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

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Message-ID: <7148dd63-3450-46e8-4cc0-8f4d607d882@linux.intel.com>
Date:   Fri, 8 Apr 2022 17:17:45 +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 03:34:27PM +0300, Ilpo Järvinen wrote:
> > On Fri, 8 Apr 2022, Andy Shevchenko wrote:
> > > On Fri, Apr 08, 2022 at 02:39:53PM +0300, Ilpo Järvinen wrote:
> 
> > 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.
> 
> No idea of impact of such change.

What I could do, is to create a separate change out of this outlined 
change alone so it would be possible to bisect to the very change, if it 
would cause a regression for somebody.

It doesn't feel very useful to have START_CHAR and STOP_CHAR match but 
who knows what is out there. I tested it briefly with stty and I know it 
"works" on the normal receive path (doesn't stop) but usefulness of such
a config seems rather doubtful.

One interesting difference is that here in the closing path, stop has 
higher precedence and on the normal path, start takes preference. I don't 
know if that's intentional or not.

-- 
 i.