RP2040/235x: Push DMA data to PIO TX FIFO through ping-pong

[holly-hacker]

This PR adds a function on embassy_rp::pio::StateMachineTx to allow pushing continuous data to a PIO state machine through a ping-pong/double buffering mechanism, allowing one buffer to be filled while the other is being sent. This method uses 2 DMA channels with CHAIN_TO allowing no downtime between transfers (assuming the user fills their buffers in time using the given callback).

Example usage to generate a 440Hz square wave over 8 digital pins at a desired sample rate:

const SAMPLE_RATE: u32 = 16_000; // 16kHz

let tx = pio.sm0.tx();

let mut data_1 = [0x0u8; 128];
let mut data_2 = [0x0u8; 128];

let mut sample_index = 0usize;
tx.dma_push_ping_pong(
    p.DMA_CH0.reborrow(),
    p.DMA_CH1.reborrow(),
    &mut data_1,
    &mut data_2,
    |buf| {
        info!("In start of fill callback, index={}", sample_index);
        if sample_index > 100_000 {
            return core::ops::ControlFlow::Break(());
        }

        for b in buf.iter_mut() {
            let time = sample_index as f32 / SAMPLE_RATE as f32;
            let wave = fast_sin(time * 440. * f32::consts::PI * 2.);
            *b = ((wave + 1.) / 2. * 256.) as u8;

            sample_index += 1;
        }

        core::ops::ControlFlow::Continue(())
    },
)
.await;

// push a zero to reset the pin state
tx.dma_push(p.DMA_CH0, &[0u8; 1], false).await;

At 16kHz, this results in a buffer of ~8ms which looks clean (or as clean as you can get with a messy 8bit R-2R "DAC" 😄) on the oscilloscope:

---

Some open questions:

• I haven't added any compiler fences which are used in noticed in dma_push. I'm not knowledgeable enough about compilers internal to know how/when the compiler will re-order instructions to know where to insert them. Feedback is appreciated, but the current implementation seems to work
• I haven't looked at task priority yet so in a program with many concurrent tasks, another task may take the cpu time needed to fill the buffer. I'm not sure if this is something that should be handled on embassy's side.
• Users can't send partial buffers which may be desired when ending a transfer. I could fix this by changing ControlFlow to ControlFlow::<(), Option<u32>> to let the user specify how much of their buffer they want to send, but that makes the API a bit ugly. Not sure what to do here, open to feedback.
• I haven't touched any code in embassy_rp::dma, which seems to contain some duplicate code to embassy_rp::dma::StateMachineTx. I'm not sure how the maintainers want to approach this.

I haven't added any examples yet. If desired, I can do that when the general design of this function is approved.

I'm pretty new to embedded dev in general, so I may have missed something obvious. Thorough code review would be appreciated.

Fixes #4190