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If a TX DMA completion interrups a forground write.
The TX DMA completion can start a dma_send and it will
then followed by the forground write's dma_send
stoping the,then in progress DMA.
By atomicaly marking the tx dma busy, the forground
write will not perform the dma_send, and will only
enqueue the data. At the next TX dma completion any
data pending in the tx queue will be sent