[PATCH v3 1/3] rust: clk: use the type-state pattern

[Daniel Almeida]

<snip>

>> 
>> The problem with what you have suggested is that the previous state is not
>> consumed if you can clone it, and consuming the previous state is a pretty key
>> element in ensuring you cannot misuse it. For example, here:
>> 
>> let enabled_clk = prepared_clk.clone().enable()?;
>> // do stuff
>> // enabled_clk goes out of scope and releases the enable
>> // ref it had
>> 
>> prepared_clk is still alive.
> 
> That was intentional in this example. Think about a prepared_clk that's
> stored in some driver-internal object, because you want to keep the clk
> prepared at all times between the probe() and unbind(). Then you have
> some sections where you want to briefly enable the clk to access
> registers, and immediately disable it when you're done. The clone()
> here guarantees that the initial prepared_clk stays valid.
> 
> If you were to disable, unprepare and put the clk when enabled_clk goes
> out of scope, you'd go

> 
> let enabled_clk = prepared_clk.enable()?;
> 
> and that would still work, it's just not the same use-case.
> 

Ok, let’s have clone() then.


>> Now, this may not be the end of the world in this
>> particular case, but for API consistency, I'd say we should probably avoid this
>> behavior.
>> 
>> I see that Alice suggested a closure approach. IMHO, we should use that
>> instead.
> 
> The closure, while being useful for the above local clk-enablement
> example, doesn't allow for passing some Clk<Enabled> guard around, like
> you would do with a lock Guard, and I believe that's a useful thing to
> have.


Wdym? You’d still get a &Clk<Enabled> that you can pass around, i.e.:

   self.prepared_clk.with_enabled(|clk: &Clk<Enabled> | {
       ... use registers, pass &Clk<Enabled> as needed
   });

This is now not about clone() vs not clone(), but more about limiting the scope of the
Enabled state, which would cater to the use-case you mentioned IIUC.

— Daniel