[alsa-devel] [RFC PATCH 1/2] regmap: add configurable lock class key for lockdep

[Nicolas Boichat]

On Fri, Jun 26, 2015 at 12:08 AM, Mark Brown <broonie at kernel.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.