On Fri, Dec 12, 2008 at 2:59 PM, Takashi Iwai <tiwai@xxxxxxx> wrote: > 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. > I hope this driver independents on low level ALSA card implementation. Because on some embedded machine, they don't use ASoC. And I want to push this driver to upstream mainline finally. >> >> >> 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. > Currently, because this sound playback is not very smooth. I guess there some underrun when I write the USB audio data chuck to the card. So maybe we need 2 buffers as a pingpang buffer to do data buffering. You know, vfs_write() is blocking operation and before it returns we can not fill the buffer if you just have one buffer. -Bryan _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
