With TCD set to loop, there is a window where the
DMA has raised Done, but not reloaded the TCD, resetting
count and clearing Done.
In this window imxrt_dmach_getcount could then return 0.
Resulting in imxrt_dma_nextrx returning RXDMA_BUFFER_SIZE.
Which is not a valid index in the FIFO.
Since the count will be set to RXDMA_BUFFER_SIZE. When the DMA
engine completes the TCD reload. The imxrt_dma_nextrx would
return 0. Therefore:
(RXDMA_BUFFER_SIZE - dmaresidual) % RXDMA_BUFFER_SIZE
accomplishes this.
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