regarding setting of fragment size in usbaudio driver

regarding setting of fragment size in usbaudio driver

[Amit Nagal]

Hi ,

i am using a OSS based usbaudio application for usb audio streaming .

my usb audio device is capture only device .

i want to set the fragment size of usbaudio alsa driver to reduce
latency problem coming in my application .

currently even if my application reads 4k data from userspace , i
observed driver will buffer for 100 ms data ( period size data )= 16 k
at 44100 frquency

and after driver  reads 16k data , read call will return with 4k data
.  so read() call remains blocked for 100ms which is causing some
latency problem .

i used SNDCTL_DSP_SETFRAGMENT  ioctl , but still driver behaviour to
buffer 100ms data remained same .

how driver's  fragment size in driver can be set to some less value in
OSS based application .

Can some patch  be implemented in kernel space to do that .?


Thanx & Regards
Amit Nagal

Re: regarding setting of fragment size in usbaudio driver

[Clemens Ladisch]

Amit Nagal wrote:
> i am using a OSS based usbaudio application for usb audio streaming .
> 
> my usb audio device is capture only device .
> 
> i want to set the fragment size of usbaudio alsa driver to reduce
> latency problem coming in my application .
> 
> currently even if my application reads 4k data from userspace , i
> observed driver will buffer for 100 ms data ( period size data )= 16 k
> at 44100 frquency
> 
> and after driver  reads 16k data , read call will return with 4k data
> .  so read() call remains blocked for 100ms which is causing some
> latency problem .

There is no extra buffer in the driver.
There seems to be something wrong with your algorithm.

Does recording with the arecord tool work?


Regards,
Clemens

Re: regarding setting of fragment size in usbaudio driver

[Amit Nagal]

Hi ,

Thanx for the reply .

Actually on my embedded target ,  i am not using arecord , but reading
from (/dev/dsp0) capture data .

i have 2 question :

i used SNDCTL_DSP_GETBLKSIZE ioctl to find the fragment size of driver
. i found it to be 16k .

1) now if the application reads 4k data from userspace , read call
will remain blocked for audio driver's fragment size usb data read (
16k)
 or app_request_size(4k) ?

2)time taken for read(4k) = time_taken(fragment size read ) or
time_taken(app_request_size=4k) ?

thanx & regards
amit nagal


On Mon, Dec 13, 2010 at 6:04 PM, Clemens Ladisch <clemens@ladisch.de> wrote:
> Amit Nagal wrote:
>> i am using a OSS based usbaudio application for usb audio streaming .
>>
>> my usb audio device is capture only device .
>>
>> i want to set the fragment size of usbaudio alsa driver to reduce
>> latency problem coming in my application .
>>
>> currently even if my application reads 4k data from userspace , i
>> observed driver will buffer for 100 ms data ( period size data )= 16 k
>> at 44100 frquency
>>
>> and after driver  reads 16k data , read call will return with 4k data
>> .  so read() call remains blocked for 100ms which is causing some
>> latency problem .
>
> There is no extra buffer in the driver.
> There seems to be something wrong with your algorithm.
>
> Does recording with the arecord tool work?
>
>
> Regards,
> Clemens
>

Re: regarding setting of fragment size in usbaudio driver

[Clemens Ladisch]

(please don't top-post)

Amit Nagal wrote:
> i used SNDCTL_DSP_GETBLKSIZE ioctl to find the fragment size of driver
> . i found it to be 16k .
> 
> 1) now if the application reads 4k data from userspace , read call
> will remain blocked for audio driver's fragment size usb data read (
> 16k)
>  or app_request_size(4k) ?

If, when read() is called, there is enough data in the buffer, it will
return that data and return immediately.  Otherwise, it will wait for
the hardware to reach the next period boundary (in OSS: fragment
boundary) and try again.

If you want to have shorter waits, use lower fragment sizes.


Regards,
Clemens

Re: regarding setting of fragment size in usbaudio driver

[Amit Nagal]

> (please don't top-post)
>
Extremely sorry for that .  will take care reagarding this from now on .

> If, when read() is called, there is enough data in the buffer, it will
> return that data and return immediately.  Otherwise, it will wait for
> the hardware to reach the next period boundary (in OSS: fragment
> boundary) and try again.
>
> If you want to have shorter waits, use lower fragment sizes.

yes ,  setting of lower fragment size is my requirement at the moment
in OSS application .
i tried to use SNDCTL_DSP_SETFRAGMENT ioctl to set lower fragment size .
but audio driver uses default fragment size only and not the fragment
size desired by application .
Is it  possible to change fragment size in kernel space ?

Thanx & Regards
Amit Nagal

Re: regarding setting of fragment size in usbaudio driver

[Clemens Ladisch]

Amit Nagal wrote:
> > If you want to have shorter waits, use lower fragment sizes.
> 
> yes ,  setting of lower fragment size is my requirement at the moment
> in OSS application .
> i tried to use SNDCTL_DSP_SETFRAGMENT ioctl to set lower fragment size .

In theory, this should work.

> but audio driver uses default fragment size only and not the fragment
> size desired by application .

Does it return an error?  If yes, try moving this call before some
other parameter setting ioctls.


Regards,
Clemens

Re: [Alsa-user] regarding setting of fragment size in usbaudio driver

[Bill Unruh]

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On Mon, 13 Dec 2010, Amit Nagal wrote:

> Hi ,
>
> Thanx for the reply .
>
> Actually on my embedded target ,  i am not using arecord , but reading
> from (/dev/dsp0) capture data .

He asked you if arecord works, not whether you are using it. 
Why are you using the OSS emulation in alsa, rather than using the native
alsa? That adds an extra layer and more code.

>
> i have 2 question :
>
> i used SNDCTL_DSP_GETBLKSIZE ioctl to find the fragment size of driver
> . i found it to be 16k .
>
> 1) now if the application reads 4k data from userspace , read call
> will remain blocked for audio driver's fragment size usb data read (
> 16k)
> or app_request_size(4k) ?

So read in all 16K into your buffer. But you do not say how your application
reads that 4k in?

Why do you not publish the fragment of you code that you use so people can see
what y ou do.
As I understand OSS, the driver reads the usb and puts the output into a
buffer. YOur call to the oss system, then reads from that buffer and delivers
it to you. That should not block, unless there is nothing in the buffer (ie
you forgot to tell the driver to start reading the sound card).

>
> 2)time taken for read(4k) = time_taken(fragment size read ) or
> time_taken(app_request_size=4k) ?
>
> thanx & regards
> amit nagal
>
>
> On Mon, Dec 13, 2010 at 6:04 PM, Clemens Ladisch <clemens@ladisch.de> wrote:
>> Amit Nagal wrote:
>>> i am using a OSS based usbaudio application for usb audio streaming .
>>>
>>> my usb audio device is capture only device .
>>>
>>> i want to set the fragment size of usbaudio alsa driver to reduce
>>> latency problem coming in my application .
>>>
>>> currently even if my application reads 4k data from userspace , i
>>> observed driver will buffer for 100 ms data ( period size data )= 16 k
>>> at 44100 frquency
>>>
>>> and after driver  reads 16k data , read call will return with 4k data
>>> .  so read() call remains blocked for 100ms which is causing some
>>> latency problem .
>>
>> There is no extra buffer in the driver.
>> There seems to be something wrong with your algorithm.
>>
>> Does recording with the arecord tool work?
>>
>>
>> Regards,
>> Clemens
>>
>
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