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. Actually, I want to remove the audio buffer here, just write the 192 audio data to sound card directly. Is that possible? 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
