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

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Message-ID: <alpine.LFD.0.98.0704270842430.9964@woody.linux-foundation.org>
Date:	Fri, 27 Apr 2007 08:52:05 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Johannes Berg <johannes@...solutions.net>,
	Alan Stern <stern@...land.harvard.edu>,
	Nick Piggin <npiggin@...e.de>, Ingo Molnar <mingo@...e.hu>,
	suspend2-devel@...ts.suspend2.net, Mike Galbraith <efault@....de>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Con Kolivas <kernel@...ivas.org>, Adrian Bunk <bunk@...sta.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Pavel Machek <pavel@....cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [linux-pm] driver power operations (was Re: suspend2 merge)

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
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