linux-kernel - Re: lockdep: incorrect deadlock warning with two GPIO expanders

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Message-ID: <20160912153356.GD10121@twins.programming.kicks-ass.net>
Date:   Mon, 12 Sep 2016 17:33:56 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bartosz Golaszewski <bgolaszewski@...libre.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Alexandre Courbot <gnurou@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-gpio <linux-gpio@...r.kernel.org>
Subject: Re: lockdep: incorrect deadlock warning with two GPIO expanders

On Mon, Sep 12, 2016 at 05:16:14PM +0200, Bartosz Golaszewski wrote:

> >>                                                - - - - -
> >>  -------             ---------  Bus segment 1 |         |
> >> |       |           |         |---------------  Devices
> >> |       | SCL/SDA   |         |               |         |
> >> | Linux |-----------| I2C MUX |                - - - - -
> >> |       |    |      |         | Bus segment 2
> >> |       |    |      |         |-------------------
> >>  -------     |       ---------                    |
> >>              |           |                    - - - - -
> >>         ------------     | MUX GPIO          |         |
> >>        |            |    |                     Devices
> >>        |    GPIO    |    |                   |         |
> >>        | Expander 1 |----                     - - - - -
> >>        |            |                             |
> >>         ------------                              | SCL/SDA
> >>                                                   |
> >>                                              ------------
> >>                                             |            |
> >>                                             |    GPIO    |
> >>                                             | Expander 2 |
> >>                                             |            |
> >>                                              ------------

> > The tricky part, and here I have absolutely no clue what so ever, is
> > being able to tell at pca953x_probe() time that this is so.
> >
> 
> AFAIK there is no clean way to tell that a GPIO is used by an I2C
> multiplexer at probe time. Linus, Alexandre could you confirm?

You cannot inspect the device tree while probing?

> It uses the fact that the two expanders we have are of different type
> (pca9534 and pca9535). The id pointer points to per-chip device info
> residing in .data which makes it suitable for mutex key.
> 
> I don't think such hack is suitable for mainline though.

Right, that works by accident rather than anything else :-)