linux-kernel - Re: [PATCH] tty: serial: imx: Add fast path when rs485 delays are 0

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Date:   Wed, 19 Jan 2022 16:20:12 +0100
From:   Harald Seiler <hws@...x.de>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Fabio Estevam <festevam@...il.com>,
        Ahmad Fatoum <a.fatoum@...gutronix.de>,
        linux-serial@...r.kernel.org, Jiri Slaby <jirislaby@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-kernel@...r.kernel.org, NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] tty: serial: imx: Add fast path when rs485 delays are 0

Hi,

On Wed, 2022-01-19 at 16:11 +0100, Uwe Kleine-König wrote:
> On Wed, Jan 19, 2022 at 03:52:03PM +0100, Harald Seiler wrote:
> > Right now, even when `delay_rts_before_send` and `delay_rts_after_send`
> > are 0, the hrtimer is triggered (with timeout 0) which can introduce a
> > few 100us of additional overhead on slower i.MX platforms.
> > 
> > Implement a fast path when the delays are 0, where the RTS signal is
> > toggled immediately instead of going through an hrtimer.  This fast path
> > behaves identical to the code before delay support was implemented.
> > 
> > Signed-off-by: Harald Seiler <hws@...x.de>
> > ---
> >  drivers/tty/serial/imx.c | 18 ++++++++++++++----
> >  1 file changed, 14 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/tty/serial/imx.c b/drivers/tty/serial/imx.c
> > index df8a0c8b8b29..67bbbb69229d 100644
> > --- a/drivers/tty/serial/imx.c
> > +++ b/drivers/tty/serial/imx.c
> > @@ -455,9 +455,14 @@ static void imx_uart_stop_tx(struct uart_port *port)
> >  	if (port->rs485.flags & SER_RS485_ENABLED) {
> >  		if (sport->tx_state == SEND) {
> >  			sport->tx_state = WAIT_AFTER_SEND;
> > -			start_hrtimer_ms(&sport->trigger_stop_tx,
> > +
> > +			if (port->rs485.delay_rts_after_send > 0) {
> > +				start_hrtimer_ms(&sport->trigger_stop_tx,
> >  					 port->rs485.delay_rts_after_send);
> > -			return;
> > +				return;
> > +			}
> > +
> > +			/* continue without any delay */
> 
> Is it right to keep the assignment sport->tx_state = WAIT_AFTER_SEND ?

I am keeping the assignment intentionally, to fall into the
if(state == WAIT_AFTER_RTS) below (which then sets the state to OFF).
I originally had the code structured like this:

	if (port->rs485.delay_rts_after_send > 0) {
		sport->tx_state = WAIT_AFTER_SEND;
		start_hrtimer_ms(&sport->trigger_stop_tx,
			 port->rs485.delay_rts_after_send);
		return;
	} else {
		/* continue without any delay */
		sport->tx_state = WAIT_AFTER_SEND;
	}

This is functionally identical, but maybe a bit more explicit.

Not sure what is more clear to read?

> >  		}
> >  
> >  		if (sport->tx_state == WAIT_AFTER_RTS ||
> > @@ -698,9 +703,14 @@ static void imx_uart_start_tx(struct uart_port *port)
> >  				imx_uart_stop_rx(port);
> >  
> >  			sport->tx_state = WAIT_AFTER_RTS;
> > -			start_hrtimer_ms(&sport->trigger_start_tx,
> > +
> > +			if (port->rs485.delay_rts_before_send > 0) {
> > +				start_hrtimer_ms(&sport->trigger_start_tx,
> >  					 port->rs485.delay_rts_before_send);
> > -			return;
> > +				return;
> > +			}
> > +
> > +			/* continue without any delay */
> 
> Here similar question here about sport->tx_state = WAIT_AFTER_RTS;

Same as above, but with WAIT_AFTER_RTS of course...

-- 
Harald

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