Re: Monitoring ALSA's write pointer (appl_ptr)?

Re: Monitoring ALSA's write pointer (appl_ptr)?

[Takashi Iwai]

At Mon, 3 Nov 2008 23:40:28 -0500,
<brian-welch@cox.net> wrote:
> 
> Hello List,
> 
> My first post, so excuse any ignorance.
> 
> Is there a definitive/safe way to monitor ALSA's write pointer in the 
> dma_area?  It seems like the ack callback and the appl_ptr is the 
> correct way, but I was told by another developer that appl_ptr doesn't 
> work in all cases.
> 
> As a future optimization for my hardware, I may want to tell ALSA to use 
> a specific area of physical memory for it's dma_area, what is the best 
> way to do that?

A good question.  The ack callback is called only when the explicit
read/write method is used.  When mmap access is used, it won't be
called.

One hack would be to check appl_ptr in the pointer callback.  This
callback is called in snd_pcm_hwsync(), too.  So, if you watch the
appl_ptr change as frequently as possible, it'd be a good place for
that.


Takashi

Re: Monitoring ALSA's write pointer (appl_ptr)?

[Brian Welch]

>> Is there a definitive/safe way to monitor ALSA's write pointer in the
>> dma_area?  It seems like the ack callback and the appl_ptr is the
>> correct way, but I was told by another developer that appl_ptr doesn't
>> work in all cases.
>>
>> As a future optimization for my hardware, I may want to tell ALSA to use
>> a specific area of physical memory for it's dma_area, what is the best
>> way to do that?
>
> A good question.  The ack callback is called only when the explicit
> read/write method is used.  When mmap access is used, it won't be
> called.
>
> One hack would be to check appl_ptr in the pointer callback.  This
> callback is called in snd_pcm_hwsync(), too.  So, if you watch the
> appl_ptr change as frequently as possible, it'd be a good place for
> that.
>
> Takashi

Thanks.  I'll try reading appl_ptr from within the pointer callback.

Brian