On Mon, 29 Nov 2010, vichy wrote: > Recently I trace scsi and usb storage driver and try to improve the > read speed of my usb stick. > I have some questions about them: > 1. below is the struct of scsi cmd, and there is a list seems for > linking multiple scsi cmd > But I found usb storage driver seems always get one cmd at a time. > why don't upper layer link more of them to send it once? usb-storage can only handle one command at a time. In fact, your USB stick can only handle one command at a time. Besides, if you look carefully at the times you'll see that the major source of delays isn't the interval between commands. It's the intervals within a single command. > 2. I have traced upper layer source code to find out where the scsi > cmd come from and I found it just pass from cmd = req->special; one > of element of request_queue that passes to scsi_request_fn? > where and how upper layer made it?(I guess it should made from block layer ) It depends on what command you're talking about. Some of them come from req->special as you saw, other SCSI commands are built specifically from req in the sd driver. See sd_prep_fn() in drivers/scsi/sd.c. Of course, req itself usually comes from the block layer -- but not always! See scsi_execute() and scsi_execute_req() in drivers/scsi/scsi_lib.c. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
