linux-kernel - Re: [PATCH] net: mdiobus: Prevent spike on MDIO bus reset signal

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Message-ID: <20210127145437.1f5227b5@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 27 Jan 2021 14:54:37 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Mike Looijmans <mike.looijmans@...ic.nl>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Andrew Lunn <andrew@...n.ch>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Heiner Kallweit <hkallweit1@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: mdiobus: Prevent spike on MDIO bus reset signal

On Wed, 27 Jan 2021 08:08:29 +0100 Mike Looijmans wrote:
> > Andrew, I don't get what you're saying.
> >
> > Here is what happens depending on the pre-existing state of the
> > reset signal:
> >
> > Reset (previously asserted):   ~~~|_|~~~~|_______
> > Reset (previously deasserted): _____|~~~~|_______
> >                                    ^ ^    ^
> >                                    A B    C
> >
> > At point A, the low going transition is because the reset line is
> > requested using GPIOD_OUT_LOW. If the line is successfully requested,
> > the first thing we do is set it high _without_ any delay. This is
> > point B. So, a glitch occurs between A and B.
> >
> > We then fsleep() and finally set the GPIO low at point C.
> >
> > Requesting the line using GPIOD_OUT_HIGH eliminates the A and B
> > transitions. Instead we get:
> >
> > Reset (previously asserted)  : ~~~~~~~~~~|______
> > Reset (previously deasserted): ____|~~~~~|______
> >                                     ^     ^
> >                                     A     C
> >
> > Where A and C are the points described above in the code. Point B
> > has been eliminated.
> >
> > Therefore, to me the patch looks entirely reasonable and correct.
> >  
> Thanks, excellent explanation.
> 
> As a bit of background, we were using a Marvell PHY where the datasheet 
> states that thou shallt not release the reset within 50 ms of power-up. 
> A pull-down on the active-low reset was thus added. Looking at the reset 
> signal with a scope revealed a short spike, visible only because it was 
> being controlled by an I2C GPIO expander. So it's indeed point "B" that 
> we wanted to eliminate.

This is all useful information - can we roll more of it into the commit
message? I'd think that calling out the part and the 50ms value could
make things more "concrete" for a reader down the line?