ALSA provides several different buffer allocation functions
depending on the bus and the architecture. All these have a
consistent API. The allocation of physically-contiguous pages is
done via
snd_malloc_xxx_pages() function, where xxx
is the bus type.
The allocation of pages with fallback is
snd_malloc_xxx_pages_fallback(). This
function tries to allocate the specified pages but if the pages
are not available, it tries to reduce the page sizes until
enough space is found.
The release the pages, call
snd_free_xxx_pages() function.
Usually, ALSA drivers try to allocate and reserve a large contiguous physical space at the time the module is loaded for the later use. This is called “pre-allocation”. As already written, you can call the following function at pcm instance construction time (in the case of PCI bus).
snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
snd_dma_pci_data(pci), size, max);
where size is the byte size to be
pre-allocated and the max is the maximum
size to be changed via the prealloc proc file.
The allocator will try to get an area as large as possible
within the given size.
The second argument (type) and the third argument (device pointer)
are dependent on the bus.
In the case of the ISA bus, pass snd_dma_isa_data()
as the third argument with SNDRV_DMA_TYPE_DEV type.
For the continuous buffer unrelated to the bus can be pre-allocated
with SNDRV_DMA_TYPE_CONTINUOUS type and the
snd_dma_continuous_data(GFP_KERNEL) device pointer,
where GFP_KERNEL is the kernel allocation flag to
use.
For the PCI scatter-gather buffers, use
SNDRV_DMA_TYPE_DEV_SG with
snd_dma_pci_data(pci)
(see the
Non-Contiguous Buffers
section).
Once the buffer is pre-allocated, you can use the
allocator in the hw_params callback:
snd_pcm_lib_malloc_pages(substream, size);
Note that you have to pre-allocate to use this function.