Re: [patch] oxygen: clean up. make precedence explicit

[Dan Carpenter <error27@gmail.com>]

On Fri, Feb 19, 2010 at 11:33:30AM +0100, Bernd Petrovitsch wrote:
> On Fre, 2010-02-19 at 13:10 +0300, Dan Carpenter wrote:
> > On Fri, Feb 19, 2010 at 09:29:05AM +0100, Clemens Ladisch wrote:
> > > > This doesn't change anything, but I think it makes the code clearer.
> > > > It silences a smatch warning:
> > > > sound/pci/oxygen/oxygen_mixer.c +91 dac_mute_put(7) warn: add some parenthesis here?
> > > 
> > > That message doesn't say why some parentheses should be added.
> > > And it's a question; how do I give it the answer "no"?  :-)
> > > 
> > > > -	changed = !value->value.integer.value[0] != chip->dac_mute;
> > > > +	changed = (!value->value.integer.value[0]) != chip->dac_mute;
> > > 
> > > This doesn't look any clearer to me; I don't think that the unary
> > > negation operator could be thought to have lower precedence than "!=".
> > 
> > Well, it's hard to argue that it's more ambiguous.  :P
> But it doesn't make the code clearer - unless you are a C novice. Unary
> operators generally bind stronger than others - be it "+", "-", "!",
> "~", "*".
> I would expect kernel programmers to know that (and I don't assume
> in-depth knowledge of operator precedence rules).
> 
> > > Why does smatch warn about this combination?  Do such errors actually
> > > happen:
> > 
> > Yep.  I have made some myself when writing smatch.
> > 
> > For example here are some related bugs in the current kernel.
> > 
> > drivers/staging/rtl8192u/ieee80211/ieee80211_wx.c
> >    721          if (!ext->ext_flags & IW_ENCODE_EXT_GROUP_KEY &&
> Well, I see potential bugs here and the if() should have been
> a) if (!(ext->ext_flags & IW_ENCODE_EXT_GROUP_KEY) &&

Yep.  This is clearly what the code should say.

The problem in the original code is that IW_ENCODE_EXT_GROUP_KEY is not 
equal to either 1 or to 0.  (So that means the condition in the original 
code is always false).

> b) if (!ext->ext_flags && IW_ENCODE_EXT_GROUP_KEY &&
> So you one has to look at the driver for the correct fix (and perhaps
> both of above are wrong).
> 
> And I don't see what parenthesis around a logical negations can help
> with the above error example.
> 

Basically often when people write:
	if (!foo == bar) { ...

What they mean is:
	if (!(foo == bar)) { ...

But if they really do mean the original code they could just write 
this so it's clear to everyone: 
	if ((!foo) == bar) { ...

To me it's like "==" vs "=".  Of course, every programmer knows the 
what the difference is but it helps to have gcc warn about adding the 
extra parenthesis.  Maybe I suck, but it definitely has helped me in 
then past.

I don't have strong feelings about this btw.  The original code in
oxygyn_mixer works fine.  I just was making some changes to smatch and
that was a new warning today.  There is no method to my madness.

regards,
dan carpenter


> 	Bernd
> -- 
> Bernd Petrovitsch                  Email : bernd@petrovitsch.priv.at
>                      LUGA : http://www.luga.at