Re: [PATCH 00/12] i2c: add and start using i2c_adapter-specific printk helpers

[Johan Hovold <johan@kernel.org>]

On Tue, Dec 23, 2025 at 04:11:08PM +0100, Bartosz Golaszewski wrote:
> On Tue, Dec 23, 2025 at 3:24 PM Johan Hovold <johan@kernel.org> wrote:
> >
> > On Tue, Dec 23, 2025 at 11:02:22AM +0100, Bartosz Golaszewski wrote:
> > > It's been another year of discussing the object life-time problems at
> > > conferences. I2C is one of the offenders and its problems are more
> > > complex than those of some other subsystems. It seems the revocable[1]
> > > API may make its way into the kernel this year but even with it in
> > > place, I2C won't be able to use it as there's currently nothing to
> > > *revoke*. The struct device is embedded within the i2c_adapter struct
> > > whose lifetime is tied to the provider device being bound to its driver.
> > >
> > > Fixing this won't be fast and easy but nothing's going to happen if we
> > > don't start chipping away at it. The ultimate goal in order to be able
> > > to use an SRCU-based solution (revocable or otherwise) is to convert the
> > > embedded struct device in struct i2c_adapter into an __rcu pointer that
> > > can be *revoked*. To that end we need to hide all dereferences of
> > > adap->dev in drivers.
> >
> > No, this is not the way to do it. You start with designing and showing
> > what the end result will look like *before* you start rewriting world
> > like you are doing here.
> 
> The paragraph you're commenting under explains exactly what I propose
> to do: move struct device out of struct i2c_adapter and protect the
> pointer storing its address with SRCU. This is a well-known design
> that's being generalized to a common "revocable" API which will
> possibly be available upstream by the time we're ready to use it.

Revocable, as presented in plumbers, is not going upstream.

> You know I can't possibly *show* the end result in a single series
> because - as the paragraph before explains - we need to first hide all
> direct dereferences of struct device in struct i2c_adapter behind
> dedicated interfaces so that we when do the conversion, it'll affect
> only a limited number of places. It can't realistically be done at
> once.

You can post an RFC converting one driver with a proper description of
the problem you're trying to solve.

Johan