[alsa-devel] Improving status timestamp accuracy

[Raymond Yau]

2016-06-04 17:31 GMT+08:00 Alan Young <consult.awy at gmail.com>:

> 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 point only increment in DMA brust size , so it is not sample accurate

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/tree/include/linux/dmaengine.h

* @DMA_RESIDUE_GRANULARITY_BURST: Residue is updated after each transferred

 *  burst. This is typically only supported if the hardware has a
progress *  register of some sort (E.g. a register with the current
read/write address *  or a register with the amount of
bursts/beats/bytes that have been *  transferred or still need to be
transferred).


if the driver point callback does not read from hardware register , it
should use


DMA_RESIDUE_GRANULARITY_DESCRIPTOR: Residue reporting is not support.
The *  DMA channel is only able to tell whether a descriptor has been
completed or *  not, which means residue reporting is not supported by
this channel. The *  residue field of the dma_tx_state field will
always be 0.




>
> 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.
>



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