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

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Date:	Mon, 14 Jun 2010 08:39:21 +0200
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
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

Hi,

Benjamin Herrenschmidt writes:
> 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
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
???
All implementations so far use spin_lock_irq_save()!

How would you be able to sleep with a mutex held?
If you hold a lock you must not sleep, no matter what sort of lock it
is.


Lothar Waßmann
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