[alsa-devel] A Restriction and rest of issues for this patchsets
o-takashi at sakamocchi.jp
Sat Apr 26 00:58:43 CEST 2014
= A Restriction
Drivers with this patchset has a restriction comparing to windows drivers.
As long as reading manual of PreSonus/Focusrite/PrismSound, these BeBoB
based devices may synchronize to the other BeBoB based devices on the
I guess this mechanism is driven by SYT in CIP packets:
- Driver select a device as 'Sample clock source' then the other
devices are 'Sample clock destination'.
- Driver handle packets from 'Sample clock source' device.
- Driver read SYT and calculate presentation timestamp for each
devices, then transfer packets to the other devices.
If you're interested, please refer to IEC 61883-6:2005 (Edition 2.0).
Actual examples are described in:
Annex D Transport characteristics
D.1.3 Embedded sample-clock jitter
D.220.127.116.11 Example one: simple two-node bus
D.18.104.22.168 Example two: three-node bus
D.22.214.171.124 Example: thirty-five-node system
Well, drivers with this patchset don't support this mechanism. The
reason is bad balance between the const to implement below requirements
and actual advantage to implement these:
- Requirement to manage states of several devices simultaneously
- Requirement to handle several streams from/to each device simultaneously
- Requirement to calculate SYT with the length between host controller
and each device
The drivers synchronized each devices independently. The drivers handle
packets from a device and read SYT, then transfer packets with the SYT
to the device.
= Rest of issues
Currently I focus on enabling ALSA to handle devices. So there are some
issues which don't cause big problems.
I know some Fireworks based devices sometimes transfer packets with
disorder. But current implementation is a lack of sorting packets. The
packets with disorder is detected as dicontinuity then streams are
forced to stop and XRUN is generated. In this case, usual ALSA PCM
applications may recover XRUN, so there may be some XRUNS to start
I need more time to investigate frequency and pattern of this issue.
Near future I want to discuss this issue again with enough data.
2.Data rate for MIDI substream
With current implementation, the maximum data rate for MIDI substream in
out-stream is one eighth of sampling rate. For example, at 48.0kHz, it's
6,000 bytes. This is excess over standard MIDI data rate, 3,125 bits per
second. So a few devices can have buffer over flow when receiving much
MIDI messages. For example, M-Audio Firewire 1814 transmits packets with
discontinuity in this case. I guess that Fireworks based devices and
some BeBoB based devices ignore MIDI messages in more than 8 data blocks
of packets and this is vendor's effort for this issue.
Drivers need a throttle for this issue but this is for future work.
Currently, the minumum number of frames per period is not less than
5msec. Drivers has a flag, SNDRV_PCM_INFO_BATCH and the minimum number
of periods in buffer is 2. As a result, its minimum latency is 10msec.
For more severe latency, further work is needed. But I feel this is not
so higher priority than other issues because actually devices are available.
4.A few devices are freezed when frequent establishing/breaking connections
One of testers reported this issue. 'PreSonus FP10' can be freezed when
frequent establishing/breaking connections. This brings a disadvantage
to PulseAudio. When PulseAudio detects the card, it starts/stops 4
streams at least. If PulseAudio can detect devices with some hacks, it
may take the device freezed.
5.Various results at bus reset
I confirm various results with Fireworks/BeBoB and its devices at bus
reset during streaming, depending on cases.
Case 1. Software do bus reset
When generating bus reset with ffado-test or jujuutils, streaming
stopped. When generating bus reset, the drivers update connections
according to IEC 61883-1. But after a few hundred msec, the device break
connections by itself. I don't know the reason exactly but I guess the
synchronizing mechanism I describe have some relations.
Case 2. Connecting devices or removing devices
This case depends on which devices are connected or removed. If they're
Fireworks/BeBoB based devices, streaming sometimes continues and
sometimes stops. If they're not Fireworks/BeBoB based device, streaming
continues. I don't know the reason exactly but I guess the synchronizing
mechanism I describe have some relations.
o-takashi at sakamocchi.jp
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