linux-kernel - Re: Locking in the clk API, part 2: clk_prepare/clk

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20110204102120.GJ30452@pengutronix.de>
Date:	Fri, 4 Feb 2011 11:21:20 +0100
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Richard Zhao <linuxzsc@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Paul Mundt <lethal@...ux-sh.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, Dima Zavin <dmitriyz@...gle.com>,
	Saravana Kannan <skannan@...eaurora.org>,
	Ben Dooks <ben-linux@...ff.org>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare

Hello Richard,

On Fri, Feb 04, 2011 at 05:54:24PM +0800, Richard Zhao wrote:
> On Tue, Feb 01, 2011 at 09:24:09PM +0000, Russell King - ARM Linux wrote:
> > On Tue, Feb 01, 2011 at 12:59:11PM -0800, Stephen Boyd wrote:
> > > On 02/01/2011 07:24 AM, Russell King - ARM Linux wrote:
> > > > I'd also be tempted at this stage to build-in a no-op dummy clock,
> > > > that being the NULL clk:
> > > >
> > > > int clk_prepare(struct clk *clk)
> > > > {
> > > > 	int ret = 0;
> > > >
> > > > 	if (clk) {
> > > > 		mutex_lock(&clk->mutex);
> > > > 		if (clk->prepared == 0)
> > > > 			ret = clk->ops->prepare(clk);
> > > > 		if (ret == 0)
> > > > 			clk->prepared++;
> > > > 		mutex_unlock(&clk->mutex);
> > > > 	}
> > > >
> > > > 	return ret;
> > > > }
> > > 
> > > I'm afraid this will hide enable/disable imbalances on some targets and
> > > then expose them on others. Maybe its not a big problem though since
> > > this also elegantly handles the root(s) of the tree.
> > 
> > You can't catch enable/disable imbalances in the prepare code, and you
> > can't really catch them in the unprepare code either.
> > 
> > Consider two drivers sharing the same struct clk.  When the second driver
> > prepares the clock, the enable count could well be non-zero, caused by
> > the first driver.  Ditto for when the second driver is removed, and it
> > calls unprepare - the enable count may well be non-zero.
> > 
> > The only thing you can check is that when the prepare count is zero,
> > the enable count is also zero.  You can also check in clk_enable() and
> > clk_disable() that the prepare count is non-zero.
> but how can we check prepare count without mutex lock? Even if prepare count
> is atomic_t, it can not guarantee the clock is actually prepared or unprepared.
> So it's important for driver writer to maintain the call sequence.
I happily point out that the prepare_count needs to be protected by a
spinlock and you need a flag that signals a prepare or unprepare is
currently running.

SCNR
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/