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? > >> 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. > 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. Takashi > > Thanks a lot > -Bryan > > > > > Takashi > > > >> > >> Thanks a lot. > >> -Bryan > >> > >> > >> On Thu, Nov 20, 2008 at 12:01 AM, Takashi Iwai <tiwai@xxxxxxx> wrote: > >> > At Wed, 19 Nov 2008 23:42:48 +0800, > >> > Bryan Wu wrote: > >> >> > >> >> On Wed, Nov 19, 2008 at 9:49 PM, Takashi Iwai <tiwai@xxxxxxx> wrote: > >> >> > At Wed, 19 Nov 2008 18:00:36 +0800, > >> >> > Bryan Wu wrote: > >> >> >> > >> >> >> Hi Takashi, > >> >> >> > >> >> >> I am developing a USB gadget driver compliant to USB Audio Class Spec 2.0. > >> >> >> So I want to open a PCM substream and do some playback of capture, > >> >> >> then close them? > >> >> >> > >> >> >> I found snd_pcm_open_substream() is for opening a substream and attach > >> >> >> it to a file. > >> >> >> But in my application, there is no need to open a file before opening > >> >> >> a substream. > >> >> >> > >> >> >> - Is there any interface for me to open a substream in kernel space > >> >> >> without attach to a file? > >> >> >> - How to playback and capture in kernel space, use snd_pcm_lib_write > >> >> >> and snd_pcm_lib_read? > >> >> >> - How to get the snd_pcm_hardware struct from low level driver, > >> >> >> because I have to get the hardware configuration of the snd pcm > >> >> >> device? > >> >> >> > >> >> >> And I am reading the code of OSS emulator in ALSA. It provides some > >> >> >> info about the kernel space sound card programming. > >> >> > > >> >> > Yes, OSS emulation code handles the PCM in the kernel. > >> >> > But, basically I don't recommend you to do this -- it's not the job of > >> >> > the sound card driver. The whole PCM stuff is handled by the PCM > >> >> > middle layer, not the driver itself. > >> >> > >> >> No, my plan is not a sound card driver. It is an USB gadget audio driver. > >> >> When an embedded system for example Blackfin board connects to a USB host (PC), > >> >> PC will recognize this USB device as a USB Audio Class device. > >> >> > >> >> Generally, there should be a sound card on the embedded system. Our > >> >> Blackfin board > >> >> has an AD1980 ALSA sound card. The USB gadget audio driver will open this sound > >> >> card and export this device to USB host PC by some USB audio class specific > >> >> descriptors. Then the PC can playback some audio stream by USB cable, USB gadget > >> >> audio driver will receive this stream and playback the data by AD1980 > >> >> ALSA playback > >> >> substream. Capture is the similar. > >> >> > >> >> > Any reason why you handle the PCM stuff completely in your driver > >> >> > code? > >> >> > > >> >> > >> >> There is USB gadget MIDI driver in kernel. But it asked the user to > >> >> use aconnect tool to > >> >> connect the virtual MIDI card to a real one. I don't want travel to > >> >> user space and it should > >> >> be more efficient in kernel space to handle all things including PCM > >> >> open/release/read/write > >> >> and Mixer control. > >> >> > >> >> Any hints about this? I really need some help from ALSA guru, cause > >> >> I'm not familiar the internal > >> >> things here. > >> > > >> > Well, the access in the kernel space is fairly similar as in the user > >> > space. It opens, issues ioctls, reads and writes. The difference is > >> > that you access via dedicated function calls instead of syscalls. > >> > There is no way to poke the driver internal from other drivers. > >> > To answer your questions... > >> > > >> >> >> - Is there any interface for me to open a substream in kernel space > >> >> >> without attach to a file? > >> > > >> > No. > >> > > >> >> >> - How to playback and capture in kernel space, use snd_pcm_lib_write > >> >> >> and snd_pcm_lib_read? > >> > > >> > Yes. But for the kernel space buffer, you'd need to fake the > >> > user-space pointer by snd_enter_user() and snd_leave_user(). See > >> > snd_pcm_oss_write3(). > >> > > >> >> >> - How to get the snd_pcm_hardware struct from low level driver, > >> >> >> because I have to get the hardware configuration of the snd pcm > >> >> >> device? > >> > > >> > Not way to peek/poke the driver internals from the outside. > >> > You'll need to negotiate via snd_pcm_kernel_ioctl() like user-space > >> > programs. > >> > > >> > > >> > HTH, > >> > > >> > Takashi > >> > > >> > > > _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
