[alsa-devel] [PATCH v2 00/10] audio timestamping evolutions

[Pierre-Louis Bossart]

Thanks for the review Jaroslav

> 1) ext_info member is not required - the standard info field
>     has enough free bits

Well this was added at Takashi's request, the initial patches didn't 
rely on this extension...I can roll back those changes if this is the 
consensus.

> 2) the whole struct snd_pcm_status is R/O -
>     _IOR('A', 0x20, struct snd_pcm_status); I believe that it's much
>     better to add new audio_tstamp_type to sw_params, but see (4)

I thought about this, but
- selecting the timestamp type with sw_params would require multiple 
system calls to achieve the same results. Every additional call or delay 
changes the accuracy of the results and correlation between data 
consumption and timing reports.
- existing code already relies on snd_pcm_status to retrieve system and 
audio timestamps, the selection through other means would make the code 
more complicated.

> 3) accuracy - I would use uint64_t and report accuracy in pico-seconds
>     (range from 0 picoseconds to 18446744 seconds); yes, use next bytes
>     from the reserved part of status struct. the __u32 might be used only
>     for flags

The timestamps are not better than nanoseconds. I don't actually know of 
any link that uses a wallclock higher than 24/48Mhz, so that's already 
~20-40ns already. It seemed overkill to me do use more than 3 
significant digits and an exponent to represent a nominal value that 
doesn't take jitter and drift into account anyway. The idea was to 
provide a qualitative value, not an actual measurement.

> 4) if there is a motivation to call / obtain timestamps for multiple
>     purposes (audio tstamp types), then offer to return all these
>     timestamps in one shot rather than do multiple queries (again, use
>     reserved bytes)

I thought about this case but I couldn't find any practical uses of 
multiple timestamps at the same time. In the absence of any atomic 
hardware snapshots of multiple counters, reading multiple values 
sequentially from different counters would actually water-down the 
accuracy and value of the timestamps returned. It's already hard-enough 
to track a single pair of audio and system counters.