On Tue, 22 May 2007, Curran, Dominic wrote: > Hi > I have two questions regarding short transfers. > My questions are best illustrated by a scenario: > > Lets say the Hosts Mass Storage driver wants to READ 12K: > > I believe that the Host Controller driver (e.g. ehci_hcd) might see > something like this coming from usbcore: > > 1) BULK OUT urb (31 bytes) [Contains READ10 command] > 2) BULK IN urb (4K) [data] > 3) BULK IN urb (8K) [data] > 4) BULK IN urb (13 bytes) [status]
Correct. However 1) must complete before 2) and 3) are submitted, and 3) must complete before 4) is submitted. This is not a fundamental limitation; it's just the way usb-storage currently works. > I have two questions regarding what happens when the transfer gets > terminated early: > > > 1) What happens if the device terminates the transfer after the 4KB > (with a zero-length packet)? > > Does the HC driver... > - set status to 0 in the 4K urb (& actual_length=4096) and Yes. > - flush all other queued urbs for that EP (in this case the 8K urb) by > completing them with an error status ? If so what is the status ? Not exactly. The 8K URB completes with an error status -EREMOTEIO. However if there were any other URBs in the endpoint queue at that time, the HCD would not flush them. The flushing would be done by the completion handler for the 8K URB; it would call usb_unlink_urb() for all the remaining URBs. > 2) What happens if the device terminates the transfer during the 4KB > (after say 3700 bytes received, with a short packet)? > > Thus: > urb->transfer_buffer_length = 4096 > urb->actual_length = 3700 > > I believe that the HC driver should do the following test: > > if( (urb->actual_length < urb->transfer_buffer_length) && > (urb->transfer_flags & URB_SHORT_NOT_OK) ) > urb->status = -EREMOTEIO; > > So if URB_SHORT_NOT_OK is set then -EREMOTEIO will be returned And if > URB_SHORT_NOT_OK is not set the 0 will be returned. > > Is this correct ? Yes. The same thing would happen to the 8K URB in your first question. > Again I assume that the 8K urb should be completed immediately ? > If so with what status ? No. The 4K URB's completion handler would see the -EREMOTEIO error and would unlink the 8K URB, which would then complete with status -ECONNRESET. > I'm having trouble understand the EHCI code; In the EHCI driver where > are these situation handled ? qh_completions() ? Part of your trouble is because some of these situations are handled outside the EHCI driver. Take a look at the usb_sg_* routines in drivers/usb/core/message.c. Also read the kerneldoc comments in drivers/usb/core/urb.c and read Documentation/usb/error-codes.txt. Alan Stern ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
