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

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Date:   Fri, 8 Apr 2022 15:34:27 +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, 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) &&
		    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.