Hi,
Now for almost all devices syncstart is emulated with a sequence of
SNDCTL_DSP_SETTRIGGER calls. I want however to have a possibility to start
several devices really simultaneously for envy24ht devices (this may be
useful for instance for accurate synchronization of input and output streams
for recording full-duplex applications). As far as I can see from audio core
code, device driver adrv_sync_control is supposed to be used for such
things. However I can't quite understand how it should work.
handle_syncstart() calls sync_control for all devices first with
SYNC_PREPARE flag, then with SYNC_TRIGGER one. These calls are independent
so device driver does not know anything about how many devices and what
exactly devices it should start simultaneoulsy. Without this knowledge
device driver won't be able to start several device at one time even if
sound card supports this.
Now I could find the only way to do what I want:
Device driver supposes that for all devices SYNC_PREPARE should be called
fisrt, then SYNC_TRIGGER for the same devices. Thus it memorizes the last
device for which SYNC_PREPARE was called. When SYNC_TRIGGER are called
driver links all devices into a chain and starts all devices when a
SYNC_TRIGGER is called for the device memorized on SYNC_PREPARE stage. But
it's quite tricky, besides I see some potential problems:
1) handle_syncstart potentially may be terminated prematurely, in this case
some devices may get SYNC_PREPARE command without SYNC_START, it's rather
confusing situation (it depends of course on sync_control implementation,
this is not a problem for the approach described above).
2) If several processes call syncstart in parallel, the order of
sync_control calls may be totally messed up, in this case it's rather
difficult to predict driver behaviour.
So my questions are:
1) Is there any sane and simple (not so tricky) way to start several devices
simultaneously with the current framework? Maybe I missed something?
2) What about potential multi process problems? Wouldn't be worth to use a
lock in handle_syncstart itself?
3) I'm not sure about that, but may be there is a way to improve the
framework itself (this probably may require changes in some drivers though)?
I understand that syncgroups may contain devices belonging to different
sound cards etc, this complicates things a lot, but anyway...
Regards,
Zakhar Levchenko
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