Re: [PATCH RFC] hwspinlock: Don't take software spinlock before hwspinlock

[Lina Iyer]

On Sat, May 09 2015 at 03:25 -0600, Ohad Ben-Cohen wrote:
> Hi Lina,
>
> On Fri, May 1, 2015 at 8:07 PM, Lina Iyer <lina.iyer@xxxxxxxxxx> wrote:
> > Some uses of the hwspinlock could be that one entity acquires the lock
> > and the other entity releases the lock. This allows for a serialized
> > traversal path from the locking entity to the other.
> >
> > For example, the cpuidle entry from Linux to the firmware to power down
> > the core, can be serialized across the context switch by locking the
> > hwspinlock in Linux and releasing it in the firmware.
> >
> > Do not force the caller of __hwspin_trylock() to acquire a kernel
> > spinlock before acquiring the hwspinlock.
>
> Let's discuss whether we really want to expose this functionality
> under the same hwspinlock API or not.
>
> In this new mode, unlike previously, users will now be able to sleep
> after taking the lock, and others trying to take the lock might poll
> the hardware for a long period of time without the ability to sleep
> while waiting for the lock. It almost sounds like you were looking for
> some hwmutex functionality.
>
> What do you think about this?

I agree, that it opens up a possiblity that user may sleep after holding
a hw spinlock.  But really, why should it prevents us from using it as a
hw mutex, if the need is legitimate?

We could make a check that the caller with NO_LOCK option calls only
with irq disabled, if thats required.

Thanks for the review.

-- Lina
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