linux-kernel - Re: [PATCH printk v2 2/8] printk: Provide debug

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Date:   Fri, 28 Jul 2023 17:22:45 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk v2 2/8] printk: Provide debug_store() for nbcon
 debugging

On Fri 2023-07-28 02:08:27, John Ogness wrote:
> To debug and validate nbcon ownership transitions, provide a new
> macro debug_store() (enabled with DEBUG_NBCON) to log transition
> information to the ringbuffer. If enabled, this macro only logs
> to the ringbuffer and does not trigger any printing. This is to
> avoid triggering recursive printing in the nbcon consoles.
> 
> --- a/kernel/printk/printk_nbcon.c
> +++ b/kernel/printk/printk_nbcon.c
> @@ -10,6 +10,35 @@
>   * the legacy style console_lock mechanism.
>   */
>  
> +/*
> + * Define DEBUG_NBCON to allow for nbcon ownership transitions to be logged
> + * to the ringbuffer. The debug_store() macro only logs to the lockless
> + * ringbuffer and does not trigger any printing.
> + */
> +#undef DEBUG_NBCON
> +
> +#ifdef DEBUG_NBCON
> +/* Only write to ringbuffer. */
> +int __debug_store(const char *fmt, ...)
> +{
> +	va_list args;
> +	int r;
> +
> +	va_start(args, fmt);
> +	r = vprintk_store(2, 7, NULL, fmt, args);

I have never used the facility number before. It seems that they are
defined in /usr/include/sys/syslog.h, for example, see
https://sites.uclouvain.be/SystInfo/usr/include/sys/syslog.h.html

They are even somehow documented in "man 3 syslog", for example, see
https://linux.die.net/man/3/syslog

We should probably use one of the LOG_LOCALX numbers, e.g.

#define        LOG_LOCAL0        (16<<3)

Also, please use LOGLEVEL_DEBUG instead of the hard coded "7".

> +	va_end(args);
> +
> +	return r;
> +}
> +#define debug_store(cond, fmt, ...)						\
> +	do {									\
> +		if (cond)							\
> +			__debug_store(pr_fmt("DEBUG: " fmt), ##__VA_ARGS__)	\
> +	} while (0)
> +#else
> +#define debug_store(cond, fmt, ...)
> +#endif
> +
>  /**
>   * nbcon_state_set - Helper function to set the console state
>   * @con:	Console to update

Best Regards,
Petr