Subject: Re: [RFC,PATCH 1/2] Add a common struct clk
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Lothar =?ISO-8859-1?Q?Wa=DFmann?= <LW@KARO-electronics.de>
Cc: Jeremy Kerr <jeremy.kerr@canonical.com>, Ben Dooks <ben-linux@fluff.org>,
       linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
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Date: Sat, 12 Jun 2010 15:12:53 +1000
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On Fri, 2010-06-11 at 11:23 +0200, Lothar Waßmann wrote:
> Hi,
> 
> > > Using a mutex in clk_enable()/clk_disable() is a bad idea, since that
> > > makes it impossible to call those functions in interrupt context.
> > 
> > Do we do this at the moment? I know at least one implementation of clk_enable 
> > uses a mutex for locking.
> > 
> You are probably talking about the Freescale i.MX51 kernel, that won't
> even boot, if you enable CONFIG_DEBUG_KERNEL, CONFIG_DEBUG_SPINLOCK,
> CONFIG_DEBUG_LOCKDEP and CONFIG_DEBUG_SPINLOCK_SLEEP.
> The mutex in the clock implementation is one of the reasons.

Regardless. Clocks generally take time to enable. I don't believe doing
clock enable/disable at hard irq context is a great idea.

If you really want to do something like that, you can always use either
threaded interrupts, or if you know your clock is off, mask & defer your
handling to a work queue yourself.

Unless we have enough case of very fast switching clocks that would
really benefit for that but from my experience, when a device can issue
interrupts, it should have its clocks on, unless it's some kind of
"wakeup" interrupt in which case it can safely be delayed.

Cheers,
Ben.

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