Re: [PATCH 2/7] tty: serial: Add poll_get_irq() to the polling interface

[Sumit Garg]

On Mon, 22 Jun 2020 at 21:26, Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> On Mon, Jun 22, 2020 at 07:56:19PM +0530, Sumit Garg wrote:
> > From: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
>
> Sumit, to some extent this mail is me yelling at myself two years ago
> although, in my defence, at the time I choose not to post it because I
> knew it wasn't right.
>
> I'm a bit concerned to see the TODO: comment critiquing this interface
> being removed (from patch 3) without this patch being changed to
> address that comment.
>

I did consider that comment but I couldn't think of a normal scenario
where request_irq() should fail. And in case it fails as well, I did
put in "WARN_ON(res);" so that the user is notified of that particular
error scenario.

>
> > Add new API: poll_get_irq() to the polling interface in order for user
> > of polling interface to retrieve allocated IRQ corresponding to
> > underlying serial device.
> >
> > Although, serial interface still works in polling mode but interrupt
> > associated with serial device can be leveraged for special purposes like
> > debugger(kgdb) entry.
> >
> > Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > ---
> >  drivers/tty/serial/serial_core.c | 18 ++++++++++++++++++
> >  include/linux/serial_core.h      |  1 +
> >  include/linux/tty_driver.h       |  1 +
> >  3 files changed, 20 insertions(+)
> >
> > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> > index 66a5e2f..1bb033c 100644
> > --- a/drivers/tty/serial/serial_core.c
> > +++ b/drivers/tty/serial/serial_core.c
> > @@ -2470,6 +2470,23 @@ static void uart_poll_put_char(struct tty_driver *driver, int line, char ch)
> >       port->ops->poll_put_char(port, ch);
> >       uart_port_deref(port);
> >  }
> > +
> > +static int uart_poll_get_irq(struct tty_driver *driver, int line)
> > +{
> > +     struct uart_driver *drv = driver->driver_state;
> > +     struct uart_state *state = drv->state + line;
> > +     struct uart_port *port;
> > +     int ret = -ENODEV;
> > +
> > +     port = uart_port_ref(state);
> > +     if (port && port->ops->poll_get_irq) {
> > +             ret = port->ops->poll_get_irq(port);
> > +             uart_port_deref(port);
> > +     }
> > +
> > +     return ret;
> > +}
> > +
> >  #endif
> >
> >  static const struct tty_operations uart_ops = {
> > @@ -2505,6 +2522,7 @@ static const struct tty_operations uart_ops = {
> >       .poll_init      = uart_poll_init,
> >       .poll_get_char  = uart_poll_get_char,
> >       .poll_put_char  = uart_poll_put_char,
> > +     .poll_get_irq   = uart_poll_get_irq,
>
> The TODO comments claimed this API was bogus because it doesn't permit
> a free and that can cause interoperation problems with the real serial
> driver. I'll cover some of that in a reply to patch 3 but for now.
>
> Anyhow, for this patch, what are the expected semantics for
> uart_poll_get_irq().

Currently, the expected use for this API is to enable uart RX
interrupts and return corresponding IRQ id.

Although, we can make this interface modular as follows:

.poll_get_irq
.poll_enable_rx_int
.poll_disable_rx_int

Your views?

>
> In particular how do they ensure correct interlocking with the real
> serial driver underlying it (if kgdb_nmi is active on a serial port
> then the underlying driver better not be active at the same time).
>

AFAIU kgdb_nmi feature, it registers a new tty driver (ttyNMI0) which
is expected to work alongside underlying tty driver (eg. ttyAMA0 with
amba-pl011). So ttyAMA0 will only become active if user-space tries to
interact with /dev/ttyAMA0 like:

# echo "Hello World!" > /dev/ttyAMA0

So I would like to understand the downsides of having both of them
active at the same time using shared IRQ (although that won't be
possible with NMI as that doesn't support shared mode)?

-Sumit

>
> Daniel.
>
>
> >  #endif
> >  };
> >
> > diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> > index 92f5eba..8b132e6 100644
> > --- a/include/linux/serial_core.h
> > +++ b/include/linux/serial_core.h
> > @@ -78,6 +78,7 @@ struct uart_ops {
> >       int             (*poll_init)(struct uart_port *);
> >       void            (*poll_put_char)(struct uart_port *, unsigned char);
> >       int             (*poll_get_char)(struct uart_port *);
> > +     int             (*poll_get_irq)(struct uart_port *);
> >  #endif
> >  };
> >
> > diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
> > index 3584462..d6da5c5 100644
> > --- a/include/linux/tty_driver.h
> > +++ b/include/linux/tty_driver.h
> > @@ -295,6 +295,7 @@ struct tty_operations {
> >       int (*poll_init)(struct tty_driver *driver, int line, char *options);
> >       int (*poll_get_char)(struct tty_driver *driver, int line);
> >       void (*poll_put_char)(struct tty_driver *driver, int line, char ch);
> > +     int (*poll_get_irq)(struct tty_driver *driver, int line);
> >  #endif
> >       int (*proc_show)(struct seq_file *, void *);
> >  } __randomize_layout;
> > --
> > 2.7.4
> >