On Tue, Oct 10, 2023 at 05:53:38AM +0800, Randy Li wrote: > Hello > > I am trying to write a driver for a USB-SPI adapter(WCH347). The protocol of > this chip would be consist with two parts, operate code and payload length > would be the first part. SPI framework could prepare the sg_table for the > second part which is the payload. > > It is worth to do dma transfer for the second part of a bulk packet, but I > have to prepend a small buffer before it. Then I have to copy the > scatterlist from the sg_table to append to a new scatter list, that costs > CPU a lot. > > I wonder if there is a way that I could construct a bulk packet with two > URBs or anyway to make the URB first try a bytes of PIO then DMA? It is not possible to do either of those things. However, it is possible to modify the sg_table by adding a new element at the start (or else create a new sg_table that starts with a new element and then includes the original sg_table). The new element can point to a buffer in which you store the operate code and payload length. Once that's finished, you can use SG for the entire URB. Alan Stern
