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@xxxxxxx>
> > > > ---
> > > > 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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