Hi Takashi, Thanks for your explanation. My request was not clear, since my understanding of ALSA is lacking and I need to understand more... The driver I currently have is here: http://www.pecore.ch/test/ep93xx-i2s.c.txt It's really messy, it performs all the format conversions in the driver (they could be done in the alsa-lib) and it lacks some functionalities, so I would like to make it shorter, better and more readable. The driver does not currently define SNDRV_PCM_INFO_MMAP and the memory is copied in the copy callback. I would like to delete the copy callback (which is not really used), since the samples could be written directly to the dma buffer by alsa-lib. How does alsa-lib know how to store the samples in the dma-buffer? How does alsa-lib know that the samples are interleaved, occupy 32 bits each one (even if the format is 24 bits) and that the left channel samples are at index #0 and the left ones at index #1? Or that between two consecutive samples in mono there should be a 4 bytes gap? I suppose this is set in the formats filed of the snd_pcm_hardware_t struct, but I did not find a clear explanation of that... Thanks again Andrea --- Takashi Iwai <tiwai@xxxxxxx> wrote: > At Tue, 17 Apr 2007 01:03:24 -0700 (PDT), > Ciaccia wrote: > > > > Hi all, > > I am trying to fix (improve?) a driver for an > embedded > > ARM device I bought months ago, and for some > reasons > > some ALSA applications work fine while other ones > > don't... > > > > Looking at the driver I got, I noticed that the > info > > field in the snd_pcm_hardware_t struct just > defines: > > > > SNDRV_PCM_INFO_INTERLEAVED | SNDRV_PCM_INFO_PAUSE > > > > while the info in the tutorial "writing an alsa > > driver" defines: > > > > SNDRV_PCM_INFO_MMAP | > > SNDRV_PCM_INFO_INTERLEAVED | > > SNDRV_PCM_INFO_BLOCK_TRANSFER | > > SNDRV_PCM_INFO_MMAP_VALID > > > > Furthermore, the driver defines a copy callback > > (actually 2, one for playback and one for > capture), > > where it copies the data from the use space to the > dma > > buffer. Since the dma buffer was preallocated with > > snd_pcm_lib_preallocate_pages_for_all and is > > accessible from the "outside", I wonder why the > > original author did this... > > > > I have some questions for you gurus: > > > > - what is the difference between > SNDRV_PCM_INFO_MMAP > > and SNDRV_PCM_INFO_MMAP_VALID? What is > > SNDRV_PCM_INFO_BLOCK_TRANSFER? > > MMAP flag is a very important flag. This indicates > that the driver > can work on mmap mode. If not given, no mmap is > allowed. > > Meanwhile, MMAP_VALID and BLOCK_TRANSFER flags are > only for kernel-OSS > emulation. For ALSA native apps, they have no > meaning. > MMAP_VALID is necessary for allowing OSS mmap mode. > BLOCK_TRANSFER is specified, it resets REALTIME > capability in OSS > ioctl. > > > - The code calls > snd_pcm_lib_preallocate_pages_for_all > > function as following: > > > > /* allocate the pcm(DMA) memory */ > > ret = snd_pcm_lib_preallocate_pages_for_all(pcm, > > SNDRV_DMA_TYPE_DEV,0, 4*128*1024, 4*128*1024); > > > > is the DMA_TYPE right for an ARM device? Is NULL a > > correct value for data in this case? > > No, it should pass the device pointer there. > Also make sure that snd_pcm_lib_malloc() is actually > used in hw_params > callback. Otherwise, pre-allocation makes no sense. > > > - In case I get rid of the copy callback, how do I > > specify the format of the stream? How does Where > can I > > find an example for that? > > The available formats are specified snd_pcm_hardware > formats. > Then in prepare callback, you can get the specified > format by the > application via runtime->format field. > > I don't understand the rest question in the above. > But, note that > the copy callback isn't used at all in mmap mode. > The copy/silenece > callbacks are for read/write mode. When the app > mmaps the buffer, it > means that the buffer is directly accessible without > read/write > calls. So, it skips copy and silence callbacks. If > any copy > operation is needed, the driver itself has to do in > some way, e.g. in > background task or in ack callback. > > > HTH, > > Takashi > __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
