At Thu, 11 Dec 2008 22:12:56 +0800, Bryan Wu wrote: > > On Thu, Dec 11, 2008 at 9:24 PM, Takashi Iwai <tiwai@xxxxxxx> wrote: > > At Thu, 11 Dec 2008 21:19:21 +0800, > > Bryan Wu wrote: > >> > >> On Thu, Dec 11, 2008 at 8:03 PM, Takashi Iwai <tiwai@xxxxxxx> wrote: > >> > At Thu, 11 Dec 2008 18:40:38 +0800, > >> > Bryan Wu wrote: > >> >> > >> >> Hi Takashi, > >> >> > >> >> I just made some progress about this USB audio gadget driver. But I > >> >> still got some questions about the audio playback. > >> >> Please kindly help me to work out here, as you're the best one to ask, -:) > >> >> > >> >> I can receive ISO transfer packets from PC host. The packet includes > >> >> 192 bytes audio data. > >> >> So I tried to use vfs_write() function to write this 192 bytes to the > >> >> opened snd card. > >> >> There is no sound. > >> >> > >> >> Then I create a buffer which is 6K bytes size and a workqueue. I will > >> >> fill the 6K buffer with the ISO packets data. > >> >> When the 6K buffer is full, in the workqueue handler I will call > >> >> vfs_write() function to write these 6K bytes data to the sound card. > >> >> This time, sound played and it works although it is not very smooth. > >> >> > >> >> So I guess the audio buffer I great is very important to playback audio. > >> >> How to choose the buffer size? If the size < 6K, there is no sound. > >> >> I guess it depends on the sound card hardware, but I failed to find > >> >> any info from hw_params and sw_params. > >> > > >> > Well, this pretty much depends on the "sound card" you are accessing. > >> > You mentioned about AD1980 but the question is rather what > >> > controller is used. The codec chip is basically independent from the > >> > DMA transfer parameter. > >> > > >> > >> Right, currently I'm trying AD1980 which using DMA transfer by > >> Blackfin BF54x processor. > > > > Is it an ASoC one? > > > > Yes, it's. Then another possibility is to create a sort of machine driver communicates with ASoC codec driver instead of accessing to the generic ALSA PCM core. > >> >> Actually, I want to remove the audio buffer here, just write the 192 > >> >> audio data to sound card directly. Is that possible? > >> > > >> > Also depends on the hardware. If the audio chip requires the DMA > >> > transfer, you'd need anyway a buffer. > >> > > >> > >> So how to determine the buffer size based on the DMA hardware configuration? > > > > This should be done via usual hw_refine / hw_params ioctls. > > > > in struct snd_pcm_hw_params, I guess only fifo_size is useful for me > to choose my buffer size, right? No, fifo_size is just a place holder, existing only for historical reasons as now. > If the fifo_size == 1K, so how big is OK for my buffer size? > > The buffer size is so tricky here, I try to make my driver is > independent with the lower card hardware. > So I need to find a algorithm to choose the buffer size here. > > >> I tried to change the period_size and buffer_size of the runtime struct. > >> It also didn't work. > > > > Well, the constraint is rather the "slave" sound card. You are > > actually creating a tunnel driver. The period size and buffer size > > are issues of the controller, thus you cannot change the parameters of > > the tunnel driver freely. > > > > Exactly. And another question is whether is it possible using > nonblocking writing in my driver? Why your driver needs to care about non-blocking write? Usually PCM core stuff cares the writing mode and other tasks. Takashi _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
