Re: [PATCH 3/3] printk: implement support for extended console drivers

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 29 Apr 2015 10:45:04 -0400 Tejun Heo <tj@kernel.org> wrote:

> printk log_buf keeps various metadata for each message including its
> sequence number and timestamp.  The metadata is currently available
> only through /dev/kmsg and stripped out before passed onto console
> drivers.  We want this metadata to be available to console drivers too
> so that console consumers can get full information including the
> metadata and dictionary, which among other things can be used to
> detect whether messages got lost in transit.
> 
> This patch implements support for extended console drivers.  Consoles
> can indicate that they want extended messages by setting the new
> CON_EXTENDED flag and they'll be fed messages formatted the same way
> as /dev/kmsg.
> 
>  "<level>,<sequnum>,<timestamp>,<contflag>;<message text>\n"
> 
> If extended consoles exist, in-kernel fragment assembly is disabled.
> This ensures that all messages emitted to consoles have full metadata
> including sequence number.  The contflag carries enough information to
> reassemble the fragments from the reader side trivially.  Note that
> this only affects /dev/kmsg.  Regular console and /proc/kmsg outputs
> are not affected by this change.
> 
> * Extended message formatting for console drivers is enabled iff there
>   are registered extended consoles.
> 
> * Comment describing /dev/kmsg message format updated to add missing
>   contflag field and help distinguishing variable from verbatim terms.

So if I'm understanding this correctly, the /dev/kmsg output is altered
(different cont handling) if some console registers with CON_EXTENDED
(and there are no such consoles yet, so the patch is a no-op).

If correct, this seems undesirable - registration of a CON_EXTENDED
console collaterally damages the /dev/kmsg interface?  If the user has
an app which depends on the original /dev/kmsg format then they'll be
wondered what-the-heck-just-happened?