UAC2: are UAC2 latency controls included in snd_pcm_status_get_delay?
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- To: "alsa-devel@xxxxxxxxxxxxxxxx" <alsa-devel@xxxxxxxxxxxxxxxx>
- Subject: UAC2: are UAC2 latency controls included in snd_pcm_status_get_delay?
- From: Pavel Hofman <pavel.hofman@xxxxxxxxxxx>
- Date: Thu, 6 Jan 2022 08:59:55 +0100
- User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0
Hi,
The UAC2 ch. "5.2.4.8 Latency Control" specifies latencies optionally
reported by the device for individual entities. Especially the overall
terminal latency TE_LATENCY_CONTROL is interesting.
Some video players include the delay reported by
snd_pcm_status_get_delay into their AV sync calculation.
A linux USB audio gadget can perform some advanced DSP, which including
the output device latency can result in a non-negligible overall
latency. The gadget userspace software could configure the actual value
to be reported by the gadget to the host in the selector UAC2_TE_LATENCY
(audio_v2.h). The UAC2-specced max value of over 4 secs is more than
sufficient for standard use cases.
But I could not find any read of the UAC2_TE_LATENCY control selector in
the snd-usb-audio driver (
https://elixir.bootlin.com/linux/latest/C/ident/UAC2_TE_LATENCY ).
Please does the driver include this value into the overall delay
reported through alsa-lib? If not, would it make sense to add such support?
Thanks a lot,
Pavel.
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