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

[Jon Hunter <jonathanh@nvidia.com>]

Hi John,

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.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>


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. So far the 
only interesting thing I have observed is that if I add 'keep_bootcon' 
to the command line the board does boot.

Jon

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