I am looking at ways to get more accurate status timestamp information for various SoC drivers. The data that is obtained by snd_pcm_status(). One route would be to implement the more accurate timestamp mechanisms that currently are only available for HDA and Skylake (which I think is the SoC version of HDA).
Looking at the code however, I think that may be unnecessary, at least for my purposes. It may not actually be practical in many cases.
A call to snd_pcm_status() result in snd_pcm_update_hw_ptr0() being called. This gets the current output position (pos) via substream->ops->pointer(substream) and then makes all the other calculations based on the result. In theory, the result of substream->ops->pointer() could be sample accurate but in practice it is very unlikely to be better than period accurate. In fact, if I read it right, it will just about always be accurate to the point of the last period interrupt. Even when a DMA driver claims support of DMA_RESIDUE_GRANULARITY_BURST, it is often the case that the actual granularity is a period.
The consequence of all that is that, for most drivers, the accuracy of a status report is period time. The result values, tstamp & audio_tstamp, are calculated using the current time and the pos estimate from above.
snd_pcm_update_hw_ptr0() is also called when there is a DMA interrupt. At that time the calculate results will be accurate, or at worst consistently inaccurate (there could be a constant offset). It would be useful if a snd_pcm_status() call would simply return the results from the point of the last interrupt, and not try to estimate a current value based on the inaccurate substream->ops->pointer() result. It could either: (a) return the result from the time of the last interrupt, in which case tstamp would be in the past and driver_tstamp would be now; or (b) update audio_tstamp based on the elapsed time since it was recorded. (b) effectively abandons the idea that a current position report will be accurate outside of an interrupt callback but, even if it is, doing so is unlikely to result in any loss of accuracy in practice (assuming a drift of better than 40ppm and period time < 100ms).
Any comments on either of these approaches? I guess (b) is more compatible with the current model.
Alan.