Re: [alsa-devel] Qustions on dmix - aplay - hw_ptr - appl_ptr

Ok I am just tracking the hw_ptr and I keep track of the last value of the hw_ptr and transfer the difference between the two.

I also solved another problem where I wasn't waiting for my device buffer to drain in the open callback. In my open callback I was querying my device buffer to set the max_buf_size variable and if I didn't wait for the previous data to transfer out it would report the wrong value.

However I am still stumped on a problem where if the file size aplay tries to play is smaller than our buffer (64KB) it randomly plays sometimes and randomly there is silence. If I modify the file to be at least 64KB it plays fine. Here is my HW params setup:

Runtime->hw.buffer_bytes_max = (1024 *64); //The size of our non-HW buffer runtime->hw.period_bytes_min = runtime->hw.buffer_bytes_max/2; runtime->hw.period_bytes_max = runtime->hw.buffer_bytes_max/2; runtime->hw.periods_min = 2; runtime->hw.periods_max = 2;

I have tried playing around the these values to only get unpredictable playback results. Sometimes playback would sound all messed up or corrupted. Perhaps I don't understand how these values are used. Also if I run aplay over and over fast enough with these smaller clips I can prevent the driver from ever calling the close callback function and this results in only hearing part of this clip sometimes and silence for the majority of the time I keep trying to run aplay quickly over and over.

Is there any way for me to avoid this un-predictable behavior with these smaller clips? I am leaning towards a setup issue or something but I am definitely stumped. Any help on this would be GREATLY appreciated. Thanks so much!

Matthew Foster

-----Original Message----- From: Takashi Iwai [mailto:tiwai@suse.de] Sent: Wednesday, April 18, 2007 6:10 AM To: matt@breakcode.com Cc: alsa-devel@alsa-project.org Subject: Re: [alsa-devel] Qustions on dmix - aplay - hw_ptr - appl_ptr

At Tue, 17 Apr 2007 10:12:07 -0700, Matthew Foster wrote:

I am a driver developer that is new to ALSA. I have read the tutorial as

an

example on how to write an ALSA driver. I appreciate the documentation you have provided online. I was wondering if you might have time to possibly answer a question. I am having trouble understanding things in the memory/buffer management area.

I am not writing a driver for a PCI device, I am writing a driver that

acts

as an interface from the ALSA to a render device. So I essentially have a circular buffer I am transferring the ALSA data to. I set up the ALSA buffer like this:

snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_CONTINUOUS, snd_dma_continuous_data(GFP_KERNEL), ALSA_BUF_SIZE, ALSA_BUF_SIZE)) < 0);

And later on in my driver after open is called I have a buffer polling

loop

that does the period notification and constantly monitors my render

(ouput)

buffer. I have my asound.conf setup like this:

pcm.mix {
    type dmix
    ipc_key 1024
    slave {
    pcm "hw:0,0"
        rate 48000
    }
}

pcm.!default {
    type plug
    slave.pcm "mix"
}

This is to enable the dmix plugin so I can run aplay more than once and

have

the two streams mixed in.

The problem I am having is that it seems that when I run aplay like this: "aplay close.wav" I only see this pointer moving:

substream->runtime->status->hw_ptrbir

And if I run aplay like this: "aplay -D plughw:0,0 close.wav" I only see this pointer moving:

substream->runtime->control->appl_ptr

This seems odd to me and currently in my code I just check which pointer moves and use that to calculate how much data I need to write to my render device. This is so I can support both use cases.

The behavior of dmix is somehow special. It mmaps a buffer to multiple instances and runs continuously without XRUN check as long as clients are connected. Dmix assumes the driver running a la DMA. That is, hw_ptr can be constantly updated in a ring buffer. Since it disables the XRUN check by adjusting stop_threshold, appl_ptr is meaningless here.

Thus, for supporting dmix on your device, you may need to ignore appl_ptr and simply transfer the period-size chunk at each time.

Currently, the support of non-DMA style hardwares is poor on ALSA, which I'd like to improve in near future. At least, we should provide a proper framework.

Another major problem is that when I play a short wav file 25KB or so and use the dmix "aplay close.wav" it randomly doesn't play at sometimes. Also if I run "aplay close.wav" and then very quickly run "aplay close.wav"

again

at the same prompt I get corruption of the data which produces screeching. In the case where I run aplay one right after the other I notice that the hw_ptr moves differently (less) than if I run it, wait, run again.

Possibly depending on the behavior above?

I am also noticing also that when I do the aplay over and over quickly

that

these values change (get lower) in my snd_pcm_prepare callback:

chip_data->buf_size = snd_pcm_lib_buffer_bytes(substream);

chip_data->period_buf_size = snd_pcm_lib_period_bytes(substream);

It seems like those value should remain constant, and I use those values

to

calculate later in my polling thread to notify of a period lapse. It's almost like the runtime is telling me the period has changed and the

buffer

size has changed. But this only happens in this case with a short wav file and running aplay over and over quickly.

This sounds odd. Do you call snd_pcm_lib_malloc() in hw_params callback?

Takashi