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

[Lars-Peter Clausen]

On 06/26/2015 04:34 AM, Nicolas Boichat wrote:
> On Thu, Jun 25, 2015 at 11:59 PM, Lars-Peter Clausen <lars@xxxxxxxxxx> wrote:
> > [...]
> > > diff --git a/include/linux/regmap.h b/include/linux/regmap.h
> > > index 116655d..09aaaf5 100644
> > > --- a/include/linux/regmap.h
> > > +++ b/include/linux/regmap.h
> > > @@ -135,6 +135,12 @@ typedef void (*regmap_unlock)(void *);
> > >     * @lock_arg:   this field is passed as the only argument of lock/unlock
> > >     *              functions (ignored in case regular lock/unlock functions
> > >     *              are not overridden).
> > > + * @lock_class_key: Custom lock class key for lockdep validator. Use that
> > > to
> > > + *                  silence false lockdep nested locking warning, when
> > > this
> > > + *                  regmap needs to access another regmap during
> > > read/write
> > > + *                  operations (directly in read/write functions, or
> > > + *                  indirectly, e.g. through bus accesses).
> >
> >
> > The recommendation when to use this is the wrong way around. The presented
> > criteria is true for all devices since the bus master might be using regmap
> > to implements its IO. Any regmap instance that might be used from within
> > another regmap instance needs a custom lock class. This includes bus masters
> > as well as resource providers like clock chips or regulators.
>
> I would have thought that it is easier to figure out that a regmap is
> going to access another one, instead of figuring out all possible uses
> of a regmap...
>
> As it stands, I could only see 2 cases where this kind of warning
> happens (I did not find any other recursive locking warning involving
> regmaps...):
> 1. rt5677: The "master" regmap is a "virtual" regmap, that, depending
> on the device mode (DSP or not), either directly access the register
> on a physical regmap on i2c bus, or does it indirectly, by doing a
> number of read/write on that same physical regmap.
> 2. drivers/media/dvb-frontends/rtl2832.c: That's Antti's case. If I
> understand correctly, regmap access require transfers on a private i2c
> bus, which, itself, uses a regmap.
>
> I think both cases are _fairly_ clear, but of course that may not
> cover everything (and I'm not sure if anyone would figure it out
> before the warning shows up...), and I'm not sure if there are cases
> that look similar but don't require a lockdep class.

When you have a generic slave driver, you don't know what the bus master is 
going to do in e.g. i2c_transfer() or spi_sync(). It might very well be 
using regmap to do its IO. Or it might be enabling/disabling a clock or 
another resource that uses regmap to do its IO.
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