Re: [PATCH] cdc-acm: acm_init: Set initial BAUD to B0

[Johan Hovold <johan@kernel.org>]

On Fri, Jul 10, 2020 at 04:05:29PM +0000, Joakim Tjernlund wrote:
> On Fri, 2020-07-10 at 14:41 +0200, Johan Hovold wrote:
> > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you recognize the sender and know the content is safe.
> > 
> > 
> > On Fri, Jul 10, 2020 at 10:46:19AM +0000, Joakim Tjernlund wrote:
> > > On Fri, 2020-07-10 at 12:34 +0200, Greg KH wrote:
> > > > 
> > > > On Fri, Jul 10, 2020 at 11:35:18AM +0200, Joakim Tjernlund wrote:
> > 
> > > > >       tty_set_operations(acm_tty_driver, &acm_ops);
> > > > > 
> > > > > -     retval = tty_register_driver(acm_tty_driver);
> > > > > +     retval = usb_register(&acm_driver);
> > > > >       if (retval) {
> > > > >               put_tty_driver(acm_tty_driver);
> > > > >               return retval;
> > > > >       }
> > > > > 
> > > > > -     retval = usb_register(&acm_driver);
> > > > > +     retval = tty_register_driver(acm_tty_driver);
> > > > >       if (retval) {
> > > > > -             tty_unregister_driver(acm_tty_driver);
> > > > > +             usb_deregister(&acm_driver);
> > > > 
> > > > Why are you switching these around?  I think I know, but you don't
> > > > really say...
> > > 
> > > I wrote:
> > >    For initial termios to reach USB core, USB driver has to be
> > >    registered before TTY driver.
> > > Found out that by trial and error. Isn't that clear enough?
> > 
> > No, that makes no sense at all since USB core does not care about
> > init_termios.
> 
> But you install acm_ops into tty:
> 	tty_set_operations(acm_tty_driver, &acm_ops);
> Perhaps there is a call into acm_ops?

No, not until the tty device has been registered by the USB driver.

> Anyhow, does it not make sense to have usb before tty as tty uses usb?

Nope, it's the other way round, and your change is therefore broken.

> Can I ask this too:
>  what is the difference between acm_tty_install and acm_tty_open ?
>  Both seems to be called at open(2)
> seems to me that install could be folded into open ?

No, their purposes are distinct and they cannot be merged.

Johan