Re: [RFC PATCH] serial: Don't assume uart_ops .throttle is always set

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Fri, 29 Apr 2022, Tomasz Mon wrote:

> On Thu, 2022-04-28 at 20:11 +0300, Ilpo Järvinen wrote:
> > uart_throttle() assumes that a driver provides a throttle function in
> > uart_ops. But not all drivers do and there seems to nothing in
> > serial_core that would set it either. Thus, check it before calling.
> 
> If the driver does not provide throttle function, it should not
> set UPSTAT_AUTORTS nor UPSTAT_AUTOXOFF in port->status.
> 
> > diff --git a/drivers/tty/serial/serial_core.c
> > b/drivers/tty/serial/serial_core.c
> > index 6a8963caf954..18c9d46e0b85 100644
> > --- a/drivers/tty/serial/serial_core.c
> > +++ b/drivers/tty/serial/serial_core.c
> > @@ -697,7 +697,8 @@ static void uart_throttle(struct tty_struct *tty)
> >  		mask |= UPSTAT_AUTORTS;
> >  
> >  	if (port->status & mask) {
> 
> This if condition is always false if driver did not set UPSTAT_AUTORTS
> nor UPSTAT_AUTOXOFF in port->status.
> 
> > -		port->ops->throttle(port);
> > +		if (port->ops->throttle)
> > +			port->ops->throttle(port);
> >  		mask &= ~port->status;
> >  	}
> > 
> 
> I think this patch is incorrect. The assumption is not ".throttle is
> always set" but rather ".throttle is set when UPSTAT_AUTORTS or
> UPSTAT_AUTOXOFF is set".

Right, I kind of ended up concluding that's likely the case after sending 
the patch when looking into 9aba8d5b0111 (SERIAL: core: add 
throttle/unthrottle callbacks for hardware assisted flow control).


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 i.