AVR Freaks

All I want is to send a contiguous byte stream via SERCOM with a 2.5MHz bit rate.  Am I asking too much?     (See also https://community.atmel.com/foru...)

Running on a SAMD11 Xplained Pro clocked at 48MHz, I've used Atmel START to configure SERCOM1 using the Master DMA driver, transmit only.  As a simple test, I want to send three bytes without any timing gap between.  Cribbing code from the examples directory, I have the following. 

It is supposed to send three bytes back-to-back and then sleep for a millisecond before repeating:

#include "atmel_start.h"
#include "atmel_start_pins.h"

static uint8_t s_pixels[] = {0x01, 0xfe, 0x55};

static void tx_complete_cb_RGB_COM(struct _dma_resource *resource) {
  /* Transfer completed */
  asm("nop");
}

static void RGB_loop(void) {
  struct io_descriptor *io;
  spi_m_dma_get_io_descriptor(&RGB_COM, &io);

  spi_m_dma_register_callback(&RGB_COM, SPI_M_DMA_CB_TX_DONE, tx_complete_cb_RGB_COM);
  spi_m_dma_enable(&RGB_COM);

  while (true) {
    io_write(io, s_pixels, sizeof(s_pixels));
    delay_ms(1);
  }
}

int main(void) {
  atmel_start_init();
  RGB_loop();
}

... but as far as my scope can tell me, it doesn't even write one byte.

What am I missing?

P.S.: I've used the same setup in Master Async and Master Sync mode, so I can eliminate hardware as the source of trouble.  The only problem with Async and Sync mode is that the emitted bytes are not contiguous.