linux-kernel - Re: console output duplicated when registering additional consoles

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Date:   Tue, 19 Nov 2019 10:36:59 -0800
From:   Jonathan Richardson <jonathan.richardson@...adcom.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        gregkh@...uxfoundation.org, jslaby@...e.com,
        sergey.senozhatsky@...il.com, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Scott Branden <scott.branden@...adcom.com>,
        Ray Jui <ray.jui@...adcom.com>,
        Srinath Mannam <srinath.mannam@...adcom.com>
Subject: Re: console output duplicated when registering additional consoles

On Tue, Nov 19, 2019 at 3:30 AM Petr Mladek <pmladek@...e.com> wrote:
>
> On Mon 2019-11-18 13:38:04, Jonathan Richardson wrote:
> > On Thu, Nov 14, 2019 at 8:33 PM Sergey Senozhatsky
> > <sergey.senozhatsky.work@...il.com> wrote:
> > >
> > > Gosh, that part of printk is really complex.
> > >
> > > On (19/11/14 10:57), Petr Mladek wrote:
> > > > For a proper solution we would need to match boot and real
> > > > consoles that write messages into the physical device.
> > > > But I am afraid that there is no support for this.
> > >
> > > Wouldn't those have same tty driver?
>
> Interesting idea. Well, do early consoles have tty driver?
>
> > > ---
> > >
> > >  kernel/printk/printk.c | 16 ++++++++++++++++
> > >  1 file changed, 16 insertions(+)
> > >
> > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > > index f1b08015d3fa..a84cb20acf42 100644
> > > --- a/kernel/printk/printk.c
> > > +++ b/kernel/printk/printk.c
> > > @@ -2690,6 +2690,19 @@ static int __init keep_bootcon_setup(char *str)
> > >
> > >  early_param("keep_bootcon", keep_bootcon_setup);
> > >
> > > +static bool known_console_driver(struct console *newcon)
> > > +{
> > > +       struct console *con;
> > > +
> > > +       for_each_console(con) {
> > > +               if (!(con->flags & CON_ENABLED))
> > > +                       continue;
> > > +               if (con->device && con->device == newcon->device)
> > > +                       return true;
> > > +       }
> > > +       return false;
> > > +}
> > > +
> > >  /*
> > >   * The console driver calls this routine during kernel initialization
> > >   * to register the console printing procedure with printk() and to
> > > @@ -2828,6 +2841,9 @@ void register_console(struct console *newcon)
> > >         if (newcon->flags & CON_EXTENDED)
> > >                 nr_ext_console_drivers++;
> > >
> > > +       if (known_console_driver(newcon))
> > > +               newcon->flags &= ~CON_PRINTBUFFER;
> > > +
> > >         if (newcon->flags & CON_PRINTBUFFER) {
> > >                 /*
> > >                  * console_unlock(); will print out the buffered messages
> >
> > Thanks.
>
> Jonathan, have you tried this patch, please?
> Does it solve your problem?

I've tried it and it doesn't solve the problem. newcon is not a known
console at the time I register my second console. The only thing I can
do is not replay the log by not setting CON_PRINTBUFFER and I
shouldn't have to do that after reading Sergey's last response.