Re: [PATCH net-next 06/12] net: dsa: rzn1-a5psw: add Renesas RZ/N1 advanced 5 port switch driver

[Vladimir Oltean]

On Fri, Apr 15, 2022 at 12:02:14PM +0100, Russell King (Oracle) wrote:
> On Fri, Apr 15, 2022 at 01:55:03PM +0300, Vladimir Oltean wrote:
> > I meant that for a DSA switch driver is mandatory to call dsa_switch_shutdown()
> > from your ->shutdown method, otherwise subtle things break, sorry for being unclear.
> > 
> > Please blindly copy-paste the odd pattern that all other DSA drivers use
> > in ->shutdown and ->remove (with the platform_set_drvdata(dev, NULL) calls),
> > like a normal person :)
> 
> Those platform_set_drvdata(, NULL) calls should be killed - the
> driver model will set the driver data to NULL after ->remove has
> been called - so having drivers also setting the driver data to
> NULL is mere duplication.

I can see why you say that, but the reverse is not true.
A driver can be removed from a device after said device has been shut
down, and DSA does things in dsa_unregister_switch() and in
dsa_switch_shutdown() that are incompatible with each other, so either
one or the other should be called, but not both.
The platform_set_drvdata(dev, NULL) from the ->remove path may be
redundant for the reason you mentioned, but it doesn't really hurt
anything, really (it's a pointer assignment), and perhaps would lead to
even more confusion (why are we setting the drvdata to NULL from
->shutdown but not also from ->remove?).

> The only case it would matter is if someone is looking up the device
> and then accessing the driver data - and one would hope that's done
> with appropriate locking or other guarantees (e.g. driver can never
> be unbound once the driver data has been set.)

No, this isn't what is happening.