#include "M5Atom.h"
#include <FastLED.h>

#define NUM_LEDS 25
#define DATA_PIN 27

CRGB leds[NUM_LEDS];

void setup() {

  Serial.begin(115200);
  delay(1000);

  Serial.println("");
  Serial.println("Initializing M5Stack Atom Matrix");
  M5.begin(true, false, true);

  // Initialize the FastLED library for this device's configuration
  Serial.println("\nInitializing FastLED");
  FastLED.addLeds<WS2812, DATA_PIN>(leds, NUM_LEDS);

  // M5Stack recommends not setting this value greater than 20
  // to avoid melting the screen/cover over the LEDs
  Serial.println("Setting brightness");
  FastLED.setBrightness(20);

  // Initialize all device LEDs to off (black)
  Serial.println("Clearing display");
  for (int num = 0; num < NUM_LEDS; num++) {
    Serial.println(num);
    leds[num] = CRGB::Black;
  }
  Serial.println("Showing LEDs");
  FastLED.show();
  
  Serial.println("Finishing setup");
  
}

I have a bunch of extra code in there so I can tell where setup fails, and it's at the point where I call FastLED.show() - everything to that point is textbook FastLED, but this just doesn't work.

It's possible, or even likely, that this is an issue with FastLED, but I simply can't find a simple, functioning example of how to code the matrix. There's a sample app buried in the SDK here: https://github.com/m5stack/M5Atom/blob/master/examples/Basics/LEDDisplay/LEDDisplay.ino, but I can't find documentation of the dis object used in that example (looking at the code, and understanding that the DisPlay library isn't even loaded by M5 I doubt it will even compile.