Trigger Callback

Trigger Callback

[Julien Bramary]

I'm an Alsa beginner and I'm trying to write an alsa driver for an embedded dac on ARM.

>From Iwai's doc:

/* trigger callback */
static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
{
    switch (cmd) {
    case SNDRV_PCM_TRIGGER_START:
            // do something to start the PCM engine
            break;
    case SNDRV_PCM_TRIGGER_STOP:
            // do something to stop the PCM engine
            break;
    default:
            return -EINVAL;
    }
}

My question here is,
What are we supposed to do to start the PCM engine?
Because, I'm currently doing nothing, and yet it starts anyway...
I'd like to understand to have more control.



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Re: Trigger Callback

[Lee Revell]

On Tue, 2006-08-01 at 20:48 +0100, Julien Bramary wrote:
> I'm an Alsa beginner and I'm trying to write an alsa driver for an embedded dac on ARM.
> 
> >From Iwai's doc:
> 
> /* trigger callback */
> static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream, int cmd)
> {
>     switch (cmd) {
>     case SNDRV_PCM_TRIGGER_START:
>             // do something to start the PCM engine
>             break;
>     case SNDRV_PCM_TRIGGER_STOP:
>             // do something to stop the PCM engine
>             break;
>     default:
>             return -EINVAL;
>     }
> }
> 
> My question here is,
> What are we supposed to do to start the PCM engine?
> Because, I'm currently doing nothing, and yet it starts anyway...
> I'd like to understand to have more control.

"PCM engine" refers to the hardware's PCM engine, not ALSA's.  It's 100%
device dependent - you need to consult the docs for your sound device to
determine how to start the PCM.

Lee


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Re: Trigger Callback

[Julien Bramary]

Lee Revell wrote:

>>  > "PCM engine" refers to the hardware's PCM engine, not ALSA's.  It's 100%
>>  > device dependent - you need to consult the docs for your sound device to
>>  > determine how to start the PCM.
>>  >
>>  > Lee
Thanks, I see now why it starts, I'm doing some inits in my dac in the
open callback... I should do those here in the trigger.



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Re: Trigger Callback

[Takashi Iwai]

At Tue, 01 Aug 2006 21:14:24 +0100,
Julien Bramary wrote:
> 
> Lee Revell wrote:
> 
> >>  > "PCM engine" refers to the hardware's PCM engine, not ALSA's.  It's 100%
> >>  > device dependent - you need to consult the docs for your sound device to
> >>  > determine how to start the PCM.
> >>  >
> >>  > Lee
> Thanks, I see now why it starts, I'm doing some inits in my dac in the
> open callback... I should do those here in the trigger.

Remember that the trigger callback is designed really as a "trigger".
That is, it should be executed as shortly as possible.  The callback
is even in the atomic context within spinlock and irq disablement, so
any function that my sleep shall cause lockup.

If you need to allocate something, it should be done in either
open, hw_params or prepare callback.  Only start and stop are done in
trigger callback.


Takashi

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Re: Trigger Callback

[Julien Bramary]

Takashi Iwai wrote:
> At Tue, 01 Aug 2006 21:14:24 +0100,
> Julien Bramary wrote:
>   
>> Lee Revell wrote:
>>
>>     
>>>>  > "PCM engine" refers to the hardware's PCM engine, not ALSA's.  It's 100%
>>>>  > device dependent - you need to consult the docs for your sound device to
>>>>  > determine how to start the PCM.
>>>>  >
>>>>  > Lee
>>>>         
>> Thanks, I see now why it starts, I'm doing some inits in my dac in the
>> open callback... I should do those here in the trigger.
>>     
>
> Remember that the trigger callback is designed really as a "trigger".
> That is, it should be executed as shortly as possible.  The callback
> is even in the atomic context within spinlock and irq disablement, so
> any function that my sleep shall cause lockup.
>
> If you need to allocate something, it should be done in either
> open, hw_params or prepare callback.  Only start and stop are done in
> trigger callback.
>
>
> Takashi
>
>   
Thanks for the reminder. I think this should be ok. The only function I 
now have in the trigger is the following 'write' (atomic enough):
   /* enable SSC data Tx */
   at91_ssc_write(AT91_SSC_CR, AT91_SSC_TXEN);

Cheers,
Julien


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