Re: DMA feature question.

[Takashi Iwai <tiwai@suse.de>]

At Sat, 26 Jun 2004 15:44:32 +0100,
James Courtier-Dutton wrote:
> 
> I have a sound card that requires access to 2 DMA areas per substream.
> 1) Is for the audio samples buffer. I can already do that with:
> 
> err = snd_pcm_lib_preallocate_pages(substream, SNDRV_DMA_TYPE_DEV, 
> snd_dma_pci_data(emu->pci), 32*1024, 32*1024);
> 
> 
> 2) Is for a table of periods..
> This table will contain:-
> { u32 Pointer_into_samples_buffer,
> u32 number_of_bytes }
> So, each record in the table is 8 bytes long, and identify each period 
> in the buffer.
> 
> So, the table will contain a list of the different periods.
> So, if there are 8 periods, the table will have 8 entries, each 8 bytes 
> long, so I will need a DMA map for 64 bytes.
> 
> Is there any way to use the snd_pcm_lib_preallocate_pages() for this 
> "table or periods".

No.  It's for the DMA buffer only.

> If not, I will just have to make (1) a bit bigger, and place the "table 
> of periods" in there.

I don't think it's a good idea.
The buffer will be mmapped to the user-space, and obviously you don't
want to expose it.

I'd suggest to allocate a table (one page should be large enough) via
snd_dma_alloc_pages() statically at the driver initialization, and
sets the max. number of periods according to the page size.


Takashi


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