linux-kernel - Re: [PATCH v3 3/5] tty: Add lookahead param to receive

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Date:   Tue, 26 Apr 2022 09:59:01 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     linux-serial <linux-serial@...r.kernel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Gilles Buloz <gilles.buloz@...tron.com>,
        Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH v3 3/5] tty: Add lookahead param to receive_buf

On Tue, Apr 26, 2022 at 10:48:40AM +0300, Ilpo Järvinen wrote:
> On Fri, 22 Apr 2022, Ilpo Järvinen wrote:
> 
> > On Fri, 22 Apr 2022, Greg KH wrote:
> > 
> > > >  /* Returns true if c is consumed as flow-control character */
> > > > -static bool n_tty_receive_char_flow_ctrl(struct tty_struct *tty, unsigned char c)
> > > > +static bool n_tty_receive_char_flow_ctrl(struct tty_struct *tty, unsigned char c,
> > > > +					 bool lookahead_done)
> > > >  {
> > > >  	if (!n_tty_is_char_flow_ctrl(tty, c))
> > > >  		return false;
> > > >  
> > > > +	if (lookahead_done)
> > > > +		return true;
> > > 
> > > Why would this function be called if this option was true?
> > 
> > Agreed, it makes sense to move the check before call (and then I also 
> > don't need to reorganize this function anymore).
> 
> I think I want to renege on this. The reason is that on flow control char, 
> two things must occur:
>   a) it must not be treated as normal char
>   b) if not yet processed, flow control actions need to be taken
> 
> When the check is inside, return value of n_tty_receive_char_flow_ctrl 
> decides a), and b) is kept internal to n_tty_receive_char_flow_ctrl().
> 
> If I more that lookahead_done check into the caller domain, things get 
> IMHO a lot more messy. Effectively, I have three options for the calling 
> domain to chose from:
> 
> 	if (I_IXON(tty)) {
> 		if (!lookahead_done) {
> 			if (n_tty_receive_char_flow_ctrl(tty, c))
> 				return;
> 		} else if (n_tty_is_char_flow_ctrl(tty, c)) {
> 			return;
> 		}
> 	}
> 
> or
> 
> 	if (I_IXON(tty)) {
> 		if ((!lookahead_done && n_tty_receive_char_flow_ctrl(tty, c)) ||
> 		    (lookahead_done && n_tty_is_char_flow_ctrl(tty, c))) {
> 			return;
> 	}
> 
> vs
> 
>         if (I_IXON(tty) && n_tty_receive_char_flow_ctrl(tty, c, lookahead_done))
>                 return;
> 
> I heavily prefer that last option.

Ok, then document the heck out of it please.

greg k-h