Re: [PATCH 4/4] serial: 8250: Support rs485 bus termination GPIO

From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To: Lukas Wunner <lukas@wunner.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jiri Slaby <jslaby@suse.com>, Rob Herring <robh+dt@kernel.org>,
	"Matwey V. Kornilov" <matwey@sai.msu.ru>,
	Giulio Benetti <giulio.benetti@micronovasrl.com>,
	Heiko Stuebner <heiko@sntech.de>,
	Christoph Muellner <christoph.muellner@theobroma-systems.com>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	devicetree@vger.kernel.org, linux-serial@vger.kernel.org
Subject: Re: [PATCH 4/4] serial: 8250: Support rs485 bus termination GPIO
Date: Wed, 6 May 2020 13:06:17 +0300	[thread overview]
Message-ID: <20200506100617.GC185537@smile.fi.intel.com> (raw)
In-Reply-To: <20200506062943.qugqwhnkismnnkrb@wunner.de>

On Wed, May 06, 2020 at 08:29:43AM +0200, Lukas Wunner wrote:
> On Tue, May 05, 2020 at 07:10:35PM +0300, Andy Shevchenko wrote:
> > On Tue, May 05, 2020 at 04:42:04PM +0200, Lukas Wunner wrote:

...

> > > +		devm_gpiod_put(dev, port->rs485_term_gpio);
> > 
> > > +	port->rs485_term_gpio = devm_gpiod_get_optional(dev, "rs485-term",
> > 
> > Using devm_*() in uart_get_rs485_mode() seems not right.
> > Why do you need this?
> 
> uart_get_rs485_mode() is called from a driver's ->probe() hook and we
> do not have a corresponding function that is called from a ->remove()
> hook where we'd be able to relinquish rs485 resources we've acquired
> on probe.
> 
> Of course I could add that but it would be more heavy-weight compared
> to simply using devm_*().  Do you disagree?
> 
> devm_gpiod_put() isn't strictly necessary here.  It is only necessary
> if one of the drivers would invoke uart_get_rs485_mode() multiple
> times, which none of them does AFAICS.  It's just a safety measure.
> I can drop it if that is preferred.

I think putting and re-requesting here is also racy. Somebody can request the
very same GPIO in between (for example crazy user space tool).

Setting the same value many times won't hurt.

> > > +		GPIOD_FLAGS_BIT_DIR_SET | GPIOD_FLAGS_BIT_DIR_OUT);
> > 
> > Parameter has a specific macro GPIOD_OUT_HIGH.
> 
> Good point.  It's also occurred to me now that reading the GPIO's
> value after changing its direction to output is nonsense.  If anything
> it ought to be read *before* changing the direction to output.

It's not a complete nonsense, depends what you actually want to achieve here.

> That would make sense in case the board has a pullup or pulldown on
> the Termination Enable pin.  In other cases the pin may just float
> and the value will be unpredictable.  However if I do not read the
> pin, I'd have to choose either high or low as initial state.  Hm.
> Let me check back with our hardware engineers today and see what they
> recommend.

-- 
With Best Regards,
Andy Shevchenko