Electronic – Question about STM32F4 DMA

As you've discovered, handling counter overflow with an interrupt handler is tricky and race-prone. Even with your priority boost, you'll get the wrong answer if interrupts are disabled while you read the counter. There's a much simpler technique which works if you're not reading the clock from interrupt context (note that I'm using the DWT cycle counter here, which is built into the processor thus fast and simple):

uint64_t last_cycle_count_64 = 0;

// Call at least every 2^32 cycles (every 59.6 seconds @ 72 MHz).
// Do not call from interrupt context!
uint64_t GetCycleCount64() {
  last_cycle_count_64 += DWT->CYCCNT - (uint32_t)(last_cycle_count_64);
  return last_cycle_count_64;
}

If you need to call it from interrupt context (or with a preemptive OS):

volatile uint64_t last_cycle_count_64 = 0;

// Call at least every 2^32 cycles (every 59.6 seconds @ 72 MHz).
uint64_t GetCycleCount64() {
  uint32_t primask;
  asm volatile ("mrs %0, PRIMASK" : "=r"(primask));
  asm volatile ("cpsid i");  // Disable interrupts.
  int64_t r = last_cycle_count_64;
  r += DWT->CYCCNT - (uint32_t)(r);
  last_cycle_count_64 = r;
  asm volatile ("msr PRIMASK, %0" : : "r"(primask));  // Restore interrupts.
  return r;
}

You may need to enable the cycle counter first:

DWT->CYCCNT |= DWT_CTRL_CYCCNTENA_Msk;  // Set bit 0.

As far as I know the STM32F4's DMA controller cannot do what you want. DMA can feed the DAC much faster than interrupt driven software (up to 10.5 Msps) so you might not need to skip samples anyway.

If DMA is not fast enough then the classical solution is to make multiple copies of your waveform, each with half as many samples as the previous one. This will use a maximum of twice as much sample memory (1 + 1/2 + 1/4 + 1/8 etc). If you have several waveforms that take up a lot of space then you might not have enough memory to store all the higher octaves. However if you only need to play one waveform at a time then you may still have enough RAM to precompute and store the octave(s) required for just the current waveform.