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

[Lina Iyer <lina.iyer@linaro.org>]

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@linaro.org> 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