SG probably stands for Scatter, Gather. It's a feature of DMA controllers where instead of single contiguous memory pointer, the DMA controller receives a list of address/size pairs and when it has data from the end device writes it to sequential elements from this list. This is called scatter as the DMA controller scatters the data from the device to different regions. The reverse is called gather where the DMA controller again receives a list of address/size pairs and reads from these blocks sequentially and sends the data to the device in a way gathering the data. The nice thing about SGDMA is that it reduces the processor's load by reducing the number of interrupts it needs to handle.
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