linux-kernel - Re: [PATCH v2 1/2] serial: 8250: Add flag so drivers can avoid THRE probe

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Date:   Wed, 5 Apr 2017 13:36:24 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Joel Stanley <joel@....id.au>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Mark Rutland <mark.rutland@....com>,
        Rob Herring <robh+dt@...nel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Jeremy Kerr <jk@...abs.org>, openbmc@...ts.ozlabs.org
Subject: Re: [PATCH v2 1/2] serial: 8250: Add flag so drivers can avoid THRE probe

On Wed, Apr 5, 2017 at 7:03 AM, Joel Stanley <joel@....id.au> wrote:
> The probing of THRE irq behaviour assumes the other end will be reading
> bytes out of the buffer in order to probe the port at driver init. In
> some cases the other end cannot be relied upon to read these bytes, so
> provide a flag for them to skip this step.
>

> Bit 16 was chosen as the flags are a int and the top bits are taken.

19 ?

>
> Acked-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Signed-off-by: Joel Stanley <joel@....id.au>
> ---
>  drivers/tty/serial/8250/8250_port.c | 2 +-
>  include/linux/serial_core.h         | 1 +
>  2 files changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 6119516ef5fc..60a6c247340f 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -2229,7 +2229,7 @@ int serial8250_do_startup(struct uart_port *port)
>                 }
>         }
>
> -       if (port->irq) {
> +       if (port->irq && !(up->port.flags & UPF_NO_THRE_TEST)) {
>                 unsigned char iir1;
>                 /*
>                  * Test for UARTs that do not reassert THRE when the
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 58484fb35cc8..260245deec94 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -195,6 +195,7 @@ struct uart_port {
>  #define UPF_NO_TXEN_TEST       ((__force upf_t) (1 << 15))
>  #define UPF_MAGIC_MULTIPLIER   ((__force upf_t) ASYNC_MAGIC_MULTIPLIER /* 16 */ )
>
> +#define UPF_NO_THRE_TEST       ((__force upf_t) (1 << 19))
>  /* Port has hardware-assisted h/w flow control */
>  #define UPF_AUTO_CTS           ((__force upf_t) (1 << 20))
>  #define UPF_AUTO_RTS           ((__force upf_t) (1 << 21))
> --
> 2.11.0
>



-- 
With Best Regards,
Andy Shevchenko