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machine/rp2: stop SPI1 transmit from restarting SPI0 DMA - #6010
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deadprogram merged 1 commit intoOct 24, 2026
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Each SPI1 transmit restarted the SPI0 DMA channel, so SPI0 sent bytes from memory past the end of its previous buffer. The SPI DMA channels now chain to themselves, and SPI1 no longer affects SPI0. Fixes tinygo-org#6009
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Tested and it does what it claims @davecheney thank you very much. Now merging.
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On RP2040 and RP2350, each SPI1 transmit restarted the SPI0 DMA channel. SPI0 then sent bytes from memory after the end of its last buffer. That memory can be freed, reused, or outside the allocation. This has been present since DMA SPI was added in 3b4e543.
With this change, SPI1 transmits no longer affect SPI0.
The traces below are from a Pico 2 W with both buses at 1 MHz. The test sends 64 bytes on SPI0, then 2 bytes on SPI1. Without the fix, SPI0 sends 512 extra clock pulses carrying bytes 0x40 to 0x7f, the unsent half of the buffer. With the fix, SPI0 stays idle.
Tested on a Pico 2 W. I did not test on a Pico (RP2040), but it uses the same code.
Fixes #6009