linux-kernel - Re: [PATCH 1/3] drivers: misc: add omap

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Message-Id: <201010192258.27988.arnd@arndb.de>
Date:	Tue, 19 Oct 2010 22:58:27 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	linux-arm-kernel@...ts.infradead.org
Cc:	"Ohad Ben-Cohen" <ohad@...ery.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Hari Kanigeri <h-kanigeri2@...com>, Suman Anna <s-anna@...com>,
	Benoit Cousson <b-cousson@...com>,
	Tony Lindgren <tony@...mide.com>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org, Simon Que <sque@...com>,
	akpm@...ux-foundation.org, linux-omap@...r.kernel.org,
	"Krishnamoorthy, Balaji T" <balajitk@...com>
Subject: Re: [PATCH 1/3] drivers: misc: add omap_hwspinlock driver

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
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