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

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Date:   Thu, 23 Jun 2022 18:08:56 +0200
From:   Lino Sanfilippo <LinoSanfilippo@....de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        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 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:

	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.


Regards,
Lino