At Tue, 08 May 2007 13:47:47 +0200, Markus Korber wrote: > > Hi, > > I'm working with a custom sound chip with its own DMA hardware. The > figure shows how the I2S DMA expects data to be prepared: > > IRQ1 +----------------+ > | page_3 (right) |\ > +----------------+ > double buffer 1 > | page_2 (left) |/ > IRQ0 +----------------+ > | page_1 (right) |\ > +----------------+ > double buffer 0 > | page_0 (left) |/ > +----------------+ > > The hardware has 2 double buffers, each containing left and right sample > regions, i.e. it alternately transmits samples from page_0 and page_1, > i.e. double buffer 0, then generates an interrupt and starts > transmitting page_2 and page_3. After that another interrupt is > generated and the process begins again with double buffer 0. > > So my question is, how to tell ALSA about this memory layout? Is it > possible to operate in MMAP mode with SNDRV_PCM_INFO_NONINTERLEAVED? > But then, how to tell the application that it should either fill double > buffer 0 or 1? Or should I use RW mode and copy samples manually to > another buffer? Or something else? SG-DMA? page()-callback? 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? 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. 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). 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. 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. Takashi _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
