On Thu, 7 May 2009, Alan Cox wrote: > > Write routine: > > Copy bytes into the available URB buffers, submitting URBs as > > they get filled. At the end, if the next URB is partially full > > then submit it only if NIF < P. > > There are good known algorithms for this and essentially the logic is > about speed and time estimation. You don't need to queue a partially > filled URB until you reach the time point that the hardware is in danger > of having sent everything already queued. So under load once you've > stuffed 4K down the pipe you know at 38400 baud that you can relax for > quite some time. Agreed. An implicit point of my earlier message was that we might be able to avoid tricky time-and-speed estimates by relying on the fact that the USB-Serial device will NAK an URB until it has sufficient buffer space to accept the data (i.e., flow control is built in at the USB protocol level). So instead of relaxing for some difficult-to-estimate time, we can simply relax until the next URB completion occurs. As long as a certain minimum number of URBs are still in flight, we can assume that the hardware isn't in danger of sending all the queued data. This assumption might not be valid at the start of a transfer, since an URB can't complete for a minimum of 1 ms (at full speed), but it should stabilize quickly. > > Allocating URBs on the fly adds a lot of complication. There has to be > > a minimum number of pre-allocated URBs; otherwise write_room could > > never return a positive value. If you allocate additional URBs > > later on, when would you free them? > > write_room can allocate URBs I think (would need to be atomic allocations) > > write_room() > if no_urbs && urbs_queued < max_queued > allocate an urb > return space_in_last_urb; That would work. It doesn't seem to be much better than allocating URBs in the write routine. After all, as long as allocations always succeed the details don't matter much. A more interesting question is whether to deallocate URBs as they complete or to reuse them. A simple approach (which Oliver no doubt has already considered) is: write_complete(urb): if (write_urb->transfer_length > 0) submit(write_urb); write_urb = urb; else urb_free(urb); 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
