From: Mike Turquette <mturquette@linaro.org>
To: Thomas Gleixner <tglx@linutronix.de>,
Ulf Hansson <ulf.hansson@linaro.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>,
linaro-kernel@lists.linaro.org, patches@linaro.org,
linux-kernel@vger.kernel.org,
Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
David Brown <davidb@codeaurora.org>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4] clk: allow reentrant calls into the clk framework
Date: Wed, 27 Mar 2013 07:25:19 -0700 [thread overview]
Message-ID: <20130327142519.4014.82657@quantum> (raw)
In-Reply-To: <alpine.LFD.2.02.1303271141470.22263@ionos>
Quoting Thomas Gleixner (2013-03-27 04:09:17)
> On Wed, 27 Mar 2013, Ulf Hansson wrote:
> > On 27 March 2013 10:55, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> > > On 27 March 2013 15:10, Thomas Gleixner <tglx@linutronix.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.
>
The issue arises any time a clk_ops callback calls a function that
unwittingly re-enters the clock framework. I think the easiest example
to understand and perhaps the most common in practice is a clock which
is controlled via an i2c transfer.
Viresh's example makes the mistake of calling
clk_enable(i2c_controller), but it must also call
clk_prepare(i2c_controller) which is missing in the call graph above.
That nested call to clk_prepare is where the reentrancy comes from.
This has nothing to do with the prepare/enable locking split and leaves
that relationship intact.
> > >
> > > 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.
>
I'll submit a v5 which I hope will end the pain and suffering this patch
has caused you.
Regards,
Mike
> Thanks,
>
> tglx
next prev parent reply other threads:[~2013-03-27 14:25 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-03-27 7:09 [PATCH v4] clk: allow reentrant calls into the clk framework Mike Turquette
2013-03-27 9:08 ` Laurent Pinchart
[not found] ` <20130327150634.4014.64797@quantum>
2013-03-27 17:12 ` Thomas Gleixner
2013-03-27 9:40 ` Thomas Gleixner
2013-03-27 9:55 ` Viresh Kumar
2013-03-27 10:03 ` Ulf Hansson
2013-03-27 11:09 ` Thomas Gleixner
2013-03-27 14:25 ` Mike Turquette [this message]
2013-03-27 9:59 ` Laurent Pinchart
2013-03-27 11:24 ` Thomas Gleixner
[not found] ` <20130327164716.4014.97638@quantum>
2013-03-27 17:09 ` Thomas Gleixner
2013-03-27 22:56 ` Russell King - ARM Linux
2013-03-28 4:45 ` [PATCH v5 0/2] reentrancy in the common " Mike Turquette
2013-03-28 4:45 ` [PATCH 1/2] clk: abstract locking out into helper functions Mike Turquette
2013-03-28 9:31 ` Thomas Gleixner
2013-03-28 4:45 ` [PATCH 2/2] clk: allow reentrant calls into the clk framework Mike Turquette
2013-03-28 9:33 ` Thomas Gleixner
2013-03-28 15:23 ` Mike Turquette
2013-03-28 10:44 ` [PATCH v5 0/2] reentrancy in the common " Laurent Pinchart
2013-03-28 20:59 ` [PATCH v6 " Mike Turquette
2013-03-28 20:59 ` [PATCH 1/2] clk: abstract locking out into helper functions Mike Turquette
2013-04-02 9:23 ` Ulf Hansson
2013-03-28 20:59 ` [PATCH 2/2] clk: allow reentrant calls into the clk framework Mike Turquette
2013-04-02 9:35 ` Ulf Hansson
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