[alsa-devel] PulseAudio and SNDRV_PCM_INFO_BATCH

[Lars-Peter Clausen]

On 06/15/2015 01:39 PM, Raymond Yau 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.html
>>>
>>> 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.
>>
>
> DSP_CAP_REALTIME bit tells if the device/operating system supports precise
> reporting of output pointer position using SNDCTL_DSP_GETxPTR. Precise
> means that accuracy of the reported playback pointer (time) is around few
> samples. Without this capability the playback/recording position is
> reported using precision of one fragment.
>
> DSP_CAP_BATCH bit means that the device has some kind of local storage for
> recording and/or playback. For this reason the information reported by
> SNDCTL_DSP_GETxPTR is very inaccurate.
>
> Are those alsa cap have the similar meaning of those oss cap ?

I would say historically SNDRV_PCM_INFO_BATCH had the same meaning as 
DSP_CAP_BATCH, but it has also come to have the inverse meaning to 
DSP_CAP_REALTIME. By default applications can assume that the pointer is 
accurate down to a few samples, but if SNDRV_PCM_INFO_BATCH is set it is 
less accurate and can be everything up to period size precession.

SNDRV_PCM_INFO_BLOCK_TRANSFER is different from all of them though.