linux-kernel - Re: [PATCH 3/3] printk/console: Always have a preferred console

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Date:   Thu, 15 Jun 2017 16:54:43 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Aleksey Makarov <aleksey.makarov@...aro.org>,
        Sabrina Dubroca <sd@...asysnail.net>,
        Sudeep Holla <sudeep.holla@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] printk/console: Always have a preferred console

On Wed 2017-06-14 18:11:28, Sergey Senozhatsky wrote:
> On (06/13/17 14:54), Petr Mladek wrote:
> 
> > This patch modifies the code that enables the configured consoles.
> > It sets the CON_CONSDEV flag also when we register the first
> > console. It causes that one of the registered consoles will
> > always have CON_CONSDEV flag set.
> 
> hm.... my impression was that we shouldn't set CON_CONSDEV if the
> console has no ->device. but then, once again, unregister_console()
> does not care and register_console() cares only in one place. so I'm
> a bit in doubt.

Great catch! It helped me to get even better picture.

If I get it correctly, the console driver for /dev/console is
found in tty_lookup_driver() by console_device(). Where
console_device() skips console drivers without any device.

CON_CONSDEV is basically handled only in register_console()
and unregister_console(). The driver with this flag is kept at
the beginning of the console_drivers list. It causes that
console_device() will return the console with this flag.

Also it is in sync with the commit cd3a1b8562d28490b33 ("printk: don't
prefer unsuited consoles on registration"). It added the check
for existing device into the fallback code in console_register()
so that console_device() might find a valid one.

BTW: The flag CON_CONSDEV first appeared in Linux 2.1.92.
It replaced CON_FIRST. Documentation/serial-console.txt
was modified so that the last console on the command line
will be used for /dev/console instead of the first one.

In each case, it seems that CON_CONSDEV is closely related
to an existing device and does not make much sense without it.

Heh, the handling in unregister_console() looks buggy. Well,
it probably does not break anything. The function does not change
the order of drivers and console_device() searches
the entire list until it finds a console with device.
Anyway, it would make sense to clean it as well.

Thanks a lot for review.

Best Regards,
Petr