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

[Nicolin Chen]

On Thu, Jan 14, 2016 at 08:29:42AM -0800, Caleb Crome wrote:
> Tuning the SSI fifo watermark & maxburst settings needs to be
> optimized differently depending on the demands on the system.  The
> current default of 2 is too low for high data-rate systems.  This
> patch maintains exactly the same behavior by default (i.e defaults to
> 2), but adds device tree options to set maxburst & fifo depth from the
> device tree.  This is necessary because a setting of 2 simply doesn't
> work at higher data rates.

> @@ -61,6 +61,16 @@ Optional properties:
>  - fsl,mode:         The operating mode for the AC97 interface only.
>                      "ac97-slave" - AC97 mode, SSI is clock slave
>                      "ac97-master" - AC97 mode, SSI is clock master
> +- fsl,fifo-watermark: Sets the fifo watermark.  The default is
> +                    fifo_depth-2 words, meaning 'initiate dma transfer
> +                    when 2 words are left in the fifo'.  At higher
> +                    data rates (48kHz, 16-channels for example), this
> +                    causes silent but deadly DMA xruns and channel
> +                    slips.  For 15 word FIFOs (like on MX5, MX6) 8 is
> +                    a good value when running at high data rates
> +- fsl,dma-maxburst: sets the max number of words to transfer in DMA.
> +                    This defaults to the same value as
> +                    fsl,fifo-watermark.

I think DT maintainers may not give a consent towards these two
properties as they are not to describe the hardware but to hack
software configurations. (And it seems you haven't CCed them.)

I forgot which values you've figured out for these two properties,
but I think those two values should work for normal cases as well:
as SSI only has limited FIFO depth, it won't hurt (increasing too
much latency) even if using a higher watermark configuration imo.
So it could be a good idea to use optimized settings for all use
cases and let other users test it.

Nicolin