[RFC 1/1] clk: Add notifier support in clk_prepare_enable/clk_disable_unprepare

[bilhuang@nvidia.com (Bill Huang)]

On Sat, 2013-03-16 at 01:09 +0800, Russell King - ARM Linux wrote:
> On Tue, Mar 12, 2013 at 10:40:04PM -0700, Bill Huang wrote:
> > That will be too bad, it looks like we deadlock in the mechanism, we
> > cannot change existing drivers behavior (that means some call
> > clk_disable/enable directly, some are not), and we cannot hook notifier
> > in clk_disable/enable either, that means there seems no any chance to
> > get what we want, any idea?
> 
> Look, the whole point is:
> 
> - Drivers can call clk_enable/clk_disable from their atomic regions to
>   control the clock.  Drivers which do this also call clk_prepare/
>   clk_unprepare from a schedulable context to perform any operations
>   necessary to allow the clock to be used.
> 
> - Drivers which only ever control the clock from a schedulable context
>   *can* use clk_prepare_enable()/clk_disable_unprepare() to control
>   their clock, which simplifies the coding in the driver.
> 
> The whole point here is to cater for what is found on different SoCs and
> not need to keep rewriting the drivers between different SoCs.
> 
> So, the idea is that:
> 
> - clk_prepare() does whatever is needed to prepare a clock for use which
>   may require waiting for the clock to be in a state which it can be
>   enabled.  In other words, if there is a PLL, the PLL is setup and
>   we wait for it to report that it has locked.
> 
> - clk_enable() is about turning the clock output on so that the device
>   receives the clock.
> 
> Now, in the case of a PLL directly feeding a device, it's entirely possible
> that clk_prepare() ends up providing the clock signal to the device, and
> clk_enable() does absolutely nothing.
> 
> Or, if the clock has a gate on it, it's entirely possible that clk_prepare()
> does nothing, and clk_enable() unmasks the gate to allow the clock to be
> provided to the device - which can happen from atomic contexts.
> 
> The whole point about the separation of these two functions is that device
> driver writers _can_ code their drivers for both situations and not care
> about how the SoC implements the clocking at all.
> 
> Why did we end up with this split in the first place?  Because we ran into
> the problem that some SoCs required a sleeping clk_enable() and others
> didn't, and the whole thing was turning into an incompatible mess.
> 
> So, please.  Realise that clk_prepare() and clk_enable() are the _official_
> APIs, and that clk_prepare_enable() is merely a helper function for drivers
> to allow them to automate the calling of those two functions in succession
> with _no_ _further_ _processing_ at all.
> 
> So, if your hooks need to be callable from schedulable contexts, then you
> need to put them inside clk_prepare().  If your hooks are callable from
> atomic contexts, then they can go into clk_enable().  But what you can
> not do is put them into clk_prepare_enable().

Thanks a lot for good point.