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

[Yang, Libin]

Hi Takashi,

> -----Original Message-----
> From: Takashi Iwai [mailto:tiwai at suse.de]
> Sent: Monday, October 24, 2016 4:41 PM
> To: Yang, Libin <libin.yang at intel.com>
> Cc: libin.yang at linux.intel.com; alsa-devel at alsa-project.org; Lin, Mengdong
> <mengdong.lin at intel.com>
> Subject: Re: [alsa-devel] [RFC PATCH v3 2/3] ALSA: hda - add DP mst audio
> support
> 
> On Mon, 24 Oct 2016 09:09:07 +0200,
> Yang, Libin wrote:
> >
> >
> > > -----Original Message-----
> > > From: Takashi Iwai [mailto:tiwai at suse.de]
> > > Sent: Friday, October 21, 2016 8:33 PM
> > > To: Yang, Libin <libin.yang at intel.com>
> > > Cc: libin.yang at linux.intel.com; alsa-devel at alsa-project.org; Lin,
> > > Mengdong <mengdong.lin at intel.com>
> > > Subject: Re: [alsa-devel] [RFC PATCH v3 2/3] ALSA: hda - add DP mst
> > > audio support
> > >
> > > 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?
> >
> > The patch of checking AC_JACK_PORT_NONE is from Stephen Warren, who
> is
> > from nvidia. I have checked with Mengdong, and she didn't know the
> > patch. But it seems to be similar with intel_not_share_xxx() function.
> 
> Well, ignoring the unused port there is basically irrelevant with the behavior
> in intel_not_share_*().  It was introduced in commit
> 116dcde638065a6b94344ca570e1e79c8e857826
>     ALSA: HDA: Remove unconnected PCM devices for Intel HDMI
> 
> Meanwhile intel_not_share_*() stuff was introduced much later in commit
> 7ef166b831237e67b2ea83ce0c933c46ddd6eb26
>     ALSA: hda - Avoid choose same converter for unused pins

Get it. It is used to reduce the user confusion. Thanks.

Regards,
Libin

> 
> 
> Takashi