24.10.2014 01:44, Andrew Eikum wrote:
On Fri, Oct 24, 2014 at 12:38:16AM +0600, Alexander E. Patrakov wrote:
24.10.2014 00:08, Andrew Eikum wrote:
I have a patch in-hand which improves the current driver
significantly, but it isn't enough to fix the USB devices problem. If
someone wants to review the winealsa driver with the patch applied and
discuss solutions, let me know. I'd love to get this problem solved.
If you send me the patch, I promise to have a look at it. But, see
my other reply why there cannot be a full solution within the ALSA
API.
I've attached the patch here. It's been a while since I've worked on
this, so I'm fuzzy on the details. It should apply on any recent Wine
version, including today's.
I have looked at the patch, but have not tested it. I cannot say just by
reading the code whether this is an improvement. I think that the most
important thing here is the removal of the "keep no more than 3 ALSA
periods" logic.
Still, the patch was useful as it forced me to look at the buffering
logic in Wine.
There's a comment above alsa_write_data() describing how the buffer is
laid out and what the offsets point to. alsa_write_data() is where we
actually send data to ALSA. AudioClient_Stop() and _Start() are where
the stream is rewound and paused. AudioClient_Initialize() is where
the device setup occurs. AudioClock_GetPosition() returns the number
of played frames.
Note that we explicitly toggle handle_underrun to TRUE in
make_handle_underrun_config(). Toggling this off does seem to have an
impact on USB devices. The underruns aren't fixed in either setting,
but it's worth playing with.
As I said, the underruns happen because the pulse plugin provides
incorrect information about the actual period size to Wine.
If I remember correctly, the problem with USB devices is that
snd_pcm_avail_update() updates too infrequently. To keep low latency,
we set up a buffer size of four ALSA periods (about 40ms, I think).
But, snd_pcm_avail_update() updates less frequently than that, so we
constantly hit underruns. Some Windows applications behave very badly
with large latencies.
I think the issue was more complicated than that, but I've forgotten
the details in the meantime...
AFAICS we can confirm or disprove this by logging last_period_time.
I believe disabling BATCH mode on USB devices "fixes" everything with
this patch applied, but I presume there was a reason it was introduced
in the first place (see PA bug 66962). I could be wrong here, I never
totally understood the issue.
That bug is exactly what I am talking about.
--
Alexander E. Patrakov
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