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

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Date:   Thu, 23 Jun 2022 11:45:57 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
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 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.

> This patch at least prevents the driver from allocating and holding a GPIO descriptor across
> the drivers lifetime that will never be used.

But it's not your issue, if DTS defines it, so the platform has an
idea about its usage.

> Furthermore it simplifies the code in patch 2 when we want to set the GPIO, since we can
> skip the check whether or not the termination GPIO is supported by the driver.

That's fine.

-- 
With Best Regards,
Andy Shevchenko