linux-kernel - Re: Locking in the clk API

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Message-ID: <20110127211151.GC1597@n2100.arm.linux.org.uk>
Date:	Thu, 27 Jan 2011 21:11:51 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-sh <linux-sh@...r.kernel.org>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...gle.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Richard Zhao <linuxzsc@...il.com>
Subject: Re: Locking in the clk API

On Thu, Jan 27, 2011 at 09:07:29PM +0000, Alan Cox wrote:
> > > For internal tree purposes, does .set_termios need to be atomic? Can it  
> > > grab mutexes instead of spinlock?
> > 
> > I think I already answered that question above where I said "protect
> > against the interrupt handler accessing the port->* stuff".
> 
> I'm not sure you answered it correctly however as the locking nowdays is
> a bit different.
> 
> Architecturally the termios handling doesn't need a spin lock nor is it
> called under one. In fact it's vital this is the case because of USB.

It needs to protect against the read_status_mask and ignore_status_mask
being read half-way through an update by the interrupt handler, otherwise
you can end up with god-knows-what coming through from each termios
change.

Eg, you could see an effective CREAD flip when the state of CREAD wasn't
actually changed.
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