[alsa-devel] [PATCH RFC 0/9] audio timestamping evolutions

[Raymond Yau]

>
>
>>
>> #define PERIOD 6000
>>
>> #define PLAYBACK_BUFFERS 4
>>
>> unsigned char buffer_p[PERIOD*4*4];
>> unsigned char buffer_c[PERIOD*4*4];
>>
>> I don't understand why you hard coded PERIOD and PLAYBACK_BUFFERS
>> when you use snd_pcm_set_params
>
>
> Thanks for pointing this out. I have no idea what this is still there, i
think the idea was to use a fixed-size buffer to make sure the timing is
predictable, it's been that way for 2+ years. Will fix.
>
>

https://git.kernel.org/cgit/linux/kernel/git/tiwai/sound.git/commit/sound/pci/hda?id=2ae66c26550cd94b0e2606a9275eb0ab7070ad0e

Seem you assume hda controller support arbitrary period size since 6000
frames is not a multiple of 32 frames

If your intel hda controller support arbritary period size , the difference
between two successive audio time in your example 1 should be 125 ms as
same as example 3 when using usb audio in [alsa-devel] [PATCH RFC 5/9]
ALSA: core: selection of audio_tstamp type and accuracy reports

Refer to  US 20100131783
System and Method of Dynamically Switching Queue Threshold

2. Description of the Prior Art

The DMA 150 has a queue, such as a first-in first-out buffer (“FIFO”) for
maintaining the stream on the HDA link 16 by storing sufficient amount of
data, such that no data under run or overrun occurs. Before sending out
data to the HDA link 16, the HDAC 15 will issue a bus master cycle to
request next stream data from the system memory 13 whenever the amount of
the stream data in the FIFO is less than a threshold value. The FIFO
threshold value and the burst length are associated with the FIFO size, as
shown in Table 1, where h represents a hexadecimal number, and DW
represents a double word (or 4-byte data).

TABLE 1

FIFO size FIFO threshold Burst length

40h DW 31h DW 10h DW
30h DW 21h DW 10h DW
20h DW 19h DW 8h DW
10h DW dh DW 4h DW
8h DW 7h DW 2h DW
4h DW 4h DW 1h DW
Others 4h DW 1h DW

This look like period size should be multiple of brust length

>>
>> since period size and buffer size can be obtained by
>> snd_pcm_get_params() after calling snd_pcm_set_params()
>>
>> if ((err = snd_pcm_set_params(handle_p,
>>                                SND_PCM_FORMAT_S16,
>>                                SND_PCM_ACCESS_RW_INTERLEAVED,
>>                                2,
>>                                48000,
>>                                0,
>>                                500000)) < 0) { /* 0.5sec */
>>
>

http://lists.freedesktop.org/archives/pulseaudio-discuss/2014-October/021898.html

Not all distrubtion use 4Mb buffer, this mean  that most ubuntu users only
have 64k