Hide boot on-screen errors, or get TV-out working on ATI Radeon X300

[Tom H]

On Mon, Dec 8, 2014 at 2:45 AM, Marius Gedminas <marius at pov.lt> wrote:
> On Sun, Dec 07, 2014 at 02:33:47PM -0500, John Hupp wrote:
>>
>> I created /etc/sysctl.d/20-quiet-printk.conf with content:
>> kernel.printk = 3 3 3 3
>> ...
>> Though I was curious to know a bit more about the mechanisms in
>> play, I didn't dig much further than that.  I wondered, for
>> instance, what behavior the default '4 4 1 7' specified, but never
>> did find out.  The files in sysctl.d are installed by procps (see
>> the List of Files link at
>> http://packages.ubuntu.com/trusty/admin/procps). And these, in turn,
>> are part of the kernel configuration governed by sysctl (see
>> http://manpages.ubuntu.com/manpages/trusty/en/man8/sysctl.8.html).
>>
>> Still looking for the meaning of '4 4 1 7' vs '3 3 3 3' I came to
>> https://www.kernel.org/doc/Documentation/printk-formats.txt, but my
>> eyes glazed over just skimming that, and I stopped there.
>
> This is a bit unobvious, but you can find the description of the
> kernel.printk sysctl setting in
> http://manpages.ubuntu.com/manpages/trusty/en/man5/proc.5.html:
>
>     /proc/sys/kernel/printk
>         The four values in this file are console_loglevel,  default_mes‐
>         sage_loglevel,     minimum_console_level,    and    default_con‐
>         sole_loglevel.  These values influence  printk()  behavior  when
>         printing or logging error messages.  See syslog(2) for more info
>         on the different loglevels.  Messages  with  a  higher  priority
>         than  console_loglevel will be printed to the console.  Messages
>         without an explicit  priority  will  be  printed  with  priority
>         default_message_level.   minimum_console_loglevel is the minimum
>         (highest)  value  to  which   console_loglevel   can   be   set.
>         default_console_loglevel   is   the   default   value  for  con‐
>         sole_loglevel.
>
> (/proc/sys is one of the possible ways of changing systcl settings,
> which I guess explain why this is documented in proc(5).)

You can find the same explanation of the four _loglevels in
https://www.kernel.org/doc/Documentation/sysctl/kernel.txt
in a more readable format (to my eyes).

As mentioned in "man 5 proc", you'll find the explanation of the
priorities in "man 2 syslog"; you can also find it in
"/usr/include/x86_64-linux-gnu/.../syslog.h" (on x86_64/amd64).

"man 2 syslog" on 15.04 also includes the text of the kernel.org
document above (clarified/updated).

If you're not on 15.04, you can get the latest man-pages (manpages in
Ubuntu) from
https://www.kernel.org/pub/linux/docs/man-pages/
untar it, and run "man man-pages-3.75/man2/syslog.2"

"3 4 1 3" is a more standard setting than "3 3 3 3".

It's better to set 4 (WARNING, the vanilla default) rather than 3
(ERR) for messages that don't have a priority, since, if a message
really is an error, it'll (should?) be tagged as such.

By setting minimum_console_loglevel to 3, you're potentially making
printk more verbose on the console.