linux-kernel - Re: [PATCH net-next v2 06/10] arm64: dts: allwinner: a527: cubie-a5e: Add ethernet PHY reset setting

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Message-ID: <aJy_qUbmqoOG-GBC@shell.armlinux.org.uk>
Date: Wed, 13 Aug 2025 17:39:05 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Chen-Yu Tsai <wens@...nel.org>
Cc: Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Jernej Skrabec <jernej@...nel.org>,
	Samuel Holland <samuel@...lland.org>, netdev@...r.kernel.org,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-sunxi@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Andre Przywara <andre.przywara@....com>
Subject: Re: [PATCH net-next v2 06/10] arm64: dts: allwinner: a527:
 cubie-a5e: Add ethernet PHY reset setting

On Wed, Aug 13, 2025 at 11:51:18PM +0800, Chen-Yu Tsai wrote:
> On Wed, Aug 13, 2025 at 11:12 PM Russell King (Oracle)
> <linux@...linux.org.uk> wrote:
> >
> > On Wed, Aug 13, 2025 at 10:55:36PM +0800, Chen-Yu Tsai wrote:
> > > diff --git a/arch/arm64/boot/dts/allwinner/sun55i-a527-cubie-a5e.dts b/arch/arm64/boot/dts/allwinner/sun55i-a527-cubie-a5e.dts
> > > index 70d439bc845c..d4cee2222104 100644
> > > --- a/arch/arm64/boot/dts/allwinner/sun55i-a527-cubie-a5e.dts
> > > +++ b/arch/arm64/boot/dts/allwinner/sun55i-a527-cubie-a5e.dts
> > > @@ -94,6 +94,9 @@ &mdio0 {
> > >       ext_rgmii_phy: ethernet-phy@1 {
> > >               compatible = "ethernet-phy-ieee802.3-c22";
> > >               reg = <1>;
> > > +             reset-gpios = <&pio 7 8 GPIO_ACTIVE_LOW>; /* PH8 */
> > > +             reset-assert-us = <10000>;
> > > +             reset-deassert-us = <150000>;
> >
> > Please verify that kexec works with this, as if the calling kernel
> > places the PHY in reset and then kexec's, and the reset remains
> > asserted, the PHY will not be detected.
> 
> I found this to be a bit confusing to be honest.
> 
> If I put the reset description in the PHY (where I think it belongs),
> then it wouldn't work if the reset isn't by default deasserted (through
> some pull-up). This would be similar to the kexec scenario.

The reason for this is quite simple. While it's logical to put it in
there, the problem is that the PHY doesn't respond on the MDIO bus
while it's reset pin is asserted.

Consequently, when we probe the MDIO bus to detect PHYs and discover
the PHY IDs, we get no response, and thus we believe there isn't a
device at the address. That means we don't create a device, and thus
there's no mdio device for the address.

There is a work-around, which is to encode the PHY ID in the DT
compatible (check the ethernet-phy binding). However, note that we
will then not read the actual PHY ID (maybe we should?) which means
if the driver wants to know e.g. the revision, or during production
the PHY changes, it will require DT to change.

-- 
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