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

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Message-ID: <20170616020054.GB465@jagdpanzerIV.localdomain>
Date:   Fri, 16 Jun 2017 11:00:54 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        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 (06/15/17 16:54), Petr Mladek wrote:
> 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.

yeah, I grep-ed and didn't find anything that would depend
on CON_CONSDEV flag (except for printk), we check ->device
in critical places.

> 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.

good to know that, thanks.  s/CON_FIRST/CON_LAST/g  :)

> 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.

yep. seems so.

	-ss