linux-kernel - Re: [PATCH 00/12] i2c: add and start using i2c

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Message-ID: <aW4PajoKqmyjkciY@hovoldconsulting.com>
Date: Mon, 19 Jan 2026 12:03:06 +0100
From: Johan Hovold <johan@...nel.org>
To: Bartosz Golaszewski <brgl@...nel.org>
Cc: Bartosz Golaszewski <bartosz.golaszewski@....qualcomm.com>,
	Wolfram Sang <wsa+renesas@...g-engineering.com>,
	Andi Shyti <andi.shyti@...nel.org>, Chen-Yu Tsai <wens@...nel.org>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Samuel Holland <samuel@...lland.org>,
	Khalil Blaiech <kblaiech@...dia.com>,
	Asmaa Mnebhi <asmaa@...dia.com>, Jean Delvare <jdelvare@...e.com>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@...nel.org>,
	Andreas Färber <afaerber@...e.de>,
	Manivannan Sadhasivam <mani@...nel.org>, linux-i2c@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-sunxi@...ts.linux.dev, linuxppc-dev@...ts.ozlabs.org,
	linux-actions@...ts.infradead.org
Subject: Re: [PATCH 00/12] i2c: add and start using i2c_adapter-specific
 printk helpers

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@...nel.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