Re: [RFC PATCH 1/2] regmap: add configurable lock class key for lockdep - Lars-Peter Clausen

From: Lars-Peter Clausen <lars@metafoo.de>
To: Mark Brown <broonie@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>,
	Nicolas Boichat <drinkcat@chromium.org>,
	Mauro Carvalho Chehab <mchehab@osg.samsung.com>,
	Antti Palosaari <crope@iki.fi>, Ingo Molnar <mingo@redhat.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, Bard Liao <bardliao@realtek.com>,
	Oder Chiou <oder_chiou@realtek.com>,
	Liam Girdwood <lgirdwood@gmail.com>,
	Jaroslav Kysela <perex@perex.cz>, Takashi Iwai <tiwai@suse.de>,
	alsa-devel@alsa-project.org,
	Anatol Pomozov <anatol.pomozov@gmail.com>
Subject: Re: [RFC PATCH 1/2] regmap: add configurable lock class key for lockdep
Date: Thu, 25 Jun 2015 17:47:41 +0200	[thread overview]
Message-ID: <558C229D.4090409@metafoo.de> (raw)
In-Reply-To: <20150625153325.GR14071@sirena.org.uk>

On 06/25/2015 05:33 PM, Mark Brown wrote:
> On Thu, Jun 25, 2015 at 05:03:00PM +0200, Lars-Peter Clausen wrote:
>> On 06/25/2015 03:21 PM, Arjan van de Ven wrote:
>
>>> wouldn't it be better to use the mutex_lock_nested() and co to explicitly
>>> express your hierarchy?
>
>> That would require that the hierarchy is known in advance. The hierarchy
>> depends on the hardware topology. Different systems will have different
>> hierarchies where the relationship between locks can change and it will be
>> hard to find a hierarchy that works across all topologies.
>
> It depends on what you use as the key for the nested locking stuff.  If
> you assign a key per regmap (casting the pointer to an integer, using an
> IDR or something).  I don't know if that creates problems for the
> locking code, I'd not expect so but then I'd not have expected the
> problem in the first place.

The maximum number of subclasses is 8 per lockclass, so a IDR that 
increments which each created regmap instance wouldn't really work.

And while on the other hand we probably wont have a hierarchy deeper than 8 
nested regmap instances it is not trivial to figure out which instance is at 
which level.

>
> 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.