linux-kernel - Re: [PATCH v4] clk: allow reentrant calls into the clk framework

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Date:	Wed, 27 Mar 2013 12:09:17 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Ulf Hansson <ulf.hansson@...aro.org>
cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	Mike Turquette <mturquette@...aro.org>,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	linux-kernel@...r.kernel.org,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	David Brown <davidb@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4] clk: allow reentrant calls into the clk framework

On Wed, 27 Mar 2013, Ulf Hansson wrote:
> On 27 March 2013 10:55, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > On 27 March 2013 15:10, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> On Wed, 27 Mar 2013, Mike Turquette wrote:
> >>
> >>> Reentrancy into the clock framework from the clk.h api is necessary
> >>> for clocks that are prepared and unprepared via i2c_transfer (which
> >>> includes many PMICs and discrete audio chips) as well as for several
> >>> other use cases.
> >>
> >> That explanation sucks.
> >>
> >> Why does an i2c clock need reentrancy? Just because it's i2c or what?
> >
> > I am noway connected to this development but was just going through
> > your mail and i think i might know the answer why is this required.
> >
> > Consider an example where an external chip has clock controller and has
> > bits which can be programmed to enable/disable clock. And this chip is
> > connected via spi/i2c to SoC.
> >
> > clk_prepare(peripheral on external chip)
> >   -> i2c_xfer(to write to external chips register)
> >       -> clk_enable(i2c controller)
> >           ->controller-xfer-routine.. and finally we enable clk here...

Which does not explain the whole issue:

    clk_prepare() takes the mutex
    clk_enable() takes the spinlock

That works today.

The issue arises, if you need to call clk_prepare(i2c) in the xfer
routine.

> >
> > Sorry if i am on the wrong side :)

Only slightly :)

> I agree with you Viresh. I guess Mike should update the commit message.
> 
> I would also like add another reason to why this is needed. For some
> clks you would like to do pinctrl operations from a clk hw. But since
> a pinctrl driver likely requires a clk to be prepared|enabled, we run
> into a clk reentrant issue.

Fair enough. This all wants to go into the changelog, so we can
understand why we have this business.

Thanks,

	tglx
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