Re: [PATCH 1/8] ALSA: pcm: add a helper function to constrain mask-type parameters

[Takashi Iwai <tiwai@suse.de>]

On Thu, 08 Jun 2017 09:28:37 +0200,
Takashi Sakamoto wrote:
> 
> Hi,
> 
> On Jun 8 2017 16:10, Takashi Iwai wrote:
> > On Thu, 08 Jun 2017 01:10:19 +0200,
> > Takashi Sakamoto wrote:
> >>
> >> Application of constraints to mask-type parameters for PCM substream is
> >> done in a call of snd_pcm_hw_refine(), while the function includes much
> >> codes and is not enough friendly to readers.
> >>
> >> This commit splits the codes to a separated function so that readers can
> >> get it easily. I leave desicion into compilers to merge the function into
> >> its callee.
> >>
> >> Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
> >> ---
> >>   sound/core/pcm_native.c | 57 ++++++++++++++++++++++++++++++++-----------------
> >>   1 file changed, 38 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> >> index 2bde07a4a87f..b3e8aab3915e 100644
> >> --- a/sound/core/pcm_native.c
> >> +++ b/sound/core/pcm_native.c
> >> ...
> >> +	struct snd_mask __maybe_unused old_mask;
> >
> > Do we really need __maybe_unused?  Drop it as much as possible.
> > IMO, it can be uglier than ifdef, since you don't know why it's
> > unused.  With ifdef, at least, you have an idea about the condition.
> 
> In kernel documentation[1], we can see below suggestion.
> 
> "20) Conditional Compilation
> ...
> If you have a function or variable which may potentially go unused in a
> particular configuration, and the compiler would warn about its
> definition going unused, mark the definition as __maybe_unused rather
> than wrapping it in a preprocessor conditional.  (However, if a
> function or variable *always* goes unused, delete it.)"
> 
> I'll follow this.

It doesn't answer my question.  Without __maybe_unused in your code,
do you get the compile warning?  If yes, why?

Well, I see the usage already in your patch for the tracepoints.
But __maybe_unused is really ugly, and should be avoided as much as
possible.

The text above doesn't recommend to use it blindly.   It's the last
resort.


thanks,

Takashi