Re: [PATCH v9 3/3] printk: fix double printing with earlycon

[Aleksey Makarov]

On 05/12/2017 04:46 PM, Petr Mladek wrote:
> On Fri 2017-05-12 14:57:29, Petr Mladek wrote:
> > On Thu 2017-05-11 17:41:58, Sergey Senozhatsky wrote:
> > > On (05/11/17 17:24), Sergey Senozhatsky wrote:
> > > > On (05/09/17 10:29), Sabrina Dubroca wrote:
> > > > [..]
> > > > > That's caused a change of behavior in my qemu setup, with this cmdline
> > > > >
> > > > >      root=/dev/sda1 console=ttyS1 console=ttyS0
> > > > >
> > > > > Before, the kernel logs appeared on ttyS1, and I logged in with ttyS0
> > > > > (with my setup, ttyS1 is a file and ttyS0 is unix socket). Now, the
> > > > > kernel logs go to ttyS0. I need to swap the two console= parameters to
> > > > > restore behavior.
>
> Do you actually need to define console=ttyS0 on the cmdline?

Exactly.  You should specify a console on the command line only
if you want kernel logs on it.  It's kernel bug that one of this
consoles does not receive kernel logs, see
Documentation/admin-guide/serial-console.rst

> IMHO, if register_console() was called for the unix socket, it
> would make logs appear on both ttyS0 and ttyS1. It seems
> that register_console() is called only for the console that
> stores logs into the file.

It's not quite accurate sentence.  It's qemu who deals with file/socket and
it is transparent to kernel.

> > > > > adding
> > > > > console=tty0 anywhere on that cmdline makes the logs appear on both
> > > > > tty0 and one ttyS* (but only one of them, and the ordering of the
> > > > > ttyS* matters).
>
> I guess that it worked this way before. I mean that the logs appeared
> on both tty0 and one of ttyS*. The only difference should be that
> the patch changed the selected ttyS*. So this is still the same problem.
>
>
> > Hmm, I have no idea how to fix this. This is the case where
> > a registered console matches more entries from the command line.
> > The fix that caused this regression fixed exactly this situation
> > and we wanted to make the preferred console first.
> >
> > I am not sure if we broke some backward compatibility or actually made
> > it more predictable in the long term.
>
> I think that we actually fixed a very old bug. The last mentioned
> console= should be the preferred one and the logs are finally
> printed there. Or do I miss anything?

Last mentioned 'console=' (preferred console) is the console that
should become /dev/console.  Its driver is returned by console_device().
In other respects the last mentioned console is not special,
so I believe it is irrelevant to the report.

Thank you
Aleksey Makarov