[alsa-devel] Buffer memory does not get allocated?

Clemens Ladisch clemens at ladisch.de
Fri Nov 5 09:23:05 CET 2010

Radivoje Jovanovic wrote:
> I am allocating memory for the buffers in my init call by doing:
> snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_CONTINUOUS,
> snd_dma_continuous_data(GFP_KERNEL), 256*1024, 256*1024);
> in my hw_params call I have:
> ret= snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
> here is the output from my hw_params call:
> called snd_pcm_lib_malloc_pages return=1
> params_buffer_bytes(hw_params)=32768
> Size of DMA area=32768
> Buffer size in bytes=262144
> Buffer virt address=9dc00000
> Buffer phys address=0
> Now I am not sure why is my buffer size bigger than my DMA area.

You did not say which values are output, but 262144 is 256*1024, so this
is the preallocated buffer.  The DMA is what is actually used with the
current hw_params.

> I assume that memory is not linear because of that

SNDRV_DMA_TYPE_CONTINUOUS gives you continguous pages.

> and that is why I do not have phys address to the buffer.

It doesn't give you the physical address; it looks as if this memory is
intended to be accessed only by the CPU.

To get coherent memory with a DMA address for a specific device, use
SNDRV_DMA_TYPE_DEV.  (This must be supported by your architecture.)


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