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

[Petr Mladek <pmladek@suse.com>]

On Sun 2025-01-05 01:32:00, John Ogness wrote:
> 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.

This is a misunderstanding. I am sorry I was not clear enough.

To be more clear. By the nested context I meant

	if (em485) {
		if (em485->tx_stopped)
			up->rs485_start_tx(up);

call by serial8250_console_write(). The original code did:

	+ up->rs485_start_tx()
	  + serial8250_em485_start_tx()
	    + serial8250_stop_rx()

, where serial8250_stop_rx() cleared the flags:

static void serial8250_stop_rx(struct uart_port *port)
{
[...]
	up->ier &= ~(UART_IER_RLSI | UART_IER_RDI);
	serial_port_out(port, UART_IER, up->ier);
[...]
}

But the flags were already cleared by:

	__serial8250_clear_IER(up);

called by serial8250_console_write() _earlier_. Which did:

static void __serial8250_clear_IER(struct uart_8250_port *up)
{
	if (up->capabilities & UART_CAP_UUE)
		serial_out(up, UART_IER, UART_IER_UUE);
	else
		serial_out(up, UART_IER, 0);

}


It means that the nested serial8250_stop_rx() did not change anything.
It was a NOP. The bits were already cleared.

Similar, the counter part. The bits were later set by

	up->rs485_stop_tx(up)

which did:

	+ serial8250_em485_stop_tx

void serial8250_em485_stop_tx(struct uart_8250_port *p, bool toggle_ier)
{
[...]
	p->ier |= UART_IER_RLSI | UART_IER_RDI;
	serial_port_out(&p->port, UART_IER, p->ier);
[...]
}

But it was after the writing the message so that it did not affect
the operation.

> 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.

I think that you do not need to investigate it further. We should keep
IER cleared when writing the message.


> > 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.

Yup, I am fine with the patch as it is.

Best Regards,
Petr