On 06/12/2015 02:29 PM, Arun Raghavan wrote:
Hi folks, I'd like to bring this one up again, since we are currently in the sub-optimal position of forcing ~100 ms latency on USB devices. The original thread is here -- http://mailman.alsa-project.org/pipermail/alsa-devel/2013-December/069666.ht...
I see two flags that are possibly of consequence here: SNDRV_PCM_INFO_BATCH and SNDRV_PCM_INFO_BLOCK_TRANSFER. I'm not sure what these mean -- the documentation mentions "double buffering" for the batch flag, and just that the block transfer means "block transfer". :-)
We've spoken about batch meaning either transfers in period size chunks, or some fixed chunk size. It seems that it would make more sense for it to mean the former, and block transfer to mean the latter.
So I guess the first thing that would be nice to have is a clear meaning of these two flags. With this done, we could potentially get to the API to report the transfer size from the driver.
Yeah, the meaning of those flags is somewhat fuzzy and may have changed over time as well. Here is my understanding of the flags, might not necessarily be 100% correct.
SNDRV_PCM_INFO_BLOCK_TRANSFER means that the data is copied from the user accessible buffer in blocks of one period. Typically these kinds of devices have some dedicated audio memory that is not accessible via normal memory access and a DMA is setup to copy data from main memory to the dedicated memory. This DMA transfers the data from the main memory to the dedicated memory in chunks of period size. But otherwise the controller might still be capable of reporting a accurate pointer position down to the sample/frame level.
So SNDRV_PCM_INFO_BLOCK_TRANSFER is mainly important for rewind handling and devices with that flag set might need additional headroom since the data up to one period after the pointer position has already been copied to the dedicated memory and hence can no longer be overwritten.
SNDRV_PCM_INFO_BATCH on the other hand has become to mean that the device is only capable of reporting the audio pointer with a coarse granularity. Typically this means a period sized granularity, but there are some other cases as well.
I did notice that there is a snd_pcm_hw_params_get_fifo_size(). Is this something we could use for the purpose of transfer size reporting, by any chance?
I think the idea with fifo_size() is mainly to aide with audio latency measurement. I.e. how many samples before the pointer() position are still inside the audio pipeline and have not yet reached the analog domain. But since there are now users that rely on it a lot of driver don't really bothered with setting it to the correct value. And for the use case here data is on the wrong side of the pointer() value as well. What we need is some kind of indication of the fuzziness of the pointer() value. E.g. how far can it be away from the actual position in the stream.
- Lars