linux-kernel - Re: [PATCH v2 2/3] tty: Add lookahead param to receive

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Date:   Fri, 8 Apr 2022 15:48:42 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
cc:     linux-serial <linux-serial@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Gilles Buloz <gilles.buloz@...tron.com>,
        Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH v2 2/3] tty: Add lookahead param to receive_buf

On Fri, 8 Apr 2022, Ilpo Järvinen wrote:

> On Fri, 8 Apr 2022, Andy Shevchenko wrote:
> 
> > On Fri, Apr 08, 2022 at 02:39:53PM +0300, Ilpo Järvinen wrote:
> > > After lookahead for XON/XOFF characters is added by the next
> > > patch, the receive side needs to ensure the flow-control
> > > actions are not retaken later on when those same characters
> > > get read by TTY.
> > > 
> > > Thus, pass lookahead count to receive_buf and skip
> > > flow-control character actions if already taken for the
> > > character in question. Lookahead count will become live after
> > > the next patch.
> > 
> > ...
> > 
> > > +		if (c == STOP_CHAR(tty)) {
> > > +			if (!lookahead_done)
> > 
> > But now it can be as below
> > 
> > 		if (c == STOP_CHAR(tty) && !lookahead_done)
> > 
> > > +				stop_tty(tty);
> > > +		} else if ((c == START_CHAR(tty) && !lookahead_done) ||
> > >  			 (tty->flow.stopped && !tty->flow.tco_stopped && I_IXANY(tty) &&
> > >  			  c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) &&
> > >  			  c != SUSP_CHAR(tty))) {
> 
> Are you sure about this? ...If I make that change to the first if, the 
> second part of the else if's condition get a chance it didn't have 
> previously.
> 
> What I'd like to do here is to take advantage of the function that was 
> added:
> 
> 		if (!n_tty_receive_char_flow_ctrl(tty, c) &&

Correcting myself.

		if (!n_tty_receive_char_flow_ctrl(tty, c, lookahead_done) &&
is the correct form as this is already 2/3, not 1/3 form of that
function.

> 		    tty->flow.stopped && !tty->flow.tco_stopped && I_IXANY(tty) &&
>         	    c != INTR_CHAR(tty) && c != QUIT_CHAR(tty) &&
> 	            c != SUSP_CHAR(tty))) {
> 			start_tty(tty);
> 			process_echoes(tty);
> 		}
> ...but it will change STOP_CHAR vs START_CHAR precedence for the case 
> where they're the same characters. I don't know if it matters.
> 
> 

-- 
 i.