Summary

Attempting to configure SPI3 and SPI4 for the STM32H5 used to fail with a linker error, something like:

[snip]
...undefined reference to `dma_SPI_4_TX'

An example of valid configuration that was failing:

diff --git a/ports/stm32/boards/NUCLEO_H563ZI/mpconfigboard.h b/ports/stm32/boards/NUCLEO_H563ZI/mpconfigboard.h
index 1e802eda5..9585ade03 100644
--- a/ports/stm32/boards/NUCLEO_H563ZI/mpconfigboard.h
+++ b/ports/stm32/boards/NUCLEO_H563ZI/mpconfigboard.h
@@ -62,6 +62,10 @@
 #define MICROPY_HW_SPI1_MISO                (pin_G9) // Arduino Connector CN7-Pin12 (D12)
 #define MICROPY_HW_SPI1_MOSI                (pin_B5) // Arduino Connector CN7-Pin14 (D11)

+#define MICROPY_HW_SPI4_SCK
+#define MICROPY_HW_SPI4_MISO
+#define MICROPY_HW_SPI4_MOSI
+
 // USRSW is pulled low. Pressing the button makes the input go high.
 #define MICROPY_HW_USRSW_PIN                (pin_C13)
 #define MICROPY_HW_USRSW_PULL               (GPIO_NOPULL)

This patch resolves that, ensuring that appropriate DMA channels are assigned to those SPI resources.

Testing

Tested SPI3 and SPI4 against a SPI peripheral (MAX14915). Also validated correct pin behaviour under a DSO.

I haven't tested if power use is higher due to more channels being employed.

Trade-offs and Alternatives

This patch does use more DMA channels. The H5 has two DMA resources, each with 8 channels. Two are needed for a bidirectional SPI channel (one each for Rx and Tx) so a whole DMA resource - 8 channels - are used to cover SPI1-4. An additional 2x channels of the second DMA resource may also be used if DAC is used.

Currently the DMA channels are statically assigned in MicroPython; an alternative would be to provide an abstraction for DMA channels and allow them to be assigned more dynamically. However, that would be a more complex implementation and difficult to provide a cross-port design.