Re: [PATCH tty-next v4 4/6] serial: 8250: Provide flag for IER toggling for RS485

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

On 2025-01-03, Petr Mladek <pmladek@suse.com> wrote:
> My understanding is that the nested IER manipulation does not
> harm.

This statement implies that it is OK for UART_IER_RLSI|UART_IER_RDI bits
to be set in UART_IER even though the console will write to UART_TX,
because the _nesting_ contexts would set those bits rather than
restoring the original value of 0x0.

I ran some tests and leaving these bits set during Tx does not appear to
cause an issue, but it is difficult to say because the context
interrupted by a nesting context will only print at most 1
character. Also, it is writing under spin_lock_irqsave() so that might
be masking any effects. Perhaps UART_IER is temporarly cleared because
of other bits that would cause problems during Tx?

I would need to create a specific test to investigate this
further. Regardless, it still is a cosmetic ugliness that bits are not
being properly restored, even if it turns out these particular bits are
not problematic during Tx.

As was with your LSR_DR comment, you are good at triggering fundamental
investigations into the history of the 8250 driver. ;-)

> All in all, the patch looks good to me.
>
> Reviewed-by: Petr Mladek <pmladek@suse.com>

Thanks for the review. I interpret it to mean that I do not need to make
any changes to this patch for v5.

John