linux-kernel - Re: [RFC PATCH 1/2] regmap: add configurable lock class key for lockdep

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Date:	Fri, 26 Jun 2015 11:16:24 +0800
From:	Nicolas Boichat <drinkcat@...omium.org>
To:	Mark Brown <broonie@...nel.org>
Cc:	Lars-Peter Clausen <lars@...afoo.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Mauro Carvalho Chehab <mchehab@....samsung.com>,
	Antti Palosaari <crope@....fi>, Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, Bard Liao <bardliao@...ltek.com>,
	Oder Chiou <oder_chiou@...ltek.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Jaroslav Kysela <perex@...ex.cz>, Takashi Iwai <tiwai@...e.de>,
	alsa-devel@...a-project.org,
	Anatol Pomozov <anatol.pomozov@...il.com>
Subject: Re: [RFC PATCH 1/2] regmap: add configurable lock class key for lockdep

On Fri, Jun 26, 2015 at 12:08 AM, Mark Brown <broonie@...nel.org> wrote:
[...]
>> >As far as I can tell we're likely to end up needing a key per regmap or
>> >something similar.
>
>> Since the number of lockdep classes itself is also limited we should avoid
>> creating extra lockdep classes when we can. I think the approach which
>> having the option of specifying a lockdep class in the regmap config will be
>> ok. The only case it can't handle if we nest instances with the same config,
>> but I don't really see valid use scases for that at the moment.
>
> Oh, ffs.  This just keeps getting better.  I hadn't been aware of that
> limitation.  We still have the problem that this needs to be something
> users can understand rather than something that's just "define something
> here in one of your drivers if you're running into problems with
> spurious warnings" here.  That's always been the biggest problem here
> (once we got past the "what is this supposed to do in the first place?"
> issues).

I found that V4L2 uses separate lockdep classes for each of their
v4l2_ctrl. This was introduced in 6cd247ef22e "[media] v4l2-ctrls:
eliminate lockdep false alarms for struct v4l2_ctrl_handler.lock"
(https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6cd247ef22e),
so we could possibly take that approach.

On my system, I have:
# cat /proc/lockdep_stats
 lock-classes:                         1241 [max: 8191]
 direct dependencies:                  7364 [max: 32768]
 indirect dependencies:               27686
 all direct dependencies:            158097
 dependency chains:                   10011 [max: 65536]
 dependency chain hlocks:             38887 [max: 327680]
 in-hardirq chains:                      92
 in-softirq chains:                     372
 in-process chains:                    9547
 stack-trace entries:                107703 [max: 524288]

So, at least on that platform, there is some room to grow...

I'm just afraid that implementing this may require creating a bunch of
macros to wrap all regmap_init_[i2c/spi/...] functions, as the lockdep
classes need to be statically allocated... Unless we find a different
solution than what V4L2 does.
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