Re: [PATCH v2] serial: uart: add hw flow control support configuration

[Peter Hurley <peter@hurleysoftware.com>]

On 05/01/2014 03:04 PM, Murali Karicheri wrote:
> 8250 uart driver currently supports only software assisted hw flow
> control. The software assisted hw flow control maintains a hw_stopped
> flag in the tty structure to stop and start transmission and use modem
> status interrupt for the event to drive the handshake signals. This is
> not needed if hw has flow control capabilities. This patch adds a
> DT attribute for enabling hw flow control for a uart port. Also skip
> stop and start if this flag is present in flag field of the port
> structure.
> 
> Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
> 
> CC: Rob Herring <robh+dt@kernel.org>
> CC: Pawel Moll <pawel.moll@arm.com>
> CC: Mark Rutland <mark.rutland@arm.com>
> CC: Ian Campbell <ijc+devicetree@hellion.org.uk>
> CC: Kumar Gala <galak@codeaurora.org>
> CC: Randy Dunlap <rdunlap@infradead.org>
> CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> CC: Jiri Slaby <jslaby@suse.cz>
> CC: Santosh Shilimkar <santosh.shilimkar@ti.com>
> ---
>  .../devicetree/bindings/serial/of-serial.txt       |    1 +
>  drivers/tty/serial/8250/8250_core.c                |    6 ++++--
>  drivers/tty/serial/of_serial.c                     |    4 ++++
>  drivers/tty/serial/serial_core.c                   |   12 +++++++++---
>  4 files changed, 18 insertions(+), 5 deletions(-)
> 

[...]

> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index b68550d..851707a 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -174,8 +174,12 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
>  			if (tty->termios.c_cflag & CBAUD)
>  				uart_set_mctrl(uport, TIOCM_RTS | TIOCM_DTR);
>  		}
> -
> -		if (tty_port_cts_enabled(port)) {
> +		/*
> +		 * if hw support flow control without software intervention,
> +		 * then skip the below check
> +		 */
> +		if (tty_port_cts_enabled(port) &&
> +		    !(uport->flags & UPF_HARD_FLOW)) {
>  			spin_lock_irq(&uport->lock);
>  			if (!(uport->ops->get_mctrl(uport) & TIOCM_CTS))
>  				tty->hw_stopped = 1;
> @@ -2772,7 +2776,9 @@ void uart_handle_cts_change(struct uart_port *uport, unsigned int status)
>  
>  	uport->icount.cts++;
>  
> -	if (tty_port_cts_enabled(port)) {
> +	/* skip below code if the hw flow control is supported */
> +	if (tty_port_cts_enabled(port) &&
> +	    !(uport->flags & UPF_HARD_FLOW)) {

Why is a modem status interrupt being generated for DCTS
if autoflow control is enabled?

This should be:

	WARN_ON_ONCE(uport->flags & UPF_HARD_FLOW);

to indicate a mis-configured driver/device.

>  		if (tty->hw_stopped) {
>  			if (status) {
>  				tty->hw_stopped = 0;
>