Re: [PATCH tty v9 1/2] serial: 8250: Switch to nbcon console, take 2

[Petr Mladek <pmladek@suse.com>]

On Tue 2026-08-18 09:14:51, Andy Shevchenko wrote:
> On Tue, Aug 18, 2026 at 06:32:12AM +0100, Jon Hunter wrote:
> > On 24/07/2026 11:36, John Ogness wrote:
> > > Implement the necessary callbacks to switch the 8250 console driver
> > > to perform as an nbcon console.
> > > 
> > > Add implementations for the nbcon console callbacks:
> > > 
> > >    ->write_atomic()
> > >    ->write_thread()
> > >    ->device_lock()
> > >    ->device_unlock()
> > > 
> > > and add CON_NBCON to the initial @flags.
> > > 
> > > All hardware access in the callbacks is within unsafe sections.
> > > The ->write_atomic() and ->write_thread() callbacks allow safe
> > > handover/takeover per byte and add a preceding newline if they
> > > take over from another context mid-line.
> > > 
> > > For the ->write_atomic() callback, a new irq_work is used to defer
> > > modem control since it may be called from a context that does not
> > > allow waking up tasks. During suspend/resume the irq_work is not
> > > used as this has been shown to cause suspend problems for some
> > > hardware. Upon resume, any pending modem control is performed.
> > > 
> > > Note: A new __serial8250_clear_IER() is introduced for direct
> > > clearing of UART_IER during console writing (which will not be
> > > holding the port lock for atomic printing or KDB/KGDB). This
> > > allows restoring a lockdep check to serial8250_clear_IER() in
> > > a follow-up commit.
> > 
> > This change is causing a boot regression for our Tegra20 and Tegra30
> > platforms. Reverting this on top of -next fixes the issue. Previously with
> > V5 I did not see a boot issue only an issue in suspend.
> 
> Wasn't v11 applied and not v9?
> 
> > So far the only
> > interesting thing I have observed is that if I add 'keep_bootcon' to the
> > command line the board does boot.
> 
> keep_bootcon defers driver taking over, that's why it works, it uses just
> simple primitives instead of full-featured driver.

It is the other way. The full-featured driver is used even with
keep_bootcon. The option causes that the simple boot driver stays
registered in parallel. You probably see duplicated lines because
of this.

Anyway, the most important effect of "keep_bootcon" is that
boot consoles prevent using the printk kthreads. Even the nbcon
consoles need to be handled in the legacy loop.

There reason is a combination of two things:

  1. Boot consoles are synchronized only using the legacy
     console_lock (console_sem). port->lock does not exist
     at boot time.

  2. There is no simple way how to match boot console drivers
     with the full-featured drivers. They use a different interface
     to access the same HW.

As a result, even the full featured console drivers have to rely
on the legacy console_lock as long as any boot console is used.

The regression seems to be when the uart 8250 port become handled
by the printk kthread.

Best Regards,
Petr

PS: I am not sure whether to continue the discussion here.
    It might be better to move it to v11. see
    https://lore.kernel.org/all/20260729120439.281252-1-john.ogness@linutronix.de/
    I am going to reply there as well.