[alsa-devel] [RFC PATCH v3 2/3] ALSA: hda - add DP mst audio support

[Takashi Iwai]

On Fri, 21 Oct 2016 14:25:28 +0200,
Yang, Libin wrote:
> 
> > > > > > Also, this rewrite changes the loop completely, so we you should
> > > > > > update the comment appropriately.  It's no longer scanning the
> > > > > > all pins (including unused ones).  And I'm not 100% sure whether
> > > > > > it's OK to change in that way; the function was written to do
> > > > > > that (touching the unused pin as well) on purpose, IIRC.
> > > > > >
> > > > > > Hm, or is it OK because we are listing always the three pins?
> > > > > > More explanation please, either in comments or in the changelog.
> > > > > >
> > > > >
> > > > > This function is a little complex now. Please let me explain it in detail.
> > > > >
> > > > > As you know, we need this function because we need fix: same cvt
> > > > > is connecting to several pins. In this situation, playback on one
> > > > > pin will also impact on other pins. Let's assume cvt 1 are
> > > > > connecting to pin 1, 2,
> > > > 3.
> > > > > When we playback on pin1, we need call this function to make sure
> > > > > pin 2, 3 is connected to other cvts.
> > > > >
> > > > > In MST mode,  we are using dynamic pcm. If no pcm is attached to
> > > > > the pin, it means there is no monitor connected to the pin. So we
> > > > > don't need to set the cvt. Let's still assume the cvt 1 are
> > > > > connecting to pin 1, 2, 3, 4, 5, 6, 7, 8, 9 (here pin is the virtual pin).
> > > > >
> > > > > Let's assume playback on pin 1 (which is connected to cvt 1).
> > > > > So we should make sure all others pins are not connected to cvt1.
> > > > >
> > > > > 1) For other pins:
> > > > >     if (!per_pin->pcm)
> > > > >         continue;
> > > > > This means no monitors is connected to the pin. So we don't care
> > > > > the cvt connection because the pin doesn't output sound to monitor.
> > > > > (when the virtual pin is connected to monitor later, this function
> > > > > will be called in present_sense(), which will make sure the cvt
> > > > > connected to the virtual pin is different from the cvt connected
> > > > > to pin 1)
> > > > >
> > > > > 2) For the following situation:
> > > > >     dev_num = snd_hda_get_num_devices(codec, per_pin->pin_nid) + 1;
> > > > >         if (per_pin->dev_id >= dev_num)
> > > > >             continue;
> > > > > The per_pin->dev_id is greater than dev_num. This means no such
> > > > > virtual pin. We should skip this situation.
> > > > >
> > > > > 3) For the following situation:
> > > > >     if (curr != mux_idx) {
> > > > >         snd_hda_set_dev_select(codec, nid, dev_id_saved);
> > > > >         continue;
> > > > >     }
> > > > > It means the cvt connected to the virtual pin is already different
> > > > > to the cvt connected to the pin 1. So we don't need to do anything.
> > > > >
> > > > > 4) Otherwise (the cvt is the same to pin 1), we need change the cvt.
> > > > >
> > > > > I will add more description for this function in the next version patch.
> > > >
> > > > One thing that is still unclear to me is how to deal with the unconfigured
> > pins.
> > > > I thought that the pins with port=non pincfg are still ignored in
> > > > hdmi_add_pin() even after your patch.  So they won't be listed in
> > > > the pins / nids arrays.
> > >
> > > Actually, MST code doesn't touch port=non code. It exists before.
> > >
> > > My understanding of this code is that port=non is HW connection
> > > related. If this pin is not connected to any jack, it means it is not
> > > lined out and the pin is useless for the special machine.
> > >
> > > >
> > > > Now, your new code only loops over the enumerated pins, i.e. these
> > > > unconfigured pins are ignored.  What if these pins are connected to
> > > > the active converters?  I thought excluding such a connection is one
> > > > of the purposes of this function, judging from the function description.
> > >
> > > If the pin is not lined out, no monitor will be connected to the pin.
> > > So even the cvt is connected to the pin, it will impact nothing. For
> > > MST mode, if it is not lined out, there will be never pcm attached to
> > > the pin. This means you will never have chance to operate the cvt
> > connected to the pin.
> > > Besides, I found if there is no monitor connected to the pin, the
> > > connection list is not actually connecting to any cvt for the pin.
> > 
> > But why the current function explicitly states that it does care about the
> > unused pins?  There must be a reason to do so.  I vaguely remember that it
> > was mentioned like that connecting to the unused pin causing some
> > unexpected behavior.
> 
> I don't know the story. Maybe in this situation, mute/unmute will be
> messed. I will check with our internal team to see if someone knows
> this reason.

Mengdong?


> You question reminds me that I should try to find a proper 
> platform to test the unused pin situation. My current test platforms 
> can't cover such situation.

Yes, a wider test coverage would be really appreciated.


Takashi