linux-kernel - Re: [RFC,PATCH 1/2] Add a common struct clk

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Date:	Sat, 12 Jun 2010 15:14:03 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Lothar Waßmann <LW@...O-electronics.de>
Cc:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Ben Dooks <ben-linux@...ff.org>
Subject: Re: [RFC,PATCH 1/2] Add a common struct clk

On Fri, 2010-06-11 at 12:08 +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.
> > IMHO if a device generates an irq its clock should already be on.  This
> > way you don't need to enable or disable a clock in irq context.
> >  
> You may want to disable a clock in the IRQ handler. The VPU driver in
> the Freescale BSP for i.MX51 does exactly this.
> Anyway I don't see any reason for using a mutex here instead of
> spin_lock_irq_save() as all other implementations do.

Because you suddenly make it impossible to sleep inside enable/disable
unless I'm mistaken about the implementation details. Some PLLs can need
milliseconds to stabilize (especially if they need to be powered up
first). Doing that with a lock held is a BAD IDEA.

Cheers,
Ben.


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