linux-kernel - Re: [PATCH 1/8] serial: core: only get RS485 termination gpio if supported

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Date:   Thu, 23 Jun 2022 19:32:35 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Vladimir Zapolskiy <vz@...ia.com>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        devicetree <devicetree@...r.kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lukas Wunner <lukas@...ner.de>, p.rosenberger@...bus.com,
        Lino Sanfilippo <l.sanfilippo@...bus.com>
Subject: Re: [PATCH 1/8] serial: core: only get RS485 termination gpio if
 supported

On Thu, Jun 23, 2022 at 06:08:56PM +0200, Lino Sanfilippo wrote:
> On 23.06.22 at 11:45, Andy Shevchenko wrote:
> > On Thu, Jun 23, 2022 at 4:00 AM Lino Sanfilippo <LinoSanfilippo@....de> wrote:
> >> On 22.06.22 at 19:04, Andy Shevchenko wrote:
> >>> On Wed, Jun 22, 2022 at 05:46:52PM +0200, Lino Sanfilippo wrote:
> >>>> From: Lino Sanfilippo <l.sanfilippo@...bus.com>
> >>>>
> >>>> In uart_get_rs485_mode() only try to get a termination GPIO if RS485 bus
> >>>> termination is supported by the driver.
> >>>
> >>> I'm not sure I got the usefulness of this change.
> >>> We request GPIO line as optional, so if one is defined it in the DT/ACPI, then
> >>> they probably want to (opportunistically) have it>
> >>>
> >>> With your change it's possible to have a DTS where GPIO line defined in a
> >>> broken way and user won't ever know about it, if they are using platforms
> >>> without termination support.
> >>
> >> This behavior is not introduced with this patch, also in the current code the driver
> >> wont inform the user if it does not make use erroneous defined termination GPIO.
> >
> > It does. If a previously stale GPIO resource may have deferred a probe
> > and hence one may debug why the driver is not working, after this
> > change one may put a stale GPIO resource into DT/ACPI and have nothing
> > in the result. Meaning the change relaxes validation which I consider
> > is not good.
> >
> 
> Ok I see the point. So what about changing it to:

You mean adding below after the existing code in the module?

> 	if (port->rs485_term_gpio &&
> 	    !(port->rs485_supported->flags & SER_RS485_TERMINATE_BUS)) {
> 		dev_warn(port->dev,
> 			"%s (%d): RS485 termination gpio not supported by driver\n",
> 			port->name, port->line);
> 		devm_gpiod_put(dev, port->rs485_term_gpio);
> 		port->rs485_term_gpio = NULL;
> 	}
> 
> This would also be consistent to the warnings we print in uart_sanitize_serial_rs485() for invalid
> RS485 settings.

Probably it's okay, but I dunno we have much on this to gain. Users may start
complaining of this (harmless) warning. I leave it to others to comment.

-- 
With Best Regards,
Andy Shevchenko