dmesg : no output on 1 of 2 7-stable boxes

[Julian H. Stacey]

Oliver Fromme wrote:
> Hello Julian,
> 
> I'm sorry this is a late reply, but I noticed your post
> on the freebsd-stable list just now.
>  > One of 2 laptops running 7-stable shows nothing with dmesg, (other is OK).
> Did you try "dmesg -a"?

Thanks Oliver,
Yes, but after posting I think, (Been a while can't remember),
Symptom stopped after "cd /usr/local/lib; mv wine wine.MV" (a bad port !).
It was filling console buf with junk.

> The dmesg buffer is a circular buffer containing both
> kernel output and console output.  However, "dmesg"
> displays only the kernel output.  If there was lots of
> console output, it filled all of the dmesg buffer,
> so "dmesg" displays nothing (all of the kernel output
> was overwritten by console output).  "dmesg -a" will
> display everything, i.e. kernel + console output.

Some of that description could usefuly be added to `man dmesg` ?
Might some commiter wnat to hack a few words in ?  Or do we neeed
a formal send-pr (that a commiter would rephrase likely anyway, so
best skip the send-pr ? ).

> If "dmesg -a" doesn't print anything either, I'm afraid
> I have no idea what might be wrong.  Well, you could try
> "sysctl -b kern.msgbuf" which will retrieve the raw
> contents of the dmesg buffer.
> 
>  >    - I tried loader.conf        kern.msgbuf=64000
> 
> I think it must be a multiple of the pages size, i,e,
> 4K = 4096 on FreeBSD/i386.  I usually set it to 65536
> or 131072.

Ah! Certainly plausible re. 2 base N.
On 7.0PRERELEASE I see
	kern.msgbuf= sundry text diagnostsics
so seems inappropriate to set that var to a number.  I see
	kern.consmsgbuf_size: 8192
so tried in /boot/loader.conf
	kern.consmsgbuf_size=4096
but even reducing size failed to vary size (enlarging had earlier failed) 
Some clue in sys/kern/tty_cons.c:666
I See 
	kern.msgbuf_clear: Clear kernel message buffer
	kern.msgbuf: Contents of kernel message buffer
Puzzled on syntax to set size in loader.conf ?
-- 
Julian Stacey.  BSD Unix Linux Net Consultant, Munich.  http://berklix.com