Date:	Tue, 22 May 2012 11:06:45 -0700
From:	"Turquette, Mike" <mturquette@...com>
To:	Peter De Schrijver <pdeschrijver@...dia.com>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Cc:	Saravana Kannan <skannan@...eaurora.org>,
	Andrew Lunn <andrew@...n.ch>,
	Grant Likely <grant.likely@...retlab.ca>,
	Jamie Iles <jamie@...ieiles.com>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	Magnus Damm <magnus.damm@...il.com>,
	Deepak Saxena <dsaxena@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Arnd Bergman <arnd.bergmann@...aro.org>,
	Stephen Warren <swarren@...dotorg.org>,
	"linux-arm-msm@...r.kernel.org" <linux-arm-msm@...r.kernel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Rob Herring <rob.herring@...xeda.com>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Zhao <richard.zhao@...aro.org>,
	Shawn Guo <shawn.guo@...escale.com>,
	Paul Walmsley <paul@...an.com>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Amit Kucheria <amit.kucheria@...aro.org>
Subject: Re: [PATCH] clk: Fix race conditions between clk_set_parent() and clk_enable()

On Tue, May 22, 2012 at 6:58 AM, Peter De Schrijver
<pdeschrijver@...dia.com> wrote:
> On Tue, May 15, 2012 at 08:20:44PM +0200, Saravana Kannan wrote:
>> On 05/11/2012 09:59 PM, Saravana Kannan wrote:
>> > Without this patch, the following race conditions are possible.
>> >
>> > Race condition 1:
>> > * clk-A has two parents - clk-X and clk-Y.
>> > * All three are disabled and clk-X is current parent.
>> > * Thread A: clk_set_parent(clk-A, clk-Y).
>> > * Thread A:<snip execution flow>
>> > * Thread A: Grabs enable lock.
>> > * Thread A: Sees enable count of clk-A is 0, so doesn't enable clk-Y.
>> > * Thread A: Releases enable lock.
>> > * Thread B: Calls clk_enable(clk-A), which in turn enables clk-X.
>> > * Thread A: Switches clk-A's parent to clk-Y in hardware.
>> >
>> > clk-A is now enabled in software, but not clocking in hardware.
>> >
>> > Race condition 2:
>> > * clk-A has two parents - clk-X and clk-Y.
>> > * All three are disabled and clk-X is current parent.
>> > * Thread A: clk_set_parent(clk-A, clk-Y).
>> > * Thread A:<snip execution flow>
>> > * Thread A: Switches parent in hardware to clk-Y.
>> > * Thread A: Grabs enable lock.
>> > * Thread A: Sees enable count of clk-A is 0, so doesn't disable clk-X.
>> > * Thread A: Releases enable lock.
>> > * Thread B: Calls clk_enable(clk-A)
>> > * Thread B: Software state still says parent is clk-X.
>> > * Thread B: So, enables clk-X and then itself.
>> > * Thread A: Updates parent in software state to clk-Y.
>> >
>
> This looks correct to me. Is there any usecase where enabling/disabling a
> clock would require sleeping but changing the parent would not?
>

clk_enable & clk_disable must never sleep.  clk_prepare and
clk_unprepare may sleep.

This patch would require the WM831x clock driver to set
CLK_SET_PARENT_GATE for the clkout mux.  The latest version of this
driver sent out by Mark does not do this and I imagine that is because
the hardware has no requirement for the mux clock to gate while
switching parents.  Holding the spinlock across the .set_parent
callback breaks this driver since .set_parent results in an i2c
transaction.

Mark, do you have an opinion on the requirements that this patch
imposes on your driver?

Regards,
Mike

> Cheers,
>
> Peter.
>
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