Alan Stern wrote: > On Thu, 30 Apr 2020, Thinh Nguyen wrote: > >> Hi, >> >> Alan Stern wrote: >>> On Thu, 30 Apr 2020, Thinh Nguyen wrote: >>> >>>> Peter Chen wrote: >>>>> If 'transfer size' here is software concept, why controller needs to know? The general >>>>> controller internal logic doesn't care class protocol, it only cares TRB chain software prepares. >>>> While some controllers don't have the concept of this, DWC_usb3x does. >>>> It has a notion of starting and ending a transfer. While a transfer is >>>> started, the endpoint uses a resource. It releases that resource when >>>> the transfer completes. So far, dwc3 implemented in such a way that bulk >>>> transfers are always in-progress and don't complete. That's fine so far, >>>> but it's not the case with streams. >>> This is peculiar. I haven't heard of any other controller doing this. >>> >>> What does the controller use this resource for? Would anything go >>> wrong if you told the controller that each transfer was a single >>> usb_request? >> It's no problem. Each transfer can be a single request. Just set the >> request->is_last. (Refer to [patch 2/5] for f_tcm). >> >> The issue here is that the controller needs to know when a stream >> completes so it can start on a different stream. In the controller > Why does it need to know this? Why can't it start on a different > stream at any time? By design, whenever the controller needs to start on a different stream, it will issue a START_TRANSFER command along with a stream ID. It cannot issue START_TRANSFER command again until the previous transfer completes or ended. > >> driver, this is done by setting a certain control bit in the TRB >> indicating the last TRB of a transfer. This knowledge can only come from >> the function driver, which is why we need this "is_last" field. > What's wrong with just assuming _every_ usb_request is the last one of > its transfer? Then you wouldn't have to add a new flag or change the > existing function drivers. That will affect performance. The driver will then need to service multiple interrupts and restart the transfer multiple times. BR, Thinh
