On 11/6/18 15:27, Takashi Iwai wrote:
I guess that the biggest issue is the understanding of PCM period
wakeup; let me cite the description of a part of your patch:
"But since 3 snd_pcm_period_elapsed(), 3 periods should be available
and it should have been able to write.
If rsnd_pointer during the start was 0x120 which is 3 periods
then 0x248 - 0x120 = 128 it could go on with write."
This assumption can't be applied.
However, the current implementation of dmix is designed to achieve the
lowest latency by putting the data at the exact position being played
back now. This, of course, doesn't guarantee the period wakeup = one
period free to fill like the above. So it's the design choice.
The current code has a workaround for the case of nperiods <= 2, by
setting the initial slave_appl_ptr to the next period start. This
guarantees that the period wakeup = one period to be filled. But its
cost is the start latency; the playback doesn't start immediately but
wait until the next period start.
Takashi
Thanks for this explanation. That helps me to understand.
Now, I am thinking about a solution to align the slave_hw_ptr and
slave_appl_ptr to the slave_period (round down) only on start up.
So we keep this low latency. (At start up it is the same behavior as
known from old sound drivers)
As known from the past in worst case it will drop up to slave_period-1
frames.
If the application uses big periods and the drop is not acceptable a
small slave_period using the var_period feature could be configured.
But I think it is also not acceptable for an application which is using
really big periods to wait sometimes one period longer.
There is always a higher probability of an under run whenever a poll()
wakeup will not result in a write.
For example on systems with high load this could be an issue.
If you are anyway not happy with this solution may be we can think about
an option to disable this feature
like "min_drop_on_start true"
But then we also should describe the drawbacks of this option like
* Delay of snd_pcm_wait() up to 2*periods
* Higher probability of Xruns
Best regards
Timo
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