Q1: Linked streams and drivers

Q1: Linked streams and drivers

[Eliot Blennerhassett]

Greetings,

I know there is some mechanism for linking streams together so that a
single trigger command starts/stops all streams.

Is the underlying driver notified of the link? and if so how?

Reason is that in our hardware we have the ability to link streams at
the hardware (on card firmware actually) level, so that one command
starts multiple streams.

thanks

Eliot
AudioScience Inc.

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Re: Q1: Linked streams and drivers

[Takashi Iwai]

At Thu, 01 Feb 2007 16:31:01 +1300,
Eliot Blennerhassett wrote:
> 
> Greetings,
> 
> I know there is some mechanism for linking streams together so that a
> single trigger command starts/stops all streams.
> 
> Is the underlying driver notified of the link? and if so how?

A PCM substream has a linked list of link members (via struct
snd_pcm_group).  The macro, snd_pcm_group_for_each(), gives you the
loop for checking each list memeber.

When a driver supports the hardware-linked substreams, it needs to
check whether multiple streams exist in that substream.  Set
snd_pcm_trigger_done() at each stream, and trigger once.

You can see examples in the driver implementations.  Grep
snd_pcm_group_for_each() under alsa-kernel/pci directory.


Takashi

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Re: Q1: Linked streams and drivers

[Eliot Blennerhassett]

On Friday 02 February 2007 03:03, Takashi Iwai wrote:
> At Thu, 01 Feb 2007 16:31:01 +1300,

>
> A PCM substream has a linked list of link members (via struct
> snd_pcm_group).  The macro, snd_pcm_group_for_each(), gives you the
> loop for checking each list memeber.
>
> When a driver supports the hardware-linked substreams, it needs to
> check whether multiple streams exist in that substream.  Set
> snd_pcm_trigger_done() at each stream, and trigger once.
>
> You can see examples in the driver implementations.  Grep
> snd_pcm_group_for_each() under alsa-kernel/pci directory.

Thanks Takashi,

I see that the above loops are in the trigger callbacks of the drivers.

But I wonder if the linked status known at hwparams or prepare callback.
This is because I need to send a message to our card to group each pair of 
streams,  I think this should not be in the trigger callback.

I.e.  group(stream0,stream1); group(stream0,stream2) etc

After this, start and stop commands on any stream in the group operate on them 
all simultaneously, and the simultaneous start is sample accurate.

I'd prefer to do this earlier in the setup process than at trigger callback.
-- 
--
Eliot Blennerhassett
AudioScience Inc.

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Re: Q1: Linked streams and drivers

[Takashi Iwai]

At Fri, 2 Feb 2007 09:19:34 +1300,
Eliot Blennerhassett wrote:
> 
> On Friday 02 February 2007 03:03, Takashi Iwai wrote:
> > At Thu, 01 Feb 2007 16:31:01 +1300,
> 
> >
> > A PCM substream has a linked list of link members (via struct
> > snd_pcm_group).  The macro, snd_pcm_group_for_each(), gives you the
> > loop for checking each list memeber.
> >
> > When a driver supports the hardware-linked substreams, it needs to
> > check whether multiple streams exist in that substream.  Set
> > snd_pcm_trigger_done() at each stream, and trigger once.
> >
> > You can see examples in the driver implementations.  Grep
> > snd_pcm_group_for_each() under alsa-kernel/pci directory.
> 
> Thanks Takashi,
> 
> I see that the above loops are in the trigger callbacks of the drivers.
> 
> But I wonder if the linked status known at hwparams or prepare callback.

Not at the prepare time.  Since snd_pcm_hw_params() in alsa-lib
invokes the prepare too, and usually snd_pcm_link() is called after
hw_params set up, the link state is practically unknown before
trigger.

> This is because I need to send a message to our card to group each pair of 
> streams,  I think this should not be in the trigger callback.
> 
> I.e.  group(stream0,stream1); group(stream0,stream2) etc
> 
> After this, start and stop commands on any stream in the group operate on them 
> all simultaneously, and the simultaneous start is sample accurate.
> 
> I'd prefer to do this earlier in the setup process than at trigger callback.

We'd need to introduce some notification in this case.

One way is to add a new callback for link notification.  This is
cleaner but would affect all drivers.

Another way is to call ops->ioctl callback from snd_pcm_link() with a
special type, SNDRV_PCM_IOCTL1_LINK, for example.

In both cases, handling an error from the notify callback is a bit
difficult because of complex locking and linked lists.  Otherwise, the
implementation would be relatively easy, I guess.

Anyways, snd_pcm_link() should check the PCM state more carefully,
accept linking only in either SETUP and PREPARE state, too.


Takashi

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