Re: IO plugin and multiple poll descriptors

[Takashi Iwai <tiwai@suse.de>]

At Wed, 18 May 2005 14:05:43 +0200,
Marcel Holtmann wrote:
> 
> Hi Takashi,
> 
> > > > > > for the IO plugin we can specify one descriptor for poll().
> > > > > > 
> > > > > > 	io.poll_fd = fd;
> > > > > > 	io.poll_events = POLLIN;
> > > > > > 
> > > > > > I like to use more than one, because in the Bluetooth cases I always
> > > > > > have to deal with a signal and a separate media channel. Is it possible
> > > > > > to add support for that or do I must work around it.
> > > > > 
> > > > > Currently, the alsa-lib handles only one poll_fd internally although
> > > > > API allows the multiple poll_fds.  So, it's not quite easy to add the
> > > > > multiple fds...
> > > > 
> > > > are there any plans to change this in the future?
> > > > 
> > > > Maybe it is a good idea to retrieve the poll_fd through a callback. I
> > > > think of something like you already do in the library:
> > > > 
> > > > 	poll_descriptors(snd_pcm_ioplug_t *io, struct pollfd *pfds)
> > > > 	poll_descriptors_count(snd_pcm_ioplug_t *io)
> > > > 
> > > > Or do you have a good idea on how to workaround it?
> > > 
> > > I agree, adding the appropriate new callbacks would be the simplest
> > > solution.  I'll implement it soon if no one is against it.
> > 
> > I added the new callbacks to CVS.
> > The following two fields are added to snd_pcm_ioplug_callback_t:
> > 
> > 	/**
> > 	 * poll descriptors count; optional
> > 	 */
> > 	int (*poll_descriptors_count)(snd_pcm_ioplug_t *io);
> > 	/**
> > 	 * poll descriptors; optional
> > 	 */
> > 	int (*poll_descriptors)(snd_pcm_ioplug_t *io, struct pollfd *pfd, unsigned int space);
> > 
> > If they are NULL, poll_fd is used as well as in the former version.
> 
> should we set poll_fd to -1 or leave it untouched when using the new
> callbacks.

It doesn't matter.  If the callback is defined, it's always used.
-1 would be better to catch possible bugs, though.

> We can set poll_events through the callback now, right? I like to have
> different events for different descriptors. One POLLIN and one POLLOUT.

Yes.  The callback has to set poll_events, too.

> What happens at the moment when I really return two poll_fd.
> 
> > This change breaks the compatibility with older versions.
> > For the future compatibility, I added a new field to
> > snd_pcm_ioplug_t.
> 
> Does it? If the new callbacks are not used you will fall back to poll_fd
> and so no difference at all.

Well, I inserted in the middle, not at the end.

> And you never released a final version with IOPLUG support.

Yes.  I don't care the compatibility *now* at all.
But, as I wrote, this is for *future* compatibility.  Once after
1.0.9-final is out, we should be careful about it.


Takashi


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