Interrupt handler

Interrupt handler

[Giuliano Pochini]

When an interrupt arrives and I call snd_pcm_period_elapsed(), what
function gets from alsa middle layer the new block to play and sets up
the hardware ?

Bye.




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RE: Interrupt handler

[Giuliano Pochini]

> When an interrupt arrives and I call snd_pcm_period_elapsed(), what
> function gets from alsa middle layer the new block to play and sets up
> the hardware ?

Ok, nobody answered, perhaps my question was too stupid :))
I still haven't found in the docs how can I get from ALSA the
address of the blocks to play/record, or how to tell ALSA where
it the space I allocated so it knows where it has to write the
audio data to be played.
I can build a very generic scatter-gather list. It has three
instructions: addr/length, generate interrupt, jump. Per example
I can write an sg list like this to emulate the cyclic buffer of
many soundblaster cards:

start:
addr0/len0
Irq
addr1/len1
Irq
goto start



Bye.



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Re: Interrupt handler

[Takashi Iwai]

At Mon, 17 Mar 2003 10:17:04 +0100 (CET),
Giuliano Pochini wrote:
> 
> 
> > When an interrupt arrives and I call snd_pcm_period_elapsed(), what
> > function gets from alsa middle layer the new block to play and sets up
> > the hardware ?
> 
> Ok, nobody answered, perhaps my question was too stupid :))

both of Jaroslav and I were absent since the last friday to attend at
the LAD meeting...

> I still haven't found in the docs how can I get from ALSA the
> address of the blocks to play/record, or how to tell ALSA where
> it the space I allocated so it knows where it has to write the
> audio data to be played.

first, the data is not necessarily as a block on ALSA.
and second, you don't have to copy the data as long as the hardware
supports DMA on the host memory.  the alsa middle layer does this
job.

usually in the interrupt handler, you need to just send "ack" to the
middle layer by calling snd_pcm_period_elapsed().  then, the
middle-layer checks the current position by calling pointer callback,
and copy/send/set-silence in the necessary area.

the pointer callback, thus, must return the current position of the
hardware pointer, i.e. the position offset (in frames) in the DMA
buffer, where the DMA is working right now (or the last finished
positoin, if you cannot get the current DMA pointer due to the
hardware limitation).


Takashi


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Re: Interrupt handler

[Giuliano Pochini]

On 17-Mar-2003 Takashi Iwai wrote:
> usually in the interrupt handler, you need to just send "ack" to the
> middle layer by calling snd_pcm_period_elapsed().  then, the
> middle-layer checks the current position by calling pointer callback,
> and copy/send/set-silence in the necessary area.

I didn't realize the role of .pointer callback. Tnx.

In hw_params callback I build the hardware sg list according
to sgbuf->table. Then I have to set the card to play the list
in a neverending loop and to issue an irq at the and of each
period. Is it right ?


Bye.



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Re: Interrupt handler

[Takashi Iwai]

At Mon, 17 Mar 2003 15:22:45 +0100 (CET),
Giuliano Pochini wrote:
> 
> 
> On 17-Mar-2003 Takashi Iwai wrote:
> > usually in the interrupt handler, you need to just send "ack" to the
> > middle layer by calling snd_pcm_period_elapsed().  then, the
> > middle-layer checks the current position by calling pointer callback,
> > and copy/send/set-silence in the necessary area.
> 
> I didn't realize the role of .pointer callback. Tnx.
> 
> In hw_params callback I build the hardware sg list according
> to sgbuf->table. Then I have to set the card to play the list
> in a neverending loop and to issue an irq at the and of each
> period. Is it right ?

yes.

also, don't forget to unlock the spinlock during calling
snd_pcm_period_elapsed() if a single lock is used for callbacks.
as mentioned above, snd_pcm_period_elapsed() will call pointer
callback.  if you call snd_pcm_period_elapsed() inside the spinlock,
this may result in a deadlock.


Takashi


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Re: Interrupt handler

[Giuliano Pochini]

On mar, 2003-03-18 at 13:32, Takashi Iwai wrote:
> also, don't forget to unlock the spinlock during calling
> snd_pcm_period_elapsed() if a single lock is used for callbacks.

Yes, I'm following the tutorial and it's clear about that.

Now I have another weird problem. This is a peice of my hw_param
function:

static int pcm_hw_params(){
  printk("pcm_hw_params bytes=%d\n",params_buffer_bytes(hw_params));
  for (per=0; per<params_periods(hw_params); per++) {
    printk("pcm_hw_params Add%d (%x - %d)\n",
            per,
            sgbuf->table[per].addr,
            params_period_bytes(hw_params));
  }
}

Mar 19 00:48:33 localhost kernel: pcm_hw_params bytes=44100
Mar 19 00:48:33 localhost kernel: pcm_hw_params Add0 (3a1d000 - 11026)
Mar 19 00:48:33 localhost kernel: pcm_hw_params Add1 (3a1c000 - 11026)
Mar 19 00:48:33 localhost kernel: pcm_hw_params Add2 (3a1b000 - 11026)
Mar 19 00:48:33 localhost kernel: pcm_hw_params OK

Look at the physical addresses. They're spaced by 4KB, but the periods
are 11KB long and 11026*3<44100 !! You can imagine how beatiful sound I
get... What am I missing ?


Bye.




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Re: Interrupt handler

[Giuliano Pochini]

> Look at the physical addresses. They're spaced by 4KB, but the periods
> are 11KB long and 11026*3<44100 !! You can imagine how beatiful sound I
> get... What am I missing ?

Ehm, I found the problem, ignore my previous msg.


Bye.




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