On 17.06.2015 20:09, David Henningsson wrote:
>
>
> On 2015-06-17 11:19, Takashi Iwai wrote:
>> Well, USB-audio has another problem.  USB-audio uses the intermediate
>> audio ring buffer, and the samples are copied to each URB buffer.  At
>> each packet complete, the driver copies the rest of sample chunk
>> again, and advances the hwptr when the packets.  So, the hwptr of
>> USB-audio is in advance of the actual sample position.  But we provide
>> the runtime delay information for user-space to correct to the more
>> accurate sample position.  So far, so good.
>>
>> A missing piece in this picture is, however, the position of the
>> not-yet-queued samples in ring buffer.  Basically you can rewrite /
>> rewind the sample at most this point, but not farther -- such
>> in-flight samples can't be modified any longer.  This can be seen a
>> kind of hardware fifo with a pretty big and non-continuously variable
>> size during operation.
>>
>> In that sense, get_fifo() looks like a candidate for giving such
>> information, indeed.  But reviving the old (and rather bad working)
>> API appears dangerous to me.  I'd prefer creating a new API function
>> instead, if any.
>>
>> BTW, because of its design like above, a large (or no) period size
>> doesn't help for saving power at all with USB-audio.  This should be
>> considered before the further discussion...
>
> Hmm...I was trying to understand this power save argument. I tried to
> figure out a "typical" URB size by just plugging my headset in, and I
> saw wMaxPacketSize being 96 and/or 192 bytes.
> Then, MAX_PACKS is set to either 6 (or 48 for USB 2.0 devices, but this
> is just a headset).
>
> Can this be correct? Does it mean that we are getting interrupts every
> 192 * 6 bytes (i e, every 6 ms for a 48kHz/stereo/16bit stream)?

At least that's how often the position gets updated in the worst case. I 
have attached a report for a Logitech USB headset and a test program 
where you can modify the buffer and period sizes. A line is logged every 
time snd_pcm_avail() returns a different value.

-- 
Alexander E. Patrakov