Re: [PATCH RFC 1/1] ASoC: fsl_ssi: Make fifo watermark and maxburst settings device tree options

On Fri, Jan 15, 2016 at 10:57 AM, Nicolin Chen [off-list ref] wrote:

On Fri, Jan 15, 2016 at 10:49:04AM -0800, Caleb Crome wrote:

quoted

quoted

The watermark is merely a threshold to trigger a DMA request. The
only relationship with the burst size is that each burst transfer
should not carry more data than the number of empty slots; FIFO
under/overflow occurs otherwise. So it's just more efficient and
safer to set an identical value to both of them. I don't think
it will cause functional problems to set TFWM to 4 and burst size
to 1 -- It just lets DMA operate in a single data transfer mode.

If there is no penalty for setting maxburst to 1 (or 2 in the case of
dual fifo I think), then should we just set both the watermark and
maxburst to 1?

I guess the real difference would be when you're in FIQ mode.  In FIQ
mode, the penalty of an interrupt per word would be pretty bad, but in
DMA mode, if we just set both to 1, we should be fine, right?

There will be much more overhead drawn by frequent DMA transfers.
I believe you understand the idea -- less burst size then more DMA
request. Each DMA transfer contains a pair of handshaking overhead
according to the bus protocol. Apparently the bus will be wasted
with lots of handshaking section instead of keep dedicated to data
transfer truly. It might work if SSI is the only user of the bus,
which we shouldn't assume.

Right, I knew there must be a trade off.  So, how about I submit a
patch with a fixed value of 4 for whichever platforms you think is
correct.

I see the 4 compatibles are:  fsl,mpc8610-ssi, fsl,imx51-ssi,
fsl,imx35-ssi, and fsl,imx21-ssi.

Shall I submit a patch that sets the value to 4 for for any/all of the
above?  Should it be different based on whether fiq is enabled.  Maybe
set it to 4 for whenever DMA is used, and to 13 whenever FIQ is used?

-Caleb