DMA Arbitration

STM32F4's DMA1/DMA2 controllers have eight streams each, and each stream multiplexes eight peripheral-request channels. Two peripherals can be wired in CubeMX onto the same physical stream — both will compile and flash, and the second one's DMA transfers will simply never fire. Nucleus catches this at nucleus check time instead of at "why is my second sensor not updating".

The model (nucleus-db/src/dma.rs)

A hand-maintained, family-parameterized map of:

  • DMA1 and DMA2, their eight streams each.
  • Each stream's eight request channels.
  • The peripheral-request table: which (peripheral, direction) — e.g. (usart2, Rx) — is servable by which (controller, stream, channel) slots.

Cited to ST's reference manuals: RM0390 Tables 28–29 (F446RE), RM0383 Tables 27–28 (F411RE). Like the clock-tree model, this can't be generated from pack XML (no DMA data there) so it's hand-maintained and seed-tested.

The solver (nucleus-compiler/src/dma.rs)

validate() is a deterministic, greedy first-fit:

  1. Collect every dma = [...] request across configured peripherals, in BTreeMap peripheral order.
  2. For each request, in order, assign the first candidate stream that's still free.
  3. If every candidate for a request is already taken, record a Conflict::DmaCollision against that stream — once per contested stream, not once per colliding pair — naming the existing holder, the new contender, and (if either side has a free alternative slot) a suggestion.

stm32.toml schema

[peripherals.i2c1]
sda = "PB7"
scl = "PB6"
dma = ["rx"]        # request DMA for specific signals…

[peripherals.uart5]
tx = "PC12"
rx = "PD2"
dma = true          # …or `true` for every direction the peripheral supports

Example: a stream collision

# tests/fixtures/dma_collision.toml — I2C1_RX and UART5_RX both map to
# DMA1 stream 0; I2C1_RX has a free alternative on stream 5, UART5_RX doesn't.
[device]
family = "STM32F446RE"

[peripherals.i2c1]
sda = "PB7"
scl = "PB6"
dma = ["rx"]

[peripherals.uart5]
tx = "PC12"
rx = "PD2"
dma = ["rx"]

$ nucleus check tests/fixtures/dma_collision.toml
tests/fixtures/dma_collision.toml: 1 conflict found:

  error: DMA collision: I2C1 and UART5 both need DMA1 stream 0 (move I2C1 to DMA1 stream 5 (channel 1))

See also