Modified code from above, troubleshooting SPI DMA writes not taking:
```rust
  cs.set_low();
    spi.write_dma(&mut xmit_buf, &mut dma);
    // dma.enable_interrupt(DmaChannel::C3, DmaInterrupt::TransferComplete);

    // todo: SPI transfer complete interrupt instead of DMA?

    defmt::info!("Pre DMA");
    // while unsafe { (*pac::SPI1::ptr()).sr.read().txe().bit_is_set() } {}
    while unsafe { (*pac::DMA1::ptr()).isr.read().tcif3().bit_is_clear() } {}
    defmt::info!("Post DMA");

    cs.set_high();  // todo: In TCComplete ISR.
    spi.stop_dma(DmaChannel::C3, &mut dma);
```
Neither the commented-out `txe()` nor `tcif3` flags are completely; it hangs there when I put in those checks. Any ideas? Thank you