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Subject: Re: [PATCH-next 3/3] serial/sysrq: Add MAGIC_SYSRQ_SERIAL_SEQUENCE
From:   Joe Perches <joe@perches.com>
To:     Dmitry Safonov <dima@arista.com>, linux-kernel@vger.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@gmail.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>,
        Vasiliy Khoruzhick <vasilykh@arista.com>,
        linux-serial@vger.kernel.org
Date:   Fri, 10 Jan 2020 08:50:03 -0800
In-Reply-To: <20200109215444.95995-4-dima@arista.com>
References: <20200109215444.95995-1-dima@arista.com>
         <20200109215444.95995-4-dima@arista.com>
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On Thu, 2020-01-09 at 21:54 +0000, Dmitry Safonov wrote:
> Many embedded boards have a disconnected TTL level serial which can
> generate some garbage that can lead to spurious false sysrq detects.

trivia:

> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
[]
> @@ -3081,6 +3081,38 @@ void uart_insert_char(struct uart_port *port, unsigned int status,
[]
> +const char sysrq_toggle_seq[] = CONFIG_MAGIC_SYSRQ_SERIAL_SEQUENCE;

static const?

> +static int uart_try_toggle_sysrq(struct uart_port *port, unsigned int ch)

This function return might read better as bool not int

> +{
> +	if (sysrq_toggle_seq[0] == '\0')
> +		return 0;
> +
> +	BUILD_BUG_ON(ARRAY_SIZE(sysrq_toggle_seq) >= sizeof(port->sysrq_seq)*U8_MAX);
> +	if (sysrq_toggle_seq[port->sysrq_seq] != ch) {
> +		port->sysrq_seq = 0;
> +		return 0;
> +	}
> +
> +	/* Without the last \0 */
> +	if (++port->sysrq_seq < (ARRAY_SIZE(sysrq_toggle_seq) - 1)) {
> +		port->sysrq = jiffies + HZ*5;
> +		return 1;
> +	}
> +
> +	schedule_work(&sysrq_enable_work);
> +
> +	port->sysrq = 0;
> +	return 1;
> +}
[]
> diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
> index 255e86a474e9..1f4443db5474 100644
> --- a/include/linux/serial_core.h
> +++ b/include/linux/serial_core.h
> @@ -243,10 +243,10 @@ struct uart_port {
>  	unsigned long		sysrq;			/* sysrq timeout */
>  	unsigned int		sysrq_ch;		/* char for sysrq */
>  	unsigned char		has_sysrq;
> +	unsigned char		sysrq_seq;		/* index in sysrq_toggle_seq */

unsigned int?

Or maybe set a maximum length of MAGIC_SYSRQ_SERIAL_SEQUENCE.


>  
>  	unsigned char		hub6;			/* this should be in the 8250 driver */
>  	unsigned char		suspended;
> -	unsigned char		unused;
>  	const char		*name;			/* port name */
>  	struct attribute_group	*attr_group;		/* port specific attributes */
>  	const struct attribute_group **tty_groups;	/* all attributes (serial core use only) */
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 330909c600b1..f6f3e82fee25 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -431,6 +431,14 @@ config MAGIC_SYSRQ_SERIAL
>  	  This option allows you to decide whether you want to enable the
>  	  magic SysRq key.
>  
> +config MAGIC_SYSRQ_SERIAL_SEQUENCE
> +	string "Char sequence that enables magic SysRq over serial"
> +	depends on MAGIC_SYSRQ_SERIAL
> +	default ""
> +	help
> +	  Specifies a sequence of characters that can follow BREAK to enable
> +	  SysRq on a serial console.
> +
>  config DEBUG_FS
>  	bool "Debug Filesystem"
>  	help