[alsa-devel] HD-Audio/HDMI: avoid missing start of audio stream on audio reconfiguation or after S3

23 Dec 2013

      Hi Takashi,
We hope to continue discussing the workaround to avoid HDMI data missing when audio configuration changes (eg. stereo ->5.1 channel) or after S3.
The basic idea is to insert silent audio stream playback on audio configuration changes.
Please see updates below ...
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-----Original Message-----
From: Takashi Iwai [mailto:tiwai@suse.de]
Sent: Thursday, December 19, 2013 9:27 PM
To: Lin, Mengdong
Cc: Wu, Fengguang; Li, Jocelyn; Girdwood, Liam R; Chehab, John
Subject: Re: Could you share some advice to check missing start of audio
stream in HDMI audio playback?
At Thu, 19 Dec 2013 12:45:03 +0000,
Lin, Mengdong wrote:
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Many thanks, Takashi and Fengguang!
Windows team met same issue. They also confirmed the latency for
monitor/AV receiver to reconfigure audio setup is not avoidable.
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And their workaround is to send silent audio data after each stream
reconfiguration, before playing back the valid audio stream.
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Do you feel this workaround should be implemented in driver or in
user-space?
Hard to say.  An easier for us would be user-space, but it can be done in
the driver (e.g. in the prepare callback), too, although the implementation
would become tricky.  We'd need a static silence page and play it in
hda_intel.c as a special requirement in the prepare, then set the BLD back
to the normal buffer at the end of azx_pcm_prepare().  A bit complex,
but certainly doable.
For Intel display engine, including Haswell/Ivybridg/Sandibridge, HW can generate silent audio data on driver request during a link underrun.
So for these platforms, we can 
- enable this "HW internal fabrication" feature in i915 driver. HW supported 32kHz, N*44,1kHz, N*48kHz.
- and add specified sleep at the end of azx_pcm_prepare() or generic_hdmi_playback_pcm_prepare()
This can avoid audio driver to prepare silent data and transmit over HD-A link. Is it doable?
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Doing this in may generate extra latency to open a stream. And the
length of the steam depends on experience.
Yes, it's purely depending on the receiver hardware.
Can we add a kernel parameter to specify the length of the silent stream?
It could be 0 by default, meaning no silent stream insertion and thus no extra latency for playback.
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And silent streams in different formats need to be prepared since there
can be wrapped compressed stream for playback.
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So I'd like such a workaround in audio middleware in user-space.
For the compress audio, it's a bit different problem, indeed.
We need to check whether HW supports compressed data format.
But at least we can apply this feature on PCM format.
Thanks
Mengdong
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Takashi
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Thanks
Mengdong
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-----Original Message-----
From: Takashi Iwai [mailto:tiwai@suse.de]
Sent: Thursday, December 19, 2013 7:55 PM
To: Wu, Fengguang
Cc: Lin, Mengdong; Li, Jocelyn; Girdwood, Liam R; Chehab, John
Subject: Re: Could you share some advice to check missing start of
audio stream in HDMI audio playback?
At Thu, 19 Dec 2013 15:38:40 +0800,
Fengguang Wu wrote:
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Mengdong,
On Thu, Dec 19, 2013 at 02:30:33PM +0800, Lin, Mengdong wrote:
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Hi Takashi/Fengguang
Sorry to interrupt. Would you please share some advice to check
a HDMI audio issue below?
For HDMI audio playback under Linux on Haswell/Valleyview, start
of an audio stream (2~4 seconds) may be missing before we can
hear the
sound, in two cases:
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     1^st playback after S3. First several seconds of audio

stream can be
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missing for some monitor, while other monitor does not have this
issue.
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     Switching from playing stereo PCM to 5.1 channel

PCM.
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About first 2
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seconds of the 5.1 channel PCM cannot be heard from the AV
receiver.
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It seems some monitor/AV receiver can drop audio data before
playing sound. We wonder if we could improve in audio/gfx driver.
Have you ever met similar issue on previous platforms?
We’ll be appreciated if you could share us some tips to check
this
issue.
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I once fixed a similar issue, hope it's useful information for you.
commit 5779191e0efd851fb0d54698c13cb4f5325caca6
Author:     Wu Fengguang fengguang.wu@intel.com
AuthorDate: Wed Nov 18 12:38:06 2009 +0800
Commit:     Takashi Iwai tiwai@suse.de
CommitDate: Wed Nov 18 07:46:19 2009 +0100
ALSA: intelhdmi - sticky stream id and format

We tracked down the first-0.5s-hdmi-audio-samples-lost

problem
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to the
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AC_VERB_SET_CHANNEL_STREAMID command. It is suspected

that
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many HDMI
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sinks need some time to adapt to the new state.

The workaround is to avoid changing stream id/format

whenever
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possible.
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Proposed by David.

Signed-off-by: David Härdeman <david@hardeman.nu>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Yeah, sounds like a similar problem.
In general, such a kind of drop is due to the receiver trying to
synchronize the stream at the beginning of the playback.  So, the
patch above essentially keeps the audio widget untouched as long as
it's left unchanged.  But, after S3, this can't be avoided.  Ditto
for changing from stereo to 5.1.  It's reconfiguring the audio setup
bits, thus it has to be reset.  That said, the problem is likely
unavoidable in both situations.
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Of course, my wile guess might be wrong.  I'd apply happily any fix
patches :)
Takashi

Lin, Mengdong

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