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Date: Wed, 11 Mar 2009 11:51:51 -0500
From: Scott Wood <scottwood@freescale.com>
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To: Timur Tabi <timur@freescale.com>
Subject: Re: [PATCH v5] introduce macro spin_event_timeout()
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Cc: linuxppc-dev@ozlabs.org, Roland Dreier <rdreier@cisco.com>
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Timur Tabi wrote:
> On Wed, Mar 11, 2009 at 12:09 AM, Roland Dreier <rdreier@cisco.com> wrote:
> 
>> Are there really cases where spinning for 1 jiffy is too long of a
>> timeout?
> 
> If the result is a timeout, then I say no.  A timeout is an error
> condition, and the code will usually terminate.
[snip]
> Two jiffies can be a very long time.

One jiffy is fine, but two is just too long?

Given that it only happens in cases of malfunctioning hardware (or a 
buggy driver), it seems reasonable as long as preemption isn't disabled 
(I'm assuming anyone that cares about a rare latency of a couple jiffies 
is using a preemptible kernel).

> Besides, if this function is
> used when interrupts are disabled, I believe that on some platforms,
> jiffies never increments.  If so, we can't use the actual 'jiffies'
> variable.

Disallow that, enforced with a call to might_sleep().

Alternatively, do something with get_cycles(), and have some sort of 
#define by which arches can say if get_cycles actually works.  In the 
absence of a working get_cycles() or equivalent, timeouts with 
interrupts disabled aren't going to happen whether we abstract it with a 
macro or not.

-Scott