On Wed, Oct 28, 2015 at 03:06:40PM -0700, Caleb Crome wrote:
- Set the watermarks for both TX and RX to 8 while using burst sizes of 6. It'd be nicer to provisionally set these numbers using hard code than your current change depending on fifo_depth as it might be an odd value.
Ah, this is fascinating you say this. fifo_depth is definitely odd, it's 15 as set in imx6qdl.dtsi:
fsl,fifo-depth = <15>; But the DMA maxburst is made even later in the code...
And odd number for burst size may course a similar problem like channel swapping in two-channel cases because the number of data FIFO is 2 -- an even number. But it seems not to be related to your problem here.
Setting the watermark to 8 and maxburst to 8 dramatically reduces the channel slip rate, in fact, i didn't see a slip for more than 30 minutes of playing. That's a new record for sure. But, eventually, there was an underrun, and the channels slipped.
Setting watermark to 8 and maxburst to 6 still had some slips, seemingly more than 8 & 8.
I feel like a monkey randomly typing at my keyboard though. I don't know why maxburst=8 worked better. I get the feeling that I was just lucky.
That's actually another possible root cause -- performance issue. burst=8 will have less bus transaction number than the case when burst=6. As you have quite a lot channels comparing to normal 2 channels, you need to feed the FIFO more frequently. If SDMA does not feed the data before the input FIFO gets underrun, a channel swapping might happen: in your case, channel slip.
There does seem to be a correlation between user space reported underruns and this channel slip, although they definitely are not 1:1
Reported by user space? Are you saying that's an ALSA underrun in the user space, not a hardware underrun reported by the IRQ in the driver? They are quite different. ALSA underrun comes from the DMA buffer gets underrun while the other one results from FIFO feeding efficiency. For ALSA underrun, enlarging the playback period size and period number will ease the problem:
period number = buffer size / period size;
An ALSA underrun may not be companied by a hardware underrun but they may co-exist.
ratio: underruns happen without slips and slips happen without underruns. The latter is very disturbing because user space has no idea something is wrong.
@@ -1260,8 +1260,8 @@ static int fsl_ssi_imx_probe(struct platform_device *pdev, * We have burstsize be "fifo_depth - 2" to match the SSI * watermark setting in fsl_ssi_startup(). */
ssi_private->dma_params_tx.maxburst = ssi_private->fifo_depth - 2;ssi_private->dma_params_rx.maxburst = ssi_private->fifo_depth - 2;
ssi_private->dma_params_tx.maxburst = 8;ssi_private->dma_params_rx.maxburst = 8;
I am actually thinking about setting a watermark to a larger number. I forgot how the SDMA script handles this number. But if this burst size means the overall data count per transaction, it might indicate that each FIFO only gets half of the burst size due to dual FIFOs.
Therefore, if setting watermark to 8, each FIFO has 7 (15 - 8) space left, the largest safe burst size could be 14 (7 * 2) actually.
Yes. That's kind of fine tunning the parameters. And for your case, you may try a larger number as the SSI is simultaneously consuming a large amount of data even though it sounds risky. But it's worth trying since you are using SSI which only has tight FIFOs not like ESAI has 128 depth.
Nicolin