[alsa-devel] [PATCH] Fix buffer position for ATI SB4x0

[Takashi Iwai]

At Fri, 6 Jun 2008 15:31:45 -0300,
Mauro Carvalho Chehab wrote:
> 
> On Fri, 06 Jun 2008 18:49:28 +0200
> Takashi Iwai <tiwai at suse.de> wrote:
> 
> > At Fri, 6 Jun 2008 11:52:32 -0300,
> > Mauro Carvalho Chehab wrote:
> > > 
> > > On Thu, 29 May 2008 16:20:21 +0200
> > > Takashi Iwai <tiwai at suse.de> wrote:
> > > 
> > > > At Thu, 29 May 2008 11:10:22 -0300,
> > > > Mauro Carvalho Chehab wrote:
> > > > > 
> > > > > ATI SB4x0 doesn't need any fix at position.
> > > > 
> > > > It's not about the position fixing but whether to use the
> > > > position-buffer.  The devices on the blacklist are the ones that have
> > > > no position buffer.  So, it would fall into LPIB mode, and this list
> > > > avoids it from the beginning.
> > > > 
> > > > > This patch solves the issue of receiving several clicks during capture on those
> > > > > devices.
> > > > > 
> > > > > Tested with a Gateway Notebook MX-6453.
> > > > 
> > > > The click noise is often a different problem.  Did you already try
> > > > the patch below?
> > > 
> > > The click seems associated to some residual samples inside the buffer. Here it
> > > is a sample of the king of click noise I'm hearing here (captured from CD input entry):
> > > 	http://linuxtv.org/~mchehab/snd.ogg
> > 
> > (I didn't check the ogg file yet, and just a wild guess)
> > 
> > Is it with dsnoop plugin?  With "default" PCM, ALSA uses dsnoop for
> > capture to allow multiplexing for HD-audio.  Does it happen with "hw"
> > or "plughw" PCM?
> 
> Results with the cdplay:
> 
> With "default" PCM (both with and without mmap):
> 	several clicks per second, very high clicks;
> 	(like a very risky analog disk)

Hm, it's obviously a problem.  I couldn't reproduce it on my machine
with HD-audio, so it might be controller/codec-specific, though.

> With "hw" or "plughw" (no mmap):
> 	less clicks (something like two clicks or three per second), with less volume.
> 	both hw and plughw produces the same effect.

If it works with hw, plughw should have no effect (i.e. no conversion
is done).  Thus it's logical that both hw and plughw have the same
result.

> With "hw" or "plughw" and mmap (-M):
> 	high quality. no noticeable clicks.
> 
> So, it seems that there are two different issues: one with dsnoop and another
> with non-mmapped captures.

Interesting.  The fact that even "hw" without mmap causes occasional
click noises implies that there is certainly a problem with the DMA
position calculation.  The dsnoop has more problems likely because it
uses smaller period size and more periods than hw, I guess.
Still not sure why the mmap mode works.

Anyway, it means that the capture position is wrongly reported, maybe
just in a reverse way of the playback position -- a few samples ahead
than the real position.  Sigh, another workaround is needed.


thanks,

Takashi