Re: [PATCH v4 3/7] serial: rsci: Enable transmit end interrupt in rsci_stop_tx()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] serial: rsci: TX restart on suspend in sci_tx_end_interrupt()
--

commit 071c63519ac773139fb715b1e8e4f93c225b5ba8
Author: Diogo Ivo <diogo.ivo.sx@renesas.com>

serial: rsci: Enable transmit end interrupt in rsci_stop_tx()

This commit enables the transmit end interrupt (CCR0_TEIE) in
rsci_stop_tx() when transmission is active (CCR0_TE is set) so that
sci_tx_end_interrupt() clears CCR0_TE once in-flight data finishes
shifting out.

> diff --git a/drivers/tty/serial/rsci.c b/drivers/tty/serial/rsci.c
> index 0391111db8ae8..05703bb31afde 100644
> --- a/drivers/tty/serial/rsci.c
> +++ b/drivers/tty/serial/rsci.c
> @@ -409,6 +409,8 @@ static void rsci_stop_tx(struct uart_port *port)
>  
>  	ctrl = rsci_serial_in(port, CCR0);
>  	ctrl &= ~CCR0_TIE;
> +	if (ctrl & CCR0_TE)
> +		ctrl |= CCR0_TEIE;
>  	rsci_serial_out(port, CCR0, ctrl);
>  }

[Severity: Medium]
Can enabling CCR0_TEIE here cause sci_tx_end_interrupt() to restart
transmission during system suspend?

uart_suspend_port() sets uport->suspended = 1 and calls ops->stop_tx()
and ops->set_mctrl(uport, 0) without setting tty->flow.stopped or
uport->hw_stopped, then drops uport->lock to wait up to 30ms for the
hardware transmitter to empty.  When the in-flight bytes finish shifting
out, the resulting TEI interrupt sees !uart_tx_stopped(port) as true and
resumes transmission if tport->xmit_fifo still has queued data:

drivers/tty/serial/sh-sci.c:sci_tx_end_interrupt() {
    ...
	if (port->x_char || (!kfifo_is_empty(&tport->xmit_fifo) &&
			     !uart_tx_stopped(port))) {
		ctrl &= ~(s->params->param_bits->teie);
		ctrl |= s->params->param_bits->tie;
		s->ops->write_reg(port, regs->control, ctrl);
		s->ops->transmit_chars(port);
    ...
}

This keeps transmitting after stop_tx() and modem control deassertion,
and if xmit_fifo does not empty within 30ms, uart_suspend_port() times
out with "Unable to drain transmitter" and calls ops->shutdown()
mid-transmission.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261113-g2ul_sci-v4-0-6815e41d90c5@renesas.com?part=3