[alsa-devel] SNDRV_PCM_INFO_MMAP - what do I need for it?

[Takashi Iwai]

At Wed, 16 Jan 2008 09:17:57 -0600,
Timur Tabi wrote:
> 
> My 8610 ASoC driver has this:
> 
> static const struct snd_pcm_hardware fsl_dma_hardware = {
> 
> 	.info   		= SNDRV_PCM_INFO_INTERLEAVED,
> 
> This means that MMAP is not enabled.
> 
> I understand what MMAP is, but not from an ALSA driver perspective.  If I change 
> the above line to this:
> 
> 	.info   		= SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_MMAP | 
> SNDRV_PCM_INFO_MMAP_VALID,
> 
> what does ALSA do differently?  Do I need to define any new functions?  From an 
> application perspective, what is different?

It means that the driver supports mmap access to the PCM buffer.

In some cases, you need to implement a proper mmap callback.  For
example, when the coheret (dma) pages are allocated via
dma_alloc_coherent and used for the PCM buffer, you need mmap callback
for ARM, at least because the default mmap handler in the PCM core
doesn't work for ARM.  In this case, prepare your own mmap callback
and pass it to pcm ops.  It should call simply dma_mmap_coherent()
appropriately.


> Also, does DMIX need MMAP support?

Yes, mandatory.


Takashi