linux-kernel - Re: [PATCH] serial: sprd: Add polling IO support

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Message-ID: <CAMz4kuLrAWDaZj2Am452BX+aVO0yCQB_cCiNUx4FAamoX6KWtA@mail.gmail.com>
Date:   Wed, 18 Sep 2019 20:10:14 +0800
From:   Baolin Wang <baolin.wang@...aro.org>
To:     Lanqing Liu <liuhhome@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>, Jiri Slaby <jslaby@...e.com>,
        lanqing.liu@...soc.com, linux-serial@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Orson Zhai <orsonzhai@...il.com>,
        Chunyan Zhang <zhang.lyra@...il.com>
Subject: Re: [PATCH] serial: sprd: Add polling IO support

Hi Lanqing,

On Wed, 18 Sep 2019 at 16:16, Lanqing Liu <liuhhome@...il.com> wrote:
>
> In order to access the UART without the interrupts, the kernel uses
> the basic polling methods for IO with the device. With these methods
> implemented, it is now possible to enable kgdb during early boot over serial.
>
> Signed-off-by: Lanqing Liu <liuhhome@...il.com>
> ---
>  drivers/tty/serial/sprd_serial.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/drivers/tty/serial/sprd_serial.c b/drivers/tty/serial/sprd_serial.c
> index 73d71a4..579ab41 100644
> --- a/drivers/tty/serial/sprd_serial.c
> +++ b/drivers/tty/serial/sprd_serial.c
> @@ -911,6 +911,24 @@ static void sprd_pm(struct uart_port *port, unsigned int state,
>         }
>  }
>
> +#ifdef CONFIG_CONSOLE_POLL
> +static int sprd_poll_get_char(struct uart_port *port)
> +{
> +       while (!(serial_in(port, SPRD_STS1) & SPRD_RX_FIFO_CNT_MASK))
> +               cpu_relax();
> +
> +       return serial_in(port, SPRD_RXD);
> +}
> +
> +static void sprd_poll_put_char(struct uart_port *port, unsigned char ch)
> +{
> +       while (serial_in(port, SPRD_STS1) & SPRD_TX_FIFO_CNT_MASK)
> +               cpu_relax();
> +
> +       serial_out(port, SPRD_TXD, ch);
> +}
> +#endif

When I tested your patch, I found only one case can work if the port
used by KGDB is same with the port selected as console, which means
this port will be powered on all the time. We had implemented the
power management for the UART ports, so I think you should enable the
clock for the port used by KGDB in poll_init(), then other ports can
be used by KGDB.

> +
>  static const struct uart_ops serial_sprd_ops = {
>         .tx_empty = sprd_tx_empty,
>         .get_mctrl = sprd_get_mctrl,
> @@ -928,6 +946,10 @@ static void sprd_pm(struct uart_port *port, unsigned int state,
>         .config_port = sprd_config_port,
>         .verify_port = sprd_verify_port,
>         .pm = sprd_pm,
> +#ifdef CONFIG_CONSOLE_POLL
> +       .poll_get_char  = sprd_poll_get_char,
> +       .poll_put_char  = sprd_poll_put_char,
> +#endif
>  };
>
>  #ifdef CONFIG_SERIAL_SPRD_CONSOLE
> --
> 1.9.1
>


-- 
Baolin Wang
Best Regards