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

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Date:	Fri, 12 Dec 2008 07:59:54 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	"Bryan Wu" <cooloney@...nel.org>
Cc:	ALSA devel <alsa-devel@...a-project.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [alsa-devel] [Question] Can I open a substream in kernel space	without attach to a file pointer?

At Thu, 11 Dec 2008 22:12:56 +0800,
Bryan Wu wrote:
> 
> On Thu, Dec 11, 2008 at 9:24 PM, Takashi Iwai <tiwai@...e.de> 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@...e.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?
> >
> 
> 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.

> >> >> 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.


Takashi
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