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printk: implement support for extended console drivers

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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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Miller <davem@davemloft.net>
Cc: Kay Sievers <kay@vrfy.org>
Reviewed-by: Petr Mladek <pmladek@suse.cz>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Tejun Heo
2015-06-25 15:01:30 -07:00
committed by Linus Torvalds
parent 0a295e67ec
commit 6fe29354be
3 changed files with 68 additions and 8 deletions
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9
Documentation/ABI/testing/dev-kmsg
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@@ -98,4 +98,13 @@ Description: The /dev/kmsg character device node provides userspace access
logic is used internally when messages are printed to the
console, /proc/kmsg or the syslog() syscall.
By default, kernel tries to avoid fragments by concatenating
when it can and fragments are rare; however, when extended
console support is enabled, the in-kernel concatenation is
disabled and /dev/kmsg output will contain more fragments. If
the log consumer performs concatenation, the end result
should be the same. In the future, the in-kernel concatenation
may be removed entirely and /dev/kmsg users are recommended to
implement fragment handling.
Users: dmesg(1), userspace kernel log consumers