Re: [PATCH v3 3/5] clk: introduce the common clock framework

[Uwe Kleine-König]

Hello,

On Fri, Dec 02, 2011 at 08:23:06PM +0000, Russell King - ARM Linux wrote:
> On Fri, Dec 02, 2011 at 10:13:10AM -0700, Paul Walmsley wrote:
> > Hi Russell,
> > 
> > On Thu, 1 Dec 2011, Russell King - ARM Linux wrote:
> > 
> > > On Wed, Nov 30, 2011 at 06:20:50PM -0700, Paul Walmsley wrote:
> > > > 1. When a clock user calls clk_enable() on a clock, the clock framework 
> > > > should prevent other users of the clock from changing the clock's rate.  
> > > > This should persist until the clock user calls clk_disable() (but see also 
> > > > #2 below).  This will ensure that clock users can rely on the rate 
> > > > returned by clk_get_rate(), as long as it's called between clk_enable() 
> > > > and clk_disable().  And since the clock's rate is guaranteed to remain the 
> > > > same during this time, code that cannot tolerate clock rate changes 
> > > > without special handling (such as driver code for external I/O devices) 
> > > > will work safely without further modification.
> > > 
> > > So, if you have a PLL whose parent clock is not used by anything else.
> > > You want to program it to a certain rate.
> > > 
> > > You call clk_disable() on the PLL clock.
> > 
> > The approach described wouldn't require the PLL to be disabled before 
> > changing its rate.  If there are no other users of the PLL, or if the 
> > other users of the PLL have indicated that it's safe for others to change 
> > the PLL's rate, the clock framework would allow the PLL rate change, even 
> > if the PLL is enabled.  (modulo any notifier activity, and assuming that 
> > the underlying PLL hardware allows its frequency to be reprogrammed while 
> > the PLL is enabled)
> > 
> > > This walks up the tree and disables the parent.  You then try to set the 
> > > rate using clk_set_rate(). clk_set_rate() in this circumstance can't 
> > > wait for the PLL to lock because it can't - there's no reference clock 
> > > for it.
> > 
> > As an aside, this seems like a good time to mention that the behavior of 
> > clk_set_rate() on a disabled clock needs to be clarified.
> 
> It's more complicated than that.  Clocks have more states than just
> enabled and disabled.
> 
> There is:
> - unprepared
> - prepared and disabled
> - prepared and enabled
> 
> Implementations can chose at which point to enable their PLLs and wait
> for them to lock - but if they want to sleep while waiting, they must
> do this in the prepare method, not the enable method (remember, enable
> is to be callable from atomic contexts.)
> 
> So, it's entirely possible that a prepared clock will have the PLLs up
> and running, which means that clk_set_rate() can also wait for the PLL
> to stablize (which would be a good idea.)
> 
> Now... we can say that PLLs should be locked when the prepare method
> returns, or whenever clk_set_rate() returns.  The problem with that is
> there's a race condition between clk_enable() and clk_set_rate() - if
> we allow clk_set_rate() to sleep waiting for the PLL to lock, a
> concurrent clk_enable() can't be prevented from returning because
> that would involve holding a spinlock...
But you can achieve that the concurrent clk_enable fails. Would that be
sane?

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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