linux-kernel - Re: [PATCH tty-next v3 4/6] serial: 8250: Specify console context for rs485_start/stop

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Message-ID: <ZyuOX4VVbfAFhMfV@pathway.suse.cz>
Date: Wed, 6 Nov 2024 16:42:23 +0100
From: Petr Mladek <pmladek@...e.com>
To: John Ogness <john.ogness@...utronix.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jirislaby@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Esben Haabendal <esben@...nix.com>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Ray Jui <rjui@...adcom.com>, Scott Branden <sbranden@...adcom.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Arnd Bergmann <arnd@...db.de>, Stefan Wahren <wahrenst@....net>,
	Uwe Kleine-König <u.kleine-koenig@...libre.com>,
	Kevin Hilman <khilman@...libre.com>,
	Markus Schneider-Pargmann <msp@...libre.com>,
	Ronald Wahl <ronald.wahl@...itan.com>, Udit Kumar <u-kumar1@...com>,
	Griffin Kroah-Hartman <griffin@...ah.com>,
	Rengarajan S <rengarajan.s@...rochip.com>,
	Lino Sanfilippo <l.sanfilippo@...bus.com>,
	Serge Semin <fancer.lancer@...il.com>,
	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH tty-next v3 4/6] serial: 8250: Specify console context
 for rs485_start/stop_tx

On Fri 2024-10-25 13:03:26, 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/stop_tx() callbacks. However, these
> callbacks will disable/enable interrupts, which is a problem
> for console write, as it must be responsible for
> disabling/enabling interrupts.

It is not clear to me what exactly is the problem. Is the main
problem calling pm_runtime*() API because it uses extra locks
and can cause deadlocks? Or is it more complicated?

IMHO, it would deserve some explanation.

> Add an argument @in_con to the rs485_start/stop_tx() callbacks
> to specify if they are being called from console write. If so,
> the callbacks will not handle interrupt disabling/enabling.
> 
> For all call sites other than console write, there is no
> functional change.
> 
> Signed-off-by: John Ogness <john.ogness@...utronix.de>

It looks like the code does what the description says. And honestly,
I do not have any idea how to improve the naming. I would keep
it as is after reading John's answers in the thread.

IMHO, one thing which makes things comlicated is that
serial8250_em485_start_tx() and serial8250_em485_stop_tx()
are not completely reversible operations. Especially,
the change done by __serial8250_stop_rx_mask_dr() is
not reverted in serial8250_em485_stop_tx(). It makes
things look tricky. But I think that it is beyond the scope
of this patchset to do anything about it.

Just 2 my cents.

Best Regaards,
Petr