[alsa-devel] [PATCH] ALSA: control: confirm to return all identical information in 'activate' event data

[Takashi Iwai]

At Mon, 09 Feb 2015 17:02:51 +0100,
Takashi Iwai wrote:
> 
> At Tue, 10 Feb 2015 01:01:15 +0900,
> Takashi Sakamoto wrote:
> > 
> > On Feb 10 2015 00:28, Takashi Iwai wrote:
> > > At Tue, 10 Feb 2015 00:02:06 +0900,
> > > Takashi Sakamoto wrote:
> > >>
> > >> When event originator doesn't set numerical ID in identical information,
> > >> the event data includes no numerical ID, thus userspace applications
> > >> cannot identify the control just by unique ID in event data.
> > >>
> > >> This commit fix this bug so as the event data includes all of identical
> > >> information.
> > >>
> > >> Signed-off-by: Takashi Sakamoto <o-takashi at sakamocchi.jp>
> > >
> > > You can't overwrite id pointer in this case.  It's a caller's object.
> > 
> > Exactly. I missed it...
> > 
> > > One way to fix would be to copy the id instance.  Another way would be
> > > to change up/down_write() with _read(), and include snd_ctl_notify()
> > > call with kctl->id inside the semaphore lock.
> > >
> > > The former would be less changes but consume the stack significantly.
> > 
> > If any sub-effects are allowed, we can use caller's data via the 
> > pointer, like:
> > memcpy(id, &kctl->id, sizeof(struct snd_ctl_elem_id);
> > Then:
> > snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_INFO, id);
> 
> That's what I suggested as the first example.

... and I think this is a better way in the end.  Simpler, better.
The stack usage is very likely acceptable.


Takashi