[alsa-devel] PulseAudio and SNDRV_PCM_INFO_BATCH

[Raymond Yau]

>>>>
>>>>
>>>> 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.

 DMA_RESIDUE_GRANULARITY_DESCRIPTOR
DMA_RESIDUE_GRANULARITY_SEGMENT
DMA_RESIDUE_GRANULARITY_BURST

Can this be regard as bad, normal and good accuracy ?

https://bugs.freedesktop.org/show_bug.cgi?id=86262

Although the resolution seem less than period size but the deviation is
quite large , can we still regard this result are accurate up to one period
?

http://lists.freedesktop.org/archives/pulseaudio-discuss/2015-June/024022.html

This is accurate up to one period

Why do we need flag for exactly one period since most of the driver use irq
should increment one period when interrupt occur ?

We only need to know those good accuracy can use timer base and those bad
accuracy need special care