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

For RS485 mode, if SER_RS485_RX_DURING_TX is not available, the
console ->write() callback needs to enable/disable Tx. It does
this by calling the ->rs485_start_tx() and ->rs485_stop_tx()
callbacks. However, some of these callbacks also disable/enable
interrupts and makes power management calls. This causes 2
problems for console writing:

1. A console write can occur in contexts that are illegal for
   pm_runtime_*(). It is not even necessary for console writing
   to use pm_runtime_*() because a console already does this in
   serial8250_console_setup() and serial8250_console_exit().

2. The console ->write() callback already handles
   disabling/enabling the interrupts by properly restoring the
   previous IER value.

Add an argument @toggle_ier to the ->rs485_start_tx() and
->rs485_stop_tx() callbacks to specify if they may disable/enable
receive interrupts while using pm_runtime_*(). Console writing
will not allow the toggling.

For all call sites other than console writing there is no
functional change.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 drivers/tty/serial/8250/8250.h            |  4 ++--
 drivers/tty/serial/8250/8250_bcm2835aux.c |  4 ++--
 drivers/tty/serial/8250/8250_omap.c       |  2 +-
 drivers/tty/serial/8250/8250_port.c       | 26 +++++++++++++----------
 include/linux/serial_8250.h               |  4 ++--
 5 files changed, 22 insertions(+), 18 deletions(-)

On Fri 2024-12-27 23:51:20, John Ogness wrote:
> For RS485 mode, if SER_RS485_RX_DURING_TX is not available, the
> console ->write() callback needs to enable/disable Tx. It does
> this by calling the ->rs485_start_tx() and ->rs485_stop_tx()
> callbacks. However, some of these callbacks also disable/enable
> interrupts and makes power management calls. This causes 2
> problems for console writing:
> 
> 1. A console write can occur in contexts that are illegal for
>    pm_runtime_*(). It is not even necessary for console writing
>    to use pm_runtime_*() because a console already does this in
>    serial8250_console_setup() and serial8250_console_exit().
>
> 2. The console ->write() callback already handles
>    disabling/enabling the interrupts by properly restoring the
>    previous IER value.

I was a bit confused by the description of the 2nd problem.
It is not clear whether it actually is a problem.

My understanding is that the nested IER manipulation does not
harm. And it is not needed at the same time. As a result,
the related pr_runtime_*() calls are not needed either.
So this is 2nd reason why the problematic pr_runtime_*() calls
can be removed in the serial8250_console_write() code path.

> Add an argument @toggle_ier to the ->rs485_start_tx() and
> ->rs485_stop_tx() callbacks to specify if they may disable/enable
> receive interrupts while using pm_runtime_*(). Console writing
> will not allow the toggling.
> 
> For all call sites other than console writing there is no
> functional change.
> 
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

It seems that the patch does what is says. I'll try to describe
it using my words to explain how I understand it.

IMHO, the main motivation is to prevent calling pm_runtime_*() in
serial8250_console_write(). It is not safe in some contexts,
especially in NMI. And it is not needed from two reasons:

  1. The console->write() callback is used only by registered consoles.
     And the pm_runtime usage counter is already bumped during
     the console registration by serial8250_console_setup().

  2. The pm_runtime_*() calls are used in the following code path

	+ serial8250_console_write()
	  + up->rs485_start_tx()
	    + serial8250_em485_start_tx()
	      + serial8250_stop_rx()

     This code is not needed because serial8250_console_write()
     always clears IER by __serial8250_clear_IER(up) anyway.

     In fact, the extra IER manipulation in serial8250_em485_start_tx()
     might be seen as a bug. Well, it is a NOP.

All in all, the patch looks good to me.

Reviewed-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr

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

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