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

Rafael J. Wysocki
2007-04-27T18:29:52Z

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