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

Linus Torvalds
2007-04-27T15:53:47Z

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*.

Linus