Re: [PATCH next v1 1/2] serial: 8250: Switch to nbcon console

[John Ogness <john.ogness@linutronix.de>]

On 2024-09-06, John Ogness <john.ogness@linutronix.de> wrote:
>> Wait! This makes the rs485 consoles much less usable for debugging.
>> They might have troubles to see the emergency and panic messages.
>>
>> Is this acceptable? Why?
>
> It is not acceptable. I am looking into making the atomic part work for
> RS485 as well.

So there are 2 things _not_ supported by the write_atomic() callback:

1. RS485 mode. This is due to the need to start up TX for the
write, which can lead to:

up->rs485_start_tx()
  serial8250_em485_start_tx()
    serial8250_stop_rx()
      serial8250_rpm_get()
        pm_runtime_get_sync()
          __pm_runtime_resume()
            spin_lock_irqsave()

Taking a spin lock is not safe from NMI and thus disqualifies this call
chain for write_atomic().

If UART_CAP_RPM is not set, the pm_runtime_get_sync() is avoided. So I
could only disable atomic RS485 if UART_CAP_RPM is set. But the OMAP
variant of the 8250 does set this capability.

2. Modem control. This is due to waiting for inputs, which can lead to:

serial8250_modem_status()
  wake_up_interruptible()

Performing wakes is not safe from scheduler or NMI and thus disqualifies
this call chain for write_atomic().

It would probably be acceptable to move serial8250_modem_status() into
an irq_work.

I would be grateful for any insights on how best to handle these 2
issues if we want full write_atomic() support for all 8250 variants.

John