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 transferwhen 2 words are left in the fifo'. At higherdata rates (48kHz, 16-channels for example), thiscauses silent but deadly DMA xruns and channelslips. For 15 word FIFOs (like on MX5, MX6) 8 isa 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 asfsl,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