Step three: the third copy happens as the DMA engine passes the data from the kernel socket buffers to the protocol engine.
步骤三：dma模块将数据由socket的缓冲区传递给协议引擎时，第3次复制发生。
GETB, PUTB, GETF, and PUTF are the primary fence and barrier-related DMA commands for single buffers.
对于单一缓冲区，GETB、PUTB、GETF和PUTF是与fence和barrier相关的主要dma命令。
Queue DMA GETs for all buffers.
对所有缓冲区的DMA GET进行排队。
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