netdev - Re: [PATCH net] net: dsa: microchip: keep compatibility with device tree blobs with no phy-mode

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Date:   Fri, 19 Aug 2022 13:57:00 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org,
        Woojung Huh <woojung.huh@...rochip.com>,
        UNGLinuxDriver@...rochip.com, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Michael Grzeschik <m.grzeschik@...gutronix.de>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        Alvin Šipraga <alsi@...g-olufsen.dk>,
        Craig McQueen <craig@...ueen.id.au>
Subject: Re: [PATCH net] net: dsa: microchip: keep compatibility with device
 tree blobs with no phy-mode

On Fri, Aug 19, 2022 at 12:19:12PM +0200, Rasmus Villemoes wrote:
> Well, there's also e.g. arch/arm/boot/dts/at91-sama5d3_ksz9477_evb.dts
> which sets the phy-mode but not the phy-handle:
> 
>                         port@0 {
>                                 reg = <0>;
>                                 label = "lan1";
>                                 phy-mode = "internal";
>                         };

Yeah, it looks like there's also that variation, curious.

> And doing that in my case seems to fix things (I wouldn't know what
> phy-handle to point at anyway), so since we're still in development, I
> think I'll do that. But if I want to follow the new-world-order to the
> letter, should I also figure out a way to point at a phy-handle?

So if by "new world order" you mean
https://patchwork.kernel.org/project/netdevbpf/cover/20220818115500.2592578-1-vladimir.oltean@nxp.com/
then no, that patch set doesn't change the requirements for *user* ports
(what you have here) but for CPU and DSA ports, where no phy-handle/
fixed-link/phy-mode means something entirely different than it means for
user ports.

To give you an idea of how things work for user ports. If a user port
has a phy-handle, DSA will connect to that, irrespective of what OF-based
MDIO bus that is on. If not, DSA looks at whether ds->slave_mii_bus is
populated with a struct mii_bus by the driver. If it is, it connects in
a non-OF based way to a PHY address equal to the port number. If
ds->slave_mii_bus doesn't exist but the driver provides
ds->ops->phy_read and ds->ops->phy_write, DSA automatically creates
ds->slave_mii_bus where its ops are the driver provided phy_read and
phy_write, and it then does the same thing of connecting to the PHY in
that non-OF based way.

So to convert a driver that currently relies on DSA allocating the
ds->slave_mii_bus, you need to allocate/register it yourself (using the
of_mdiobus_* variants), and populate ds->slave_mii_bus with it. Look at
lan937x_mdio_register() for instance.

For existing device trees which connect in a non-OF based way, you still
need to keep ds->ops->phy_read and ds->ops->phy_write, and let DSA
create the ds->slave_mii_bus. The phy_read and phy_write can be the same
between your MDIO bus ops and DSA's MDIO bus ops.

> > Fixes: 2c709e0bdad4 ("net: dsa: microchip: ksz8795: add phylink support")
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=216320
> > Reported-by: Craig McQueen <craig@...ueen.id.au>
> > Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> 
> I've also tested this patch on top of v5.19 without altering my .dts,
> and that also seems to fix things, so I suppose you can add
> 
> Fixes: 65ac79e18120 ("net: dsa: microchip: add the phylink get_caps")
> Tested-by: Rasmus Villemoes <rasmus.villemoes@...vas.dk>

So I don't intend to make you modify your device tree in this case, but
there is something to be said about U-Boot compatibility. In U-Boot,
with DM_DSA, I don't intend to support any unnecessary complexity and
alternative ways of describing the same thing, so there, phy-mode and
one of phy-handle or fixed-link are mandatory for all ports. And since
U-Boot can pass its own device tree to Linux, it means Linux DSA drivers
might need to gradually gain support for OF-based phy-handle on user
ports as well. So see what Tim Harvey has done in drivers/net/ksz9477.c
in the U-Boot source code, and try to work with his device tree format,
as a starting point.