[PATCH 01/11] usb: chipidea: Add power management support

[Russell King - ARM Linux]

On Sat, Oct 12, 2013 at 05:35:03PM +0800, Peter Chen wrote:
> This commit adds runtime and system power management support for
> chipidea core. The runtime pm support is controlled by glue
> layer, it can be enabled by flag CI_HDRC_SUPPORTS_RUNTIME_PM.

Let's look at the locking.

1. Runtime PM.  These callbacks are locked with a spinlock, which holds
   dev->power.lock.  This lock is taken either with or without disabling
   IRQs depending on whether runtime PM is IRQ safe or not.

2. Normal PM.  These callbacks are locked by holding dev->mutex.

Now, there's a little bit of protection between these two operations -
when normal PM places a device into a low power state, it 'gets' a
reference on the runtime PM to ensure no runtime PM transitions occur
while normal PM is active.  (See pm_runtime_get_noresume() in
device_prepare().)  This is only dropped when the normal PM resumes the
device.

Moreover, all runtime PM events are flushed before the suspend callback
occurs (see the pm_runtime_barrier() in __device_suspend()).

What that means is that you can't receive any runtime PM events while
you are in your suspend/resume callbacks.  So, each call is mutually
exclusive.

So, runtime PM callbacks vs normal PM callbacks for any single device
are all called with mutual exclusion - you won't have two running at
any time.

Hence, for the reasons stated previously about the non-atomic nature of
atomic_read()/atomic_set(), there's even more reasons that their use
here is just mere obfuscation: the accesses to this state tracking
variable is already guaranteed to be single-threaded by core code, so
the use of atomic_read()/atomic_set() just adds additional needless
confusion to this code.