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Subject: Re: [PATCH] net: mdiobus: Prevent spike on MDIO bus reset signal
To:     Andrew Lunn <andrew@lunn.ch>
CC:     netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        linux-kernel@vger.kernel.org
References: <20210126073337.20393-1-mike.looijmans@topic.nl>
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From:   Mike Looijmans <mike.looijmans@topic.nl>
Organization: TOPIC
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Date:   Thu, 28 Jan 2021 09:45:41 +0100
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Hi Andrew,

Response below...


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On 28-01-2021 02:56, Andrew Lunn wrote:
> On Tue, Jan 26, 2021 at 08:33:37AM +0100, Mike Looijmans wrote:
>> The mdio_bus reset code first de-asserted the reset by allocating with
>> GPIOD_OUT_LOW, then asserted and de-asserted again. In other words, if
>> the reset signal defaulted to asserted, there'd be a short "spike"
>> before the reset.
>>
>> Instead, directly assert the reset signal using GPIOD_OUT_HIGH, this
>> removes the spike and also removes a line of code since the signal
>> is already high.
>=20
> Hi Mike
>=20
> Did you look at the per PHY reset? mdiobus_register_gpiod() gets the
> GPIO with GPIOD_OUT_LOW. mdiobus_register_device() then immediately
> sets it high.
>=20
> So it looks like it suffers from the same problem.

Well, now that I have your attention...

The per PHY reset was more broken, it first probes the MDIO bus to see if t=
he=20
PHY is there, and only after that it controls the reset line. So if the res=
et=20
defaults to "asserted", the PHY will not work because it didn't respond whe=
n=20
the MDIO went looking for it. I haven't quite figured out how this was=20
supposed to work, but at least for the case of one MDIO bus, one PHY=20
configured through devicetree it didn't work as one would expect. I added a=
=20
few printk statements to reveal that this was indeed the case.

This issue also makes the PHY hardware reset useless - if the PHY is in som=
e=20
non-responsive state, the MDIO won't get a response and report the PHY as=20
missing before even attempting to assert/de-assert the reset line.

This was with a 5.4 kernel, but as far as I could see this hasn't changed=20
since then.

My solution to that was to move to the MDIO bus reset, since that at least=
=20
happened before interrogating the devices on the bus. This revealed the iss=
ue=20
with the extra "spike" while evaluating that, which is something that I cou=
ld=20
easily solve and upstream.

Probably these issues were never dicovered because usually there's a pull-u=
p=20
of some kind on the (active-low) reset signal of the PHYs. That hides the=20
spike and also hides the fact that the per-phy reset doesn't actually work.=
 We=20
only discovered the issue when we changed that to a pull-down and suddenly =
the=20
phy failed to probe.

The way that the MDIO bus is being populated seemed rather complex to me, s=
o=20
chances of breaking things are quite high there...