[alsa-devel] [Question] Can I open a substream in kernel space without attach to a file pointer?

[Takashi Iwai]

At Thu, 11 Dec 2008 21:19:21 +0800,
Bryan Wu wrote:
> 
> On Thu, Dec 11, 2008 at 8:03 PM, Takashi Iwai <tiwai at suse.de> 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 at suse.de> 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 at suse.de> 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
> >> >
> >>
> >
>