Messages in this thread | | | From | Mike Turquette <> | Subject | Re: [PATCH v4] clk: allow reentrant calls into the clk framework | Date | Wed, 27 Mar 2013 07:25:19 -0700 |
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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
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