2016-06-04 17:31 GMT+08:00 Alan Young <consult.awy@xxxxxxxxx>: > 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. > > > > > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
