[alsa-devel] [PATCH 12/13] drm: bridge/dw_hdmi-ahb-audio: add audio driver

Anssi Hannula anssi.hannula at iki.fi
Sun May 10 13:47:07 PDT 2015

10.05.2015, 22:33, Russell King - ARM Linux kirjoitti:
> On Sun, May 10, 2015 at 09:59:42PM +0300, Anssi Hannula wrote:
>> What I'd like to see is arrive at some sort of general consensus on how
>> the AES bits should be handled (i.e. should the driver always set them
>> themselves and disallow/allow the userspace to override the rate bits),
>> which could then be applied to other drivers as well.
>> But maybe that is for another time, or just a futile effort altogether...
> My personal view is that where we're dealing with PCM audio, the driver
> needs to set these bits correctly as there is nothing in userspace to
> do this.  This provides an identical interface between each audio device
> which accepts PCM samples - whether it's a SPDIF or non-SPDIF based
> device.
> For non-audio data sent via an audio device, the AES bits need to be
> conveyed from userspace, and we should respect what userspace gives us.
> (If it's wrong, it's a userspace bug, and userspace should be fixed,
> rather than trying to work around the bug by patching the kernel.)

Just to make sure I didn't misunderstand. You propose looking at the
"non-pcm" (aka "non-audio") bit in the AES to see whether driver/kernel
should force rate (and maybe other) AES bits?
Because I think that is currently the only way for the kernel/driver to
know if the data is "non-audio".

>> Indeed. I did notice there is a SND(RV)_PCM_FORMAT_SPECIAL but I guess
>> it might not be easily used for this purpose since it doesn't have a
>> specific sample width etc (but I am not familiar enough with this to say
>> whether it could work or not)...
> I spent quite a while looking at alsa-lib, wondering whether I could
> move all the conversions out to userspace, but I couldn't without
> building them _into_ alsa-lib.  This was a while back now, but from
> what I remember, plugins to alsa-lib which aren't built as part of
> alsa-lib are not able to do format conversions.

Sounds a bit strange (assuming one'd plainly reference the plugin from
<hw>.conf directly), but OK. I'm not suggesting to look into it again
unless you really want to yourself :)

>>> However, in the case of VLC, if it wants to send non-audio, it will
>>> open the IEC958 device, which will use the iec958 plugin to configure
>>> the AES bits for non-audio, and pass IEC958 data to the kernel (which
>>> still needs to be reformatted to the hardware's special format.)
>> Ah, so the AES bits are actually overridable by userspace, which is what
>> I was initially concerned with :)
>> Of course, this means that applications opening "iec958" but not setting
>> rate bits (which is common) will get the default 48kHz bits from
>> /usr/share/alsa/pcm/(iec958|hdmi).conf). Not sure how big an issue that
>> is, though. The "iec958" ALSA plugin does seem to have a FIXME comment
>> about setting AES bits according to sample rate.
> Note that VLC does set the "sample" rate appropriately:
>         switch (aout->format.i_rate)
>         {
> #define FS(freq) \
>             case freq: aes3 = IEC958_AES3_CON_FS_ ## freq; break;
>             FS( 44100) /* def. */ FS( 48000) FS( 32000)
>             FS( 22050)            FS( 24000)
>             FS( 88200) FS(768000) FS( 96000)
>             FS(176400)            FS(192000)
> #undef FS
>             default:
>                 aes3 = IEC958_AES3_CON_FS_NOTID;
>                 break;

Yep, one of the few that do.

>>> <confdir:pcm/iec958.conf>
>>> dw-hdmi-ahb-aud.pcm.iec958.0 {
>> I think you should s/iec958/hdmi/ for the above two lines. HDMI devices
>> should be using "hdmi" instead of "iec958" by convention (the latter is
>> used for optical/coaxial S/PDIF).
> Except doing that kills VLC's passthrough option (denoted by "spdif"),
> which explicitly wants the iec958 device:
>     /* Choose the IEC device for S/PDIF output:
>        if the device is overridden by the user then it will be the one.
>        Otherwise we compute the default device based on the output format. */
>     if (spdif && !strcmp (device, "default"))
>     {
> ...
>         if (asprintf (&device,
>                       "iec958:AES0=0x%x,AES1=0x%x,AES2=0x%x,AES3=0x%x",
>                       IEC958_AES0_CON_EMPHASIS_NONE | IEC958_AES0_NONAUDIO,
>                       IEC958_AES1_CON_ORIGINAL | IEC958_AES1_CON_PCM_CODER,
>                       0, aes3) == -1)
> Yes, technically an application bug, since VLC should allow the device
> to be selectable and/or detect hdmi devices.  I wonder if that's
> something which has changed between 2.0.8 and the latest vlc.

Yes, in the current version it appends the parameters to the configured
device if it contains "iec958" or "hdmi" (git log says iec958 was
un-hardcoded on the above line in Jan 2014).

So if you have selected the "HDMI Output" device from the audio dropdown
in settings, it would work.

However, the default device "default" is still mangled to "iec958", (the
code has a "TODO: hdmi" comment).

> I did consider having the hdmi output device, but also alias that to
> the iec958 device name, which I think can be done via:
> <confdir:pcm/hdmi.conf>
> dw-hdmi-ahb-aud.pcm.hdmi.0 {
> ...
> }
> <confdir:pcm/iec958.conf>
> dw-hdmi-ahb-aud.pcm.iec958.0 cards.dw-hdmi-ahb-aud.pcm.hdmi.0

If a userspace workaround is wanted, I think I'd prefer this one myself.

Personally, I don't believe a workaround is of much use here, because
using "iec958" is already broken for majority of HDMI users anyway (i.e.
those that use HDMI outputs on x86 PCs)... but I don't see the
workaround as strictly unacceptable, either.

Also, if this used "hdmi" and no workaround, i.MX6 devices with both
S/PDIF and HDMI connectors would then have just a single iec958* device
and a single hdmi* device. But I guess that doesn't matter *that* much...

> However, for HDMI sinks, I haven't seen any way in Linux to allow
> userspace to know the audio capabilities of the attached HDMI sink,
> and thus whether the video player can output compressed MPEG audio.
> Anyone know?

HDA driver exports the ELD as a mixer control, which the application can
then use to determine the capabilities.

Not the best idea IMHO (I think ELD should've preferably remained
internal since I think it should be considered an implementation detail,
plus it is not very application friendly), but it is what it is and I
guess it was the easiest way to do it from ALSA side.

I know pulseaudio uses the ELD control to:
- get receiver name
- trigger a device change event when it changes

Kodi uses it to:
- get receiver name
- get default passthrough (compressed audio) capabilities
- trigger re-enumeration and device reopening when it changes
(e.g. if the user starts audio playback before turning on their
amplifier/receiver, we might've started the playback with
less-than-optimal parameters due to ALSA driver ELD rate/channel
restrictions, since the amplifier/receiver might've passed us the
probably-more-limited monitor/TV SADs instead of its own SADs when the
receiver was in standby mode).

But I wouldn't be too surprised if those were the only users.

Anssi Hannula

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