data write in alsa

data write in alsa

[Vinod Koul]

Hi Takashi,

For alsa pcm we have two ways to write data, either we use write()
system call or use ioctl SNDRV_PCM_IOCTL_WRITEI_FRAMES.

Is there any specific reason why we have two methods and which one is
preferred and why?

This question is basically for compress API, should we add similar write
method thru ioctls, if so why?

-- 
~Vinod

Re: data write in alsa

[Takashi Iwai]

At Fri, 02 Dec 2011 12:24:41 +0530,
Vinod Koul wrote:
> 
> Hi Takashi,
> 
> For alsa pcm we have two ways to write data, either we use write()
> system call or use ioctl SNDRV_PCM_IOCTL_WRITEI_FRAMES.
> 
> Is there any specific reason why we have two methods and which one is
> preferred and why?

Well, I only remember vaguely.  IIRC, ioctl is used since it can pass
the error code more directly without using errno.  Maybe Jaroslav
knows more exact reason.

But, honestly speaking, I don't like this implementation.  For a new
interface, I'd suggest to stick with the normal read/write.  Otherwise
you'll get a mess of 32bit compat layer again.


Takashi

Re: data write in alsa

[Jaroslav Kysela]

Date 2.12.2011 08:12, Takashi Iwai wrote:
> At Fri, 02 Dec 2011 12:24:41 +0530,
> Vinod Koul wrote:
>>
>> Hi Takashi,
>>
>> For alsa pcm we have two ways to write data, either we use write()
>> system call or use ioctl SNDRV_PCM_IOCTL_WRITEI_FRAMES.
>>
>> Is there any specific reason why we have two methods and which one is
>> preferred and why?
>
> Well, I only remember vaguely.  IIRC, ioctl is used since it can pass
> the error code more directly without using errno.  Maybe Jaroslav
> knows more exact reason.

The true reason was to avoid the audio frame <-> byte conversions.

> But, honestly speaking, I don't like this implementation.  For a new
> interface, I'd suggest to stick with the normal read/write.  Otherwise
> you'll get a mess of 32bit compat layer again.

I think that for compressed data, the standard r/w ops are fine, because 
there are no "frames".

					Jaroslav

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

Re: data write in alsa

[Vinod Koul]

On Fri, 2011-12-02 at 08:16 +0100, Jaroslav Kysela wrote:
> > But, honestly speaking, I don't like this implementation.  For a new
> > interface, I'd suggest to stick with the normal read/write.
> Otherwise
> > you'll get a mess of 32bit compat layer again.
> 
> I think that for compressed data, the standard r/w ops are fine,
> because 
> there are no "frames". 
Thanks, I had the same assumptions as well, wanted to check before
posting next version

-- 
~Vinod