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

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Message-ID: <5cab27cab5a39ef5e19992bc54e57c3f6106dafe.camel@denx.de>
Date:   Wed, 19 Jan 2022 17:59:46 +0100
From:   Harald Seiler <hws@...x.de>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Ahmad Fatoum <a.fatoum@...gutronix.de>,
        linux-serial@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        linux-kernel@...r.kernel.org, NXP Linux Team <linux-imx@....com>,
        Fabio Estevam <festevam@...il.com>,
        Shawn Guo <shawnguo@...nel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        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 17:21 +0100, Uwe Kleine-König wrote:
> On Wed, Jan 19, 2022 at 04:20:12PM +0100, Harald Seiler wrote:
> > 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?
> 
> I didn't oppose to the readability thing. With your patch you skip
> starting the stop_tx timer and that would usually care for calling
> imx_uart_stop_tx and setting sport->tx_state = OFF. This doesn't happen
> with your patch any more.

Not starting the timer is the entire point of the patch - instead, the
code which would run inside the timer callback now runs immediately. To
do this, I set the tx_state to WAIT_AFTER_SEND and _don't_ do the early
return which leads into the if(tx_state == WAIT_AFTER_SEND) below.  This
is the code-path which normally runs later in the hrtimer callback.

I suppose it would have been good to provide more context lines in the
patch... Here is the relevant bit (in the changed version now):

	if (sport->tx_state == SEND) {
		sport->tx_state = WAIT_AFTER_SEND;

		if (port->rs485.delay_rts_after_send > 0) {
			start_hrtimer_ms(&sport->trigger_stop_tx,
				 port->rs485.delay_rts_after_send);
			return;
		}

		/* continue without any delay */
	}

	if (sport->tx_state == WAIT_AFTER_RTS ||
	    sport->tx_state == WAIT_AFTER_SEND) {
		/* ... actual rts toggling ... */

		sport->tx_state = OFF;
	}

Regards,
-- 
Harald

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