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

[Jon Hunter <jonathanh@nvidia.com>]

On 18/08/2026 07:14, 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?

Ah may be I replied to the wrong version. However, I am testing 
next-20260817. So whatever is in -next.
>> 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.

Yes that makes sense then.
Jon

-- 
nvpublic