Direct use of the DMA buffer?

Direct use of the DMA buffer?

[Pavel Hofman]

Hello,

Please excuse my ignorance, but is it possible for an application to
directly use/output to the DMA buffer allocated by the kernel in
hw_params? Something like using the actual DMA buffer in the
snd_pcm_writei function, without copying the data in alsa-lib/driver PCM
midle layer (?), just managing the pointers.

Only a theoretical question though, no plans on my side :)

Thanks a lot for even a short No :-)

Best regards,

Pavel.

Re: Direct use of the DMA buffer?

[Clemens Ladisch]

Pavel Hofman wrote:
> Please excuse my ignorance, but is it possible for an application to
> directly use/output to the DMA buffer allocated by the kernel in
> hw_params?

Set one of the SND_PCM_ACCESS_MMAP_* access types and then use
snd_pcm_mmap_begin/commit.


Regards,
Clemens

Re: Direct use of the DMA buffer?

[Pavel Hofman]

Dne 20.2.2012 09:11, Clemens Ladisch napsal(a):
> Pavel Hofman wrote:
>> Please excuse my ignorance, but is it possible for an application to
>> directly use/output to the DMA buffer allocated by the kernel in
>> hw_params?
> 
> Set one of the SND_PCM_ACCESS_MMAP_* access types and then use
> snd_pcm_mmap_begin/commit.
> 

Clemens, thanks a lot for the information. How does it work when the
alsa-lib chain contains plugins, such as plug, rate, etc.? I would
assume the application circumvents these.

Thanks a lot.

Pavel.

Re: Direct use of the DMA buffer?

[Jaroslav Kysela]

Date 20.2.2012 09:38, Pavel Hofman wrote:
> 
> Dne 20.2.2012 09:11, Clemens Ladisch napsal(a):
>> Pavel Hofman wrote:
>>> Please excuse my ignorance, but is it possible for an application to
>>> directly use/output to the DMA buffer allocated by the kernel in
>>> hw_params?
>>
>> Set one of the SND_PCM_ACCESS_MMAP_* access types and then use
>> snd_pcm_mmap_begin/commit.
>>
> 
> Clemens, thanks a lot for the information. How does it work when the
> alsa-lib chain contains plugins, such as plug, rate, etc.? I would
> assume the application circumvents these.

Nope. The mmap commit mechanism was introduced to keep the plugin
interface functional. The direct mmap access to the DMA buffer is only
applied for the direct hw: devices. If a plugin is used inside alsa-lib,
the application talks to the plugin chain (it means that an intermediate
buffer for samples can be allocated to let things working).

					Jaroslav

-- 
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project; Red Hat, Inc.

Re: Direct use of the DMA buffer?

[Pavel Hofman]

Dne 20.2.2012 10:21, Jaroslav Kysela napsal(a):
>>
>> Clemens, thanks a lot for the information. How does it work when the
>> alsa-lib chain contains plugins, such as plug, rate, etc.? I would
>> assume the application circumvents these.
> 
> Nope. The mmap commit mechanism was introduced to keep the plugin
> interface functional. The direct mmap access to the DMA buffer is only
> applied for the direct hw: devices. If a plugin is used inside alsa-lib,
> the application talks to the plugin chain (it means that an intermediate
> buffer for samples can be allocated to let things working).
> 

Jaroslav, thanks a lot for the clarification.

Pavel.