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

[Dann Frazier]

On Thu, May 1, 2014 at 1:04 PM, Murali Karicheri <m-karicheri2@xxxxxx> 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.

ubuntu@hwflow:~$ sudo stty -a --file /dev/ttyS0 |tr ' ' '\n' | grep crtscts
crtscts
ubuntu@hwflow:~$ ls /proc/device-tree/soc/serial@1c021000/has-hw-flow-control
/proc/device-tree/soc/serial@1c021000/has-hw-flow-control
ubuntu@hwflow:~$ python
Python 2.7.6 (default, Mar 22 2014, 22:58:30)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> UPF_HARDWARE_FLOW = 1 << 21
>>> if 0xB9200000 & UPF_HARDWARE_FLOW:
...     print "OK"
...
OK

Hope that's a reasonable test case. Test fails when booted w/o
has-hw-flow-control attribute.

Tested-by: dann frazier <dann.frazier@xxxxxxxxxxxxx>

>
> Signed-off-by: Murali Karicheri <m-karicheri2@xxxxxx>
>
> CC: Rob Herring <robh+dt@xxxxxxxxxx>
> CC: Pawel Moll <pawel.moll@xxxxxxx>
> CC: Mark Rutland <mark.rutland@xxxxxxx>
> CC: Ian Campbell <ijc+devicetree@xxxxxxxxxxxxxx>
> CC: Kumar Gala <galak@xxxxxxxxxxxxxx>
> CC: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> CC: Jiri Slaby <jslaby@xxxxxxx>
> CC: Santosh Shilimkar <santosh.shilimkar@xxxxxx>
> ---
>  .../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/Documentation/devicetree/bindings/serial/of-serial.txt b/Documentation/devicetree/bindings/serial/of-serial.txt
> index 1928a3e..7705477 100644
> --- a/Documentation/devicetree/bindings/serial/of-serial.txt
> +++ b/Documentation/devicetree/bindings/serial/of-serial.txt
> @@ -37,6 +37,7 @@ Optional properties:
>  - auto-flow-control: one way to enable automatic flow control support. The
>    driver is allowed to detect support for the capability even without this
>    property.
> +- has-hw-flow-control: the hardware has flow control capability.
>
>  Example:
>
> diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
> index 0e1bf88..b69aff2 100644
> --- a/drivers/tty/serial/8250/8250_core.c
> +++ b/drivers/tty/serial/8250/8250_core.c
> @@ -2338,9 +2338,11 @@ serial8250_do_set_termios(struct uart_port *port, struct ktermios *termios,
>          * the trigger, or the MCR RTS bit is cleared.  In the case where
>          * the remote UART is not using CTS auto flow control, we must
>          * have sufficient FIFO entries for the latency of the remote
> -        * UART to respond.  IOW, at least 32 bytes of FIFO.
> +        * UART to respond.  IOW, at least 32 bytes of FIFO. Also enable
> +        * AFE if hw flow control is supported
>          */
> -       if (up->capabilities & UART_CAP_AFE && port->fifosize >= 32) {
> +       if ((up->capabilities & UART_CAP_AFE && (port->fifosize >= 32)) ||
> +           (port->flags & UPF_HARD_FLOW)) {
>                 up->mcr &= ~UART_MCR_AFE;
>                 if (termios->c_cflag & CRTSCTS)
>                         up->mcr |= UART_MCR_AFE;
> diff --git a/drivers/tty/serial/of_serial.c b/drivers/tty/serial/of_serial.c
> index 9924660..77ec6a1 100644
> --- a/drivers/tty/serial/of_serial.c
> +++ b/drivers/tty/serial/of_serial.c
> @@ -182,6 +182,10 @@ static int of_platform_serial_probe(struct platform_device *ofdev)
>                                           "auto-flow-control"))
>                         port8250.capabilities |= UART_CAP_AFE;
>
> +               if (of_property_read_bool(ofdev->dev.of_node,
> +                                         "has-hw-flow-control"))
> +                       port8250.port.flags |= UPF_HARD_FLOW;
> +
>                 ret = serial8250_register_8250_port(&port8250);
>                 break;
>         }
> 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)) {
>                 if (tty->hw_stopped) {
>                         if (status) {
>                                 tty->hw_stopped = 0;
> --
> 1.7.9.5
>
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