Re: [PATCH 1/3] drivers: misc: add omap_hwspinlock driver

Arnd Bergmann
2010-10-19T20:58:31Z

On Tuesday 19 October 2010 22:43:34 Ohad Ben-Cohen wrote:
> > Disabling irqs might be a concern as a source of RT latency. It
> > might be better to make the caller responsible for managing local spin
> > locks and irq disable/enable.
>
> This a coming from an hardware requirement, rather than a choice the
> user should take.
>
> If a hwspinlock is taken over a long period of time, its other user
> (with which we try to achieve synchronization) might be polling the
> OMAP interconnect for too long (trying to take the hwspinlock) and
> thus preventing it to be used for other transactions.

This sounds exactly like any other spinlock.

> To prevent such lengthy polling on the interconnect, the hwspinlock
> should only be used for very short period of times, with preemption
> and interrupts disabled.

Interrupts disabled in general might go a bit too far -- they are also
short and infrequent events unless you have seriously broken drivers.

When running with CONFIG_PREEMPT, I would guess you actually want to
turn the omap_hwspinlock into a sleeping operation, though that would
require much extra work to implement. Disabling preemption while the
hwspinlock is held is of course a correct implementation technically,
but it might not be what someone enabling CONFIG_PREEMPT expects.

> That's why we don't give users the choice whether to disable
> interrupts or not - it's simply not a decision they should take.

What about those cases where you already know that interrupts are
disabled, e.g. while holding a regular spin_lock_irq or inside
of an interrupt handler?

Arnd