Re: [linux-pm] driver power operations (was Re: suspend2 merge)

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Friday, 27 April 2007 17:52, Linus Torvalds wrote:
> 
> On Fri, 27 Apr 2007, Rafael J. Wysocki wrote:
> > 
> > I think we can use 'stages' and pass them as arguments to the functions.
> 
> No, no NOOOO!
> 
> If you use stages, just describe them in the function name instead.	
> 
> > quiesce(PREPARE) -- that may be needed for drivers that allocate much memory
> > before quiescing devices (if any)
> > ...
> > quiesce(PRE_SNAPSHOT)
> > ...
> > quiesce(PRE_SNAPSHOT_IRQ_OFF)
> 
> There is *no* advantage to this (and _lots_ of disadvantages) compared to 
> saying
> 
> 	dev->snapshot_prepare(dev);
> 	dev->snapshot_freeze(dev);
> 	dev->snapshot(dev)
> 
> The latter is
>  - more readable
>  - MUCH easier for programmers to write readable code for (if-statements 
>    and case-statements are *by*definition* more complicated to parse both 
>    for humans and for CPU's - static information is good)
>  - allows for the different stages to have different arguments, and 
>    somewhat related to that, to have better static C type checking.
> 
> Look here, which one is more readable:
> 
> 	int some_mixed_function(int arg)
> 	{
> 		do_one_thing();
> 		if (arg == SLEEP)
> 			do_another_thing();
> 		else
> 			do_yet_another_thing();
> 	}
> 
> or
> 
> 	int do_sleep(void)
> 	{
> 		do_one_thing();
> 		do_another_thing();
> 	}
> 
> 	int prepare_to_sleep(void)
> 	{
> 		do_one_thing();
> 		do_yet_another_thing();
> 	}
> 
> and quite frankly, while the second case may take more lines of code, 
> anybody who says that it's not clearer what it does (because it can 
> "self-document" with function names etc) is either lying, or just a really 
> bad programmer. The second case is also likely faster and probably not 
> larger code-size-wise either, since it does static decisions _statically_ 
> (since all callers are realistically going to use a constant argument 
> anyway, and the argument really is static).
> 
> Finally, the second case is *much* easier to fix, exactly because it 
> doesn't mix up the cases. You can change the arguments, you can have 
> totally different locking, you don't need things like
> 
> 	int gfp = (arg == SLEEP) ? GFP_ATOMIC : GFP_KERNEL;
> 
> etc, and it's just more logical.
> 
> So don't overload a function. That's the *bug* with the current 
> "dev->suspend()" interface already. Don't re-create it. The current one 
> overloads two *totally*different* operations onto one function. 
> 
> Just don't do it. Not in the suspend part, not *ever*.

OK, I won't.

Greetings,
Rafael
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