> > +
> > +       dbg("%s - port %d, cmd 0x%.4x", __func__, port->number, cmd);
> > +
> > +       WARN_ON(!port->port.count);
> > +
> > +       /* pass on to the driver specific version of this function
> > +          if it is available */
> > +       if (port->serial->type->compat_ioctl) {
> > +               lock_kernel();
> > +               retval = port->serial->type->compat_ioctl(tty, file, cmd, arg);
> > +               unlock_kernel(); 
> 
> By convention, compat_ioctl functions should be called without the big kernel
> lock held.

Thanks - I didn't realize these were supposed to be different.

> You should probably define compat_ioctl to return an int as well
> so that it becomes possible to use the same function for both eventually.

The existing tty layer compat_ioctl is defined to return long; is that
also wrong?

> Then again, we can also drop the file argument, which is entirely unused
> in all the usb-serial ioctls.

I'm not sure it's worth the effort; if some future usb serial ioctl
needs the argument, we'd end up changing every existing driver back. I
assume there are some serial devices for which the file is relevant
during ioctl.

> Maybe the best way for now is to make the new compat_ioctl be
> int compat_ioctl(struct tty_struct, unsigned int cmd, unsigned long arg);
> and leave the old ioctl as it is. When we get around to pushing the
> BKL down into ->ioctl, we can change the prototype at the same time
> to warn potential out-of-tree drivers.

Let's figure out what the right compat_ioctl interface is first and
worry about fixing the existing ioctl interface later.

-- 
keith.packard@...el.com

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