linux-kernel - Re: [PATCH 2/2] tty: Implement lookahead to process XON/XOFF timely

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Date:   Wed, 6 Apr 2022 10:21:12 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     linux-serial@...r.kernel.org, Greg KH <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Gilles Buloz <gilles.buloz@...tron.com>,
        Johan Hovold <johan@...nel.org>
Subject: Re: [PATCH 2/2] tty: Implement lookahead to process XON/XOFF timely

On 05. 04. 22, 18:11, Andy Shevchenko wrote:
> On Tue, Apr 05, 2022 at 01:24:37PM +0300, Ilpo Järvinen wrote:
>> When tty is not read from, XON/XOFF may get stuck into an
>> intermediate buffer. As those characters are there to do software
>> flow-control, it is not very useful. In the case where neither end
>> reads from ttys, the receiving ends might not be able receive the
>> XOFF characters and just keep sending more data to the opposite
>> direction. This problem is almost guaranteed to occur with DMA
>> which sends data in large chunks.
>>
>> If TTY is slow to process characters, that is, eats less than given
>> amount in receive_buf, invoke lookahead for the rest of the chars
>> to process potential XON/XOFF characters.
>>
>> The guards necessary for ensuring the XON/XOFF character are
>> processed only once were added by the previous patch. All this patch
>> needs to do on that front is to pass the lookahead count (that can
>> now be non-zero) into port->client_ops->receive_buf().
> 
> ...
> 
>> +static bool __n_tty_receive_char_special(struct tty_struct *tty, unsigned char c,
>> +					 bool lookahead_done)
>> +{
>> +	if (!I_IXON(tty))
>> +		return false;
>> +
>> +	if (c == START_CHAR(tty)) {
>> +		if (!lookahead_done) {
>> +			start_tty(tty);
>> +			process_echoes(tty);
>> +		}
>> +		return true;
>> +	}
>> +	if (c == STOP_CHAR(tty)) {
>> +		if (!lookahead_done)
>> +			stop_tty(tty);
>> +		return true;
>> +	}
>> +	return false;
>> +}
> 
> Looking into this I would first make a preparatory patch that splits out
> current code into something like
> 
> static bool __n_tty_receive_char_special_no_lookahead(struct tty_struct *tty, unsigned char c)
> {
> 	...current code...
> }
> 
> Then in the patch 1 you add
> 
> static bool __n_tty_receive_char_special_lookahead(struct tty_struct *tty, unsigned char c)
> {
> 	...
> }
> 
> static bool __n_tty_receive_char_special(struct tty_struct *tty, unsigned char c,
> 					 bool lookahead_done)

This should be dubbed better. Maybe n_tty_receive_char_flow_control()?

And I would place the if (I_IXON(tty)) to the caller. I am a bit lost in 
this pseudo code, so maybe this doesn't make sense in your proposal. I 
have something like in my mind:

if (I_IXON(tty))
   return n_tty_receive_char_flow_control();

Historically, these n_tty_receive* function names were a big mess. Don't 
produce more of that by simply prepending only "__".

> {
> 	if (!I_IXON(tty))
> 		return false;
> 
> 	if (lookahead_done)
> 		return _lookahead();
> 
> 	return _no_lookahead();
> }

thanks
-- 
js
suse labs