Thus spake Takashi Iwai:
First of all, thanks for your answer. However,
At Tue, 08 May 2007 13:47:47 +0200, Markus Korber wrote:
IRQ1 +----------------+ | page_3 (right) |\ +----------------+ > double buffer 1 | page_2 (left) |/ IRQ0 +----------------+ | page_1 (right) |\ +----------------+ > double buffer 0 | page_0 (left) |/ +----------------+If I understand correctly, the samples for a channel (e.g. left) is practically split to two regions, page 0 and page 2 in the above?
Correct.
If yes, this h/w design is hard to implement mmap mode straightforwardly because the alsa-lib (and apps) assume "linear" buffers. For normal read/write without mmap, this shouldn't be that hard as you guess, though.
Are there any drawbacks if I use the R/W mode without mmap, regarding applications (mplayer), dmix, etc.?
Furthermore, in mmap mode, I think I could use an intermediate buffer (with the layout as above), where I copy the data from the (linear) mmap'd buffer to, could I? Maybe using helpers from struct pcm-indirect.h? But this is not very beautiful.
A workaround would be to use SG-buffer (if possible). The constraint is thta both double buffers above have to be aligned in PAGE_SIZE (depending on architecture, 4KB mostly).
It's a custom chip with an ARM core and an AMBA-AHB bus. PAGE_SIZE alignment wouldn't be a problem, although then buffer_size would be fixed to multiples of PAGE_SIZE, i guess?
I'm not quite sure how to implement this:
chip_pcm_new - snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_CONTINUOUS, snd_dma_continuous_data(GFP_KERNEL), PAGE_SIZE, PAGE_SIZE)
chip_hw_params - snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params)) - map sg-pages with snd_pcm_sgbuf_get_addr()
Why/How?
I mean, my hardware DMA layout is fixed (apart from the double buffer page sizes, which should be a multiple of PAGE_SIZE). So do I have to call snd_pcm_lib_malloc_pages() and/or do I need to call dma_map_sg() with a scatterlist? Or how do I tell ALSA about my DMA layout? And how is the application informed about this?
For implementing a SG buffer, you can use either mmap or page pcm_ops. The mmap is to do the whole mmap operation, so you can do all what you'd like.
My understanding is, that the application mmap's my allocated buffer (currently allocated with dma_alloc_writecombine) and the ALSA library tells it, where it is allowed to put its data. (ALSA gets this info through the pointer() callback and calls to snd_pcm_period_elapsed.) Thus, this mmap() callback is only called once in normal mmap mode.
Now, if I would use a SG approach, how would I handle this mmap operation in my callback? I guess, I have to provide similar functionality as in dma_mmap() but with 2 different base addresses?
The page is a callback used by the default ALSA mmap handler and you can return the page pointer corresponding to the given buffer offset.
This approach looks easier to me. Am I right, that here I would just call snd_pcm_sgbuf_ops_page() from my page() callback handler? What else do I need to do in this case to let the application know which buffer it should fill?
Regards, Markus Korber