linux-kernel - Re: [PATCH v2 0/4] gpio: fix an incorrect lockdep warning

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Date:   Tue, 20 Sep 2016 12:28:37 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     Peter Rosin <peda@...ntia.se>, Wolfram Sang <wsa@...-dreams.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Courbot <gnurou@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Vignesh R <vigneshr@...com>, Yong Li <yong.b.li@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Ingo Molnar <mingo@...hat.com>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/4] gpio: fix an incorrect lockdep warning

On Tue, Sep 20, 2016 at 12:07:39PM +0200, Bartosz Golaszewski wrote:
> 2016-09-20 10:48 GMT+02:00 Peter Rosin <peda@...ntia.se>:
> >
> > One pretty simple problematic case is:
> >
> >   .---.          .----.
> >   |   |          |    |-- i2c2
> >   |   |-- i2c0 --|mux0|          .----.
> >   | l |          |    |-- i2c3 --|gpio|
> >   | i |          '----'          '----'
> >   | n |             .--------------'
> >   | u |          .----.          .----.
> >   | x |          |    |-- i2c4 --|dev0|
> >   |   |-- i2c1 --|mux1|          '----'
> >   |   |          |    |-- i2c5
> >   '---'          '----'

Shees, and I suppose this is all external to SoC stuff, so people can
stick on whatever they pretty well please. I mean, its an i2c bus, just
order parts from ebay and stick on a board.

> > Accesses to dev0 will:
> >
> > 1. lock i2c1:mux_lock (depth 0)
> > 2. switch mux1 to i2c4 using gpio
> >  a lock i2c0:mux_lock (depth 0)
> >  b switch mux0 to i2c3 using whatever
> >  c access gpio
> >  d unlock i2c0:mux_lock
> > 3. access dev0
> > 4. unlock i2c1:mux_lock
> >
> > 2a will cause a lockdep splat if i2c0:mux_lock is in the same
> > lockdep class & subclass as i2c1:mux_lock. So, lockdep needs
> > separate lockdep classes depending on the i2c root adapter
> > (subclasses are needed to handle deeper trees, so they are off
> > limits). Great fun. How do I go about creating a new lockdep
> > class for every i2c root adapter instance?
> >
> 
> I feel like it's just wrong to set an arbitrary limit on the number of
> i2c branches - and this is what the result of this approach would be.
> 
> One solution that comes to mind is to have a separate, global set of
> lock classes solely for gpio expanders. I think you mentioned earlier
> that it's the only thing that can cause this kind of lockdep false
> positives. We could potentially have a limited set of lock classes and
> every expander that would need one would request it using some kind of
> API ensuring that every instance gets a separate class. But this
> sounds like a big hack too I'm afraid... And regmap would need to be
> aware of that as well.
> 
> Anyways, we're past rc7 already and 4.9 will be the next LTS kernel.
> We have real hardware here that runs on mainline linux and is
> suffering from this issue. Are there any objections against merging
> this series now and continuing the work on improving the solution for
> 4.10?

Seems sensible, I'll also see if I can come up with a better annotation
that can help here.