Re: [PATCH 3/3] tty: n_gsm: add TIOCMIWAIT support

Greg KH
2023-02-01T12:51:38Z

On Wed, Feb 01, 2023 at 09:30:28AM +0000, Starke, Daniel wrote:
> > > Add support for the TIOCMIWAIT ioctl on the virtual ttys. This enables the
> > > user to wait for virtual modem signals like RING.
> > >
> > > Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
> > > ---
> > > drivers/tty/n_gsm.c | 32 ++++++++++++++++++++++++++++++++
> > > 1 file changed, 32 insertions(+)
> > >
> > > diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> > > index 118511c1fa37..48fb7dad44cd 100644
> > > --- a/drivers/tty/n_gsm.c
> > > +++ b/drivers/tty/n_gsm.c
> > > @@ -1542,6 +1542,7 @@ static void gsm_process_modem(struct tty_struct *tty, struct gsm_dlci *dlci,
> > > if (brk & 0x01)
> > > tty_insert_flip_char(&dlci->port, 0, TTY_BREAK);
> > > dlci->modem_rx = mlines;
> > > + wake_up_interruptible(&dlci->gsm->event);
> > > }
> > >
> > > /**
> > > @@ -3848,6 +3849,35 @@ static int gsm_modem_update(struct gsm_dlci *dlci, u8 brk)
> > > return -EPROTONOSUPPORT;
> > > }
> > >
> > > +/**
> > > + * gsm_wait_modem_change - wait for modem status line change
> >
> > No need for tabs.
> >
> > Or for kernel doc for static functions, but that's not a big deal.
>
> All other functions follow the same style here. Nevertheless, I will
> replace the tabs with spaces. Originally, I had planned a code clean-up
> after committing all the changes from my original RFC.

Ah, ok, nevermind then, I didn't see that as it's not in the diffs.

Cleaning up later is fine.

> > > +static int gsm_wait_modem_change(struct gsm_dlci *dlci, u32 mask)
> > > +{
> > > + struct gsm_mux *gsm = dlci->gsm;
> > > + u32 old = dlci->modem_rx & mask;
> > > + int ret;
> > > +
> > > + if (gsm->dead)
> > > + return -ENODEV;
> > > +
> > > + do {
> > > + ret = wait_event_interruptible(gsm->event, gsm->dead
> > > + || old ^ (dlci->modem_rx & mask));
> > > + if (ret)
> > > + return ret;
> > > + if (dlci->state != DLCI_OPEN)
> > > + return -EL2NSYNC;
> > > + if (gsm->dead)
> > > + return -ENODEV;
> > > + } while ((old ^ (dlci->modem_rx & mask)) == 0);
> >
> > No way to break out of the loop if it goes for forever?
>
> I assume that this is the expected behavior for TIOCMIWAIT. The functions
> returns if:
> - the requested modem signal changed
> - the wait function got interrupted (e.g. by a signal)
> - the underlying DLCI was closed
> - the underlying ldisc device was removed

Hm, I guess you are right. But wow, reading that
wait_event_interruptible() condition is crazy, please document that
really well to explain this properly so you will be able to understand
it in a year when you next have to fix it up :)

> I can add that the function returns immediate if a mask has been passed
> which matches no modem signal if this is preferred?

I don't think that would work, try it on some existing serial ports to
see what they do.

thanks,

greg k-h