[alsa-devel] [RFC PATCH 5/5] ASoC: omap-mcbsp: Place correct constraints for streams

Peter Ujfalusi peter.ujfalusi at nokia.com
Tue Jun 1 10:19:30 CEST 2010



On Tuesday 01 June 2010 10:38:28 ext Jarkko Nikula wrote:
> On Tue, 1 Jun 2010 09:47:09 +0300
> Peter Ujfalusi <peter.ujfalusi at nokia.com> wrote:
> > I like the following naming:
> > omap_mcbsp_hwrule_min_buffersize()
> > omap_mcbsp_hwrule_max_periodsize()
> Looks clear to me.
> > Also, I think there is no point to limit the lower period size in
> > threshold mode to 32, so I will remove that as well I think.
> What was the reason why period size cannot be bigger than threshold?
> This constraint was there before your patch but I don't remember reason
> for it.

When DMA is used to push the data to McBSP on OMAP3:
The McBSP threshold means, that if threshold amount of locations (words) are 
free in the buffer, than the DMA request line will be asserted, and McBSP 
expects that DMA will transfer exactly threshold number of words in response 
to the DMA request.
So, if threshold is 1 (in register it is 0), than McBSP requests for new word, 
whenever a single location is free in the FIFO. The DMA should send 1 word per 
DMA request.

If threshold is configured to 100 (99 in register), than McBSP will asserts the 
DMA request line, when 100 locations are free. Than DMA has to send 100 words 
per DMA request.

So we need to limit the period size (which is used to configure the DMA's elem 
count - number of words per DMA request) that it shall never be bigger than the 

> Should it be opposite that period size cannot be smaller than threshold?


Well... One thing I was wondering about for a long time.
If we change the way how McBSP/DMA is used on OMAP3 (we could use the frame mode 
for that):
We place constraint on the period step to be the size of the threshold selected.
So userspace can have bigger periods than the McBSP FIFO and still gain benefit 
from the usage of the FIFO.
So period size is x * threshold.
We configure McBSP threshold
We configure DMA to push threshold amount of word per request.
In the DMA callback, we count, and when the x-th DMA request has been served, 
than we call elapsed.
If there is interest, I might look at this.
I guess this could be useful on McBSP1,3,4, and 5, which has small FIFO...


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