linux-kernel - Re: [PATCH net-next 06/16] net: pcs: xpcs: Avoid creating dummy XPCS MDIO device

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Message-ID: <eeyhdczfpgxwxbtljjc7tkjwi64avqkn2h7tehh56xq6pss3x3@7cun56p633o4>
Date:   Wed, 13 Dec 2023 18:47:19 +0300
From:   Serge Semin <fancer.lancer@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Tomer Maimon <tmaimon77@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, openbmc@...ts.ozlabs.org,
        netdev@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 06/16] net: pcs: xpcs: Avoid creating dummy XPCS
 MDIO device

On Tue, Dec 12, 2023 at 07:06:46PM +0000, Russell King (Oracle) wrote:
> On Tue, Dec 12, 2023 at 06:26:16PM +0300, Serge Semin wrote:
> > I would have used in the first place if it was externally visible, but
> > it's defined as static. Do you suggest to make it global or ...
> 
> That would be one option - I didn't make it visible when I introduced it
> beacuse there were no users for it.
> 
> > > At some point, we should implement
> > > mdiobus_get_mdiodev() which also deals with the refcount.
> > 
> > ... create mdiobus_get_mdiodev() instead?
> > 
> > * Note in the commit message I mentioned that having a getter would be
> > * better than directly touching the mii_bus instance guts.
> 
> What I'm thinking is:
> 
> /**
>  * mdiobus_get_mdiodev() - get a mdiodev for the specified bus
>  * @bus: mii_bus to get mdio device from
>  * @addr: mdio address of mdio device
>  *
>  * Return the struct mdio_device attached to the MII bus @bus at MDIO
>  * address @addr. On success, the refcount on the device will be
>  * increased, which must be dropped using mdio_device_put(), and the
>  * mdio device returned. Otherwise, returns NULL.
>  */
> struct mdio_device *mdiobus_get_mdiodev(struct mii_bus *bus, int addr)
> {
> 	struct mdio_device *mdiodev;
> 
> 	mdiodev = mdiobus_find_device(bus, addr);
> 	if (mdiodev)
> 		get_device(&mdiodev->dev);
> 	return mdiodev;
> }
> EXPORT_SYMBOL(mdiobus_get_mdiodev);
> 
> should do it, and will hold a reference on the mdiodev structure (which
> won't be freed) and also on the mii_bus (since this device is a child
> of the bus device, the parent can't be released until the child has
> been, so struct mii_bus should at least stay around.)

Right. That's exactly what had in mind. Thanks for suggesting a
ready-to-apply solution. I'll add it to the series as a separate patch
if we decide to keep the proposed in this patch change.  See my
question in the next message:
https://lore.kernel.org/netdev/wnptneaxxe2tq2rf7ac6a72xtyluyggughvmtxbbg5qto64mpa@7gchl5e4qllu/

> 
> What would help the "the bus driver has been unbound" situation is if
> we took the mdio_lock on the bus, and then set the {read,write}{,_c45}
> functions to dummy stubs when the bus is being unregistered which then
> return e.g. -ENXIO. That will probably make unbinding/unloading all
> MDIO bus drivers safe from kernel oops, although phylib will spit out
> a non-useful backtrace if it tries an access. I don't think there's
> much which can be done about that - I did propose a patch to change
> that behaviour but apparently folk like having it!
> 
> It isn't perfect - it's racy, but then accessing mdio_map[] is
> inherently racy due to no locking with mdiobus_.*register_device().
> At least if we have everyone using a proper getter function rather
> than directly fiddling with bus->mdio_map[]. We only have one driver
> that accesses it directly at the moment (mscc_ptp):
> 
>                 dev = phydev->mdio.bus->mdio_map[vsc8531->ts_base_addr];
>                 phydev = container_of(dev, struct phy_device, mdio);
> 
>                 return phydev->priv;
> 
> and that should really be using mdiobus_get_phy().

Regarding the driver bind/unbind. I guess the maintainers just forget
about that problem. Do you think it's worth reminding them about it? 

-Serge(y)

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