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

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Message-ID: <19477.53673.644128.558827@ipc1.ka-ro>
Date:	Mon, 14 Jun 2010 08:52:25 +0200
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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,

Uwe Kleine-König writes:
> Hello Lothar,
> 
> On Mon, Jun 14, 2010 at 08:39:21AM +0200, Lothar Waßmann wrote:
> > 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.
> That's wrong.  With a mutex hold you may sleep.
> 
OK, you're right. But still all other implementations (omap, mxc,
davinci,...) use spin_lock_irqsave() to protect the enable/disable
functions and don't seem to have any problem with this.
Is there any reason to change this, or make it inconsistent
for one arch?

And arch/arm/plat-s3c/clock.c has the following comment:
|/* We originally used an mutex here, but some contexts (see resume)
| * are calling functions such as clk_set_parent() with IRQs disabled
| * causing an BUG to be triggered.
| */
|DEFINE_SPINLOCK(clocks_lock);


Lothar Waßmann
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