[alsa-devel] [RFC 3/4] OMAP3: McBSP: Add interface for transmit FIFO state query

[Peter Ujfalusi]

Hello,

On Wednesday 03 March 2010 17:01:39 Ujfalusi Peter (Nokia-D/Tampere) wrote:
> On Wednesday 03 March 2010 16:18:42 ext Jarkko Nikula wrote:
> > On Wed, 3 Mar 2010 12:02:48 +0200
> > 
> > Peter Ujfalusi <peter.ujfalusi at nokia.com> wrote:
> > > The command for test:
> > > aplay -fdat -d 1 /dev/urandom
> > 
> > I recommend to use /dev/zero instead since urandom takes CPU time.
> > 
> > > [   96.290924] XBUFFSTAT: 0
> > > [   96.293487] XBUFFSTAT: 0
> > > [   96.296020] XBUFFSTAT: 0
> > > [   96.299774] XBUFFSTAT: 1
> > > 
> > > So the buffer is kept full in element mode, as it is expected.
> > 
> > I was expecting to see more variations. IRCC, the DMA in OMAP is doing
> > transfers over the memory bus using some small bursts but I'm not
> > completely sure. I'm thinking if that could explain the Eero's and
> > Liam's observations?
> 
> I kind of expecting similar result (this).
> In element mode the threshold value is 0 (1 word).
> So if one word is free in McBSP buffer, than it will assert the DMA request
> line, and the DMA will write one word to McBSP.
> So in theory the buffer must be kept full in this mode, with quite minimal
> variation.
> But I might be wrong on this...

Well, to be precise, I expected that the XBUFFSTAT would have been 1 most of the 
time, and not 0.
But in stereo we are still within one sample accuracy, so I think this is OK.

-- 
Péter