linux-kernel - Re: [PATCH] serial: mps2-uart: Initialize early console

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Date:   Tue, 19 Jun 2018 10:07:01 +0100
From:   Vladimir Murzin <vladimir.murzin@....com>
To:     Guenter Roeck <linux@...ck-us.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>, linux-serial@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Liviu Dudau <liviu.dudau@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH] serial: mps2-uart: Initialize early console

Hi Guenter,

On 19/06/18 05:54, Guenter Roeck wrote:
> The early console code for mps2-uart assumes that the serial hardware is
> enabled for transmit when the system boots. However, this is not the case
> after reset. This results in a hang in mps2_early_putchar() if the serial
> transmitter is not enabled by a boot loader or ROM monitor.

I was under impression that for earlycon there is an assumption/requirement
that the serial port must already be setup and configured. For instance, I 
see such requirement for pl011. So it looks like boot code's fault not to
enable serial (for mps2 it needs to setup BAUDDIV as well).

I'm not against the patch per se, but I'd like to hear if my understanding of
earlycon requirements is correct or not.

Cheers
Vladimir

> 
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
>  drivers/tty/serial/mps2-uart.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/tty/serial/mps2-uart.c b/drivers/tty/serial/mps2-uart.c
> index 9f8f63719126..0743a0551ce1 100644
> --- a/drivers/tty/serial/mps2-uart.c
> +++ b/drivers/tty/serial/mps2-uart.c
> @@ -448,6 +448,14 @@ static struct console mps2_uart_console = {
>  
>  #define MPS2_SERIAL_CONSOLE (&mps2_uart_console)
>  
> +static void mps2_early_init(struct uart_port *port)
> +{
> +	u8 control = readb(port->membase + UARTn_CTRL);
> +
> +	control |= UARTn_CTRL_TX_ENABLE;
> +	writeb(control, port->membase + UARTn_CTRL);
> +}
> +
>  static void mps2_early_putchar(struct uart_port *port, int ch)
>  {
>  	while (readb(port->membase + UARTn_STATE) & UARTn_STATE_TX_FULL)
> @@ -469,6 +477,7 @@ static int __init mps2_early_console_setup(struct earlycon_device *device,
>  	if (!device->port.membase)
>  		return -ENODEV;
>  
> +	mps2_early_init(&device->port);
>  	device->con->write = mps2_early_write;
>  
>  	return 0;
>