linux-kernel - Re: [PATCH net-next 2/2] drivers: net: dsa: mt7530: MT7530 optional GPIO support

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Message-ID: <20210111135509.GT1551@shell.armlinux.org.uk>
Date:   Mon, 11 Jan 2021 13:55:09 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     DENG Qingfang <dqfext@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        netdev <netdev@...r.kernel.org>, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>, linux-kernel@...r.kernel.org,
        René van Dorst <opensource@...rst.com>,
        Frank Wunderlich <frank-w@...lic-files.de>
Subject: Re: [PATCH net-next 2/2] drivers: net: dsa: mt7530: MT7530 optional
 GPIO support

On Mon, Jan 11, 2021 at 09:40:00PM +0800, DENG Qingfang wrote:
> On Mon, Jan 11, 2021 at 7:04 PM Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> >
> > FYI, Documentation/driver-api/gpio/consumer.rst says:
> >
> >   For output GPIOs, the value provided becomes the initial output value.
> >   This helps avoid signal glitching during system startup.
> >
> > Setting the pin to be an output, and then setting its initial value
> > does not avoid the glitch. You may wish to investigate whether you
> > can set the value before setting the pin as an output to avoid this
> > issue.
> >
> 
> So, setting the Output Enable bit _after_ setting the direction and
> initial value should avoid this issue. Right?

It depends on the hardware. I don't know how your hardware works, so
I can't say whether doing anything will result in correct behaviour,
or even work.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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