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

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Message-ID: <587cdb60-d7fe-3ab2-b635-02c5072e102e@axentia.se>
Date:   Sun, 18 Sep 2016 10:52:58 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Wolfram Sang <wsa@...-dreams.de>
CC:     Bartosz Golaszewski <bgolaszewski@...libre.com>,
        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>,
        Peter Zijlstra <peterz@...radead.org>,
        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 2016-09-16 19:58, Wolfram Sang wrote:
> 
>>> Looks good from my POV, but will wait for Peter to comment.
>>>
>>> If accepted, I'd think this should go via my I2C tree and I would like
>>> to ask Linus to ack patch 4. D'accord, everyone?
>>
>> Since it is not clear if "Peter" is me or PeterZ (I suspect PeterZ...),
> 
> Nope, I meant you :) I really value your input, it especially helps me
> on topics like locking, nesting, muxing... etc. Much appreciated, thanks
> a lot for doing that!
> 
>> I'm just adding that it all looks fine by me as well, just to prevent
>> this from being held up by a misunderstanding.
> 
> OK. I read this as Acked-by.
> 
>> It does unconditionally add a new function to i2c-core that is only
>> ever used if lockdep is enabled, but it is tiny and I'm not bothered
>> by that memory waste.
> 
> Same here. And if it prevents us from false positive lockdep reports, I
> am all for fixing it.

Except it doesn't, when I think some more about it...

If you have two gpio-expanders on the same depth but on different i2c
branches you still end up with a splat if one is used to control a mux
to reach the other.

The only way to solve it for good, that I see, is to have every instance
of the gpio-expander mutex in its own class. That might lead to many
lockdep classes but then again, how many gpio expanders could there be
in a system? A dozen or two seems extreme, so maybe that is the correct
approach anyway?

Cheers,
Peter