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

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Date:	Sun, 13 Jun 2010 23:27:15 +0100
From:	Ben Dooks <ben-linux@...ff.org>
To:	Lothar Wa?mann <LW@...O-electronics.de>
Cc:	Uwe Kleine-K?nig <u.kleine-koenig@...gutronix.de>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org,
	Ben Herrenchmidt <benh@...nel.crashing.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, Jun 11, 2010 at 12:08:01PM +0200, Lothar Wa?mann 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.

Hmm, then again the VPU driver may just be a bit wrong here.

We could protect each clock with a spinlock, but that would end up
with a problem of spinning where we have clocks that takes 100s of
usec or so to init. See all PLLs on S3C devices, where it can take
100-300uS to get a stable clock out of the device.

-- 
Ben (ben@...ff.org, http://www.fluff.org/)

  'a smiley only costs 4 bytes'
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