[alsa-devel] clarification on mmap

[Takashi Iwai]

At Tue, 24 Feb 2009 10:24:19 +0530,
Harsha, Priya wrote:
> 
> Thanks Takashi. Then what would be the best way to know when the
> mmaped buffer has been filled. So that I can take action to send it
> to the hardware? 

In the current ALSA design, mmap transfer is completely asynchronous.
The driver doesn't take care when app_ptr is updated.  It's checked
only when the hw_ptr is updated in snd_pcm_period_elapsed().  So, in
general, what the card driver needs is to provide the ISR calling
snd_pcm_period_elapsed() appropriately and, if possible, to provide
the accurate PCM pointer callback to give the updated hw_ptr.

In mmap mode, other any transfer to the hardware is supposed to be
done by the hardware (DMA) more or less automatically.  If you need to
do it by yourself, mmap isn't always the right answer.

> Should I use the runtime->control->appl_ptr or
> runtime->status->appl_ptr to get the position the app has filled? 

appl_ptr exists only in runtime->control.


HTH,

Takashi

> -----Original Message-----
> From: Takashi Iwai [mailto:tiwai at suse.de] 
> Sent: Tuesday, February 24, 2009 12:55 AM
> To: Harsha, Priya
> Cc: alsa-devel at alsa-project.org
> Subject: Re: clarification on mmap
> 
> At Mon, 23 Feb 2009 22:21:24 +0530,
> Harsha, Priya wrote:
> > 
> > Thank you. Can I use .ack callback to know that the mmaped buffer
> > has been filled by the user?
> 
> Not really.  It's just for explicit read/write modes.
> 
> > How would I know how much the user has
> > written into the buffer that time?
> 
> You can check appl_ptr at any time.  This corresponds to the position
> the app has filled.
> 
> 
> Takashi
> 
> > Would I need to have the pointers
> > calculated and tracked myself or is there a field in the structures
> > that I can read and find out? 
> > 
> > -----Original Message-----
> > From: Takashi Iwai [mailto:tiwai at suse.de] 
> > Sent: Monday, February 23, 2009 8:57 PM
> > To: Harsha, Priya
> > Cc: alsa-devel at alsa-project.org
> > Subject: Re: clarification on mmap
> > 
> > At Mon, 23 Feb 2009 18:59:11 +0530,
> > Harsha, Priya wrote:
> > > 
> > > Hi,
> > > 
> > > I have a question on mmap. If I give my .info to be _MMAP and
> > > _MMAP_VALID. Will ALSA framework internally take care of mmap-ing
> > > the kernel buffer that has been pre-allocated in the .probe call? Do
> > > I need to do anything special to mmap a kernel buffer into user
> > > space? Just accessing the runtime->dma_area would allow me to access
> > > user data right? 
> > 
> > Yes.  The buffers allocated via preallocator are supposed to be
> > mmappable, so you can simply pass _MMAP* flag there.
> > 
> > 
> > Takashi
> > 
>