Re: [Question] Can I open a substream in kernel space without attach to a file pointer?

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.

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.


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


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