[alsa-devel] .pointer return value

[Giuliano Pochini]

On Thu, 23 Apr 2009 19:34:08 +0530
"Harsha, Priya" <priya.harsha at intel.com> wrote:

> Can anyone educate me on what value should be returned to .pointer function?
> 	Should I be passing the number of frames played till that point from the time stream started or thnumber of frames played such that the bytes is within the buf_size?

The latter: it's the index of the last transferred frame in the ring buffer.


> And if I am using, snd_pcm_indirect_playback_pointer, what should be my input to it?
> 	Should I be passing the number of bytes played till that point from the time stream started or the number of bytes within buf_size?

In practice, they're both fine. After a quick look at the sources
in /include/sound/pcm-indirect.h:

----
static inline snd_pcm_uframes_t
snd_pcm_indirect_playback_pointer(struct snd_pcm_substream *substream,
				struct snd_pcm_indirect *rec, unsigned int ptr) {
        int bytes = ptr - rec->hw_io;
        if (bytes < 0)
                bytes += rec->hw_buffer_size;
        rec->hw_io = ptr;
----

It cares about the difference from the last position to the current one and
it adds hw_buffer_size if ptr wrapped around.

Side note:
I can guess it may not behave correctly in case of long latencies when
bytes < -hw_buffer_size. Perhaps should the "if" be changed to "while" ?



-- 
Giuliano.