i2s: Sleep when TX FIFO fills

[gbenson]

i2sWrite spins calling runtime.Gosched in a tight loop if the TX FIFO fills. This pull request makes it precalculate how long the FIFO takes to become writable and time.Sleep for slightly longer than that time instead. It also joins the FIFOs to bump the TX FIFO from 4 to 8 words.

On TinyGo at least, I'm 99% sure that time.Sleep yields the CPU In the same way as runtime.Gosched (i.e. it won't deadlock when running on a single core) and, one core or two, with 44.1kHz audio the sleep time is 3,109 cycles at 133MHz (3,507 at 150MHz) so you get a lot of CPU back (or burn less power, if you just wait it out).

[soypat]

Yes! Very nice. I've been thinking about how to add precalculated sleep to the rest of these drivers, especially when doing DMA. Might be worth doing something with the dma and deadline abstractions in the future. For now this is an excellent addition. Thank you!

[gbenson]

Actually this is causing me problems now :( If two other things are happening when the sleep finishes it isn't coming back to the sleeping routine fast enough to avoid the audio dropping out. Or, at least I think that's what's happening. I have three goroutines: one filling audio buffers, one writing them, and one updating a little I2C screen, and I'm now getting clicks whenever the screen updates. Please could you revert it while I figure things out? Sorry!

[gbenson]

(I added runtime.Gosched() all through the screen updater, so it might be how TinyGo schedules. I want to tell it "the buffer filler and the I2S writer have absolute priority, this other one for the display can run if you have time", but I don't know how:(

[soypat]

OK, changes reverted- If you want strict synchronization you should follow an event-loop architecture or develop a priority thread abstraction over the goroutines. I guess the change maybe does not belong in this repo since audio requires strict timing and the best way to meet strict schedules is to just thrash runtime.Gosched. This topic is worthy of a new discussion on tinygo-org/tinygo