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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jan 19, 2022 at 05:59:46PM +0100, Harald Seiler wrote:
> 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@denx.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;
> 	}
> 

Uwe, any thoughts about if this patch should be taken or not?

thanks,

greg k-h