a bulk urb with both PIO and DMA buffer?

[Randy Li <ayaka@xxxxxxxxxxx>]

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?

Sincerely

Randy