Locking in the clk API

[Jeremy Kerr]

Hi all,

I'd like to define some locking semantics for the clk API, so that device 
driver can have some assurance about when it is safe to call various clk_ 
functions from atomic contexts.

Vincent - you mentioned in a conversation that your code had some specific 
requirements for clock enable/disable at interrupt time - could you reply with 
an outline of this? Also, Sascha and Uwe: do you have any thoughts from your 
work with the common struct clk?

== Requirements ==

To get things started, here are some basic requirements from the external API 
side:

1) clk_enable: the clock should be outputting a valid clock signal before
   returning from this function.

    - drivers may require valid clock signal to be present for subsequent
      device interactions.

3) clk_disable: may be called from atomic context

    - Vincent: this is what I recall from our conversation, is it still true?

4) clk_set_rate: clock should change to the new rate before this returns

5) clk_get_rate: may be called from atomic context

6) in general, drivers shouldn't require details about the clock
   implementation

And from the clock implementation side:

7) interactions with clock hardware may require sleeping (eg, clocks on an i2c
   bus)

8) clk_enable() may require enabling a parent, which may also require
   sleeping. Ideally, we shouldn't have to care about the parent's
   implementation.

I'm sure there are others, please feel free to add to this list.

== Implementation ==

At present, we can satisfy these with:

* clk_enable: may sleep

* clk_disable: may not sleep, but it's possible to make the global
  clk_disable() atomic and defer the actual disable (clk->ops.disable()) to a
  non-atomic context.

* clk_get_rate: may not sleep

* clk_set_rate: may sleep

As we build up our requirements, we can adjust as suitable.

I'm excluding clk_{get,set}_parent at present, as I'm not sure we want these 
as part of the device-driver API (ie, they require knowledge of the platform 
clock infrastructure).

Thanks,


Jeremy
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