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[Bug 1859378] Re: xhci Control Transfer requiring a Status TRB before st
Benjamin David Lunt
[Bug 1859378] Re: xhci Control Transfer requiring a Status TRB before starting transfer
Mon, 13 Jan 2020 13:53:04 -0000
I believe it is required to send an error event. It checked for the
STATUS TRB and found that it was missing, therefore it must send an
This is (not so clearly) stated in the specification and I have quoted
this in a previous comment.
I took it as:
If the controller checks for the error and then does not processes the transfer
due to the check, the controller is required to place an Error Event on the
Therefore, QEMU either needs to follow through with the check and place
an Error on the Event Ring *or* not do the 'check and return'.
Anyway, this is just my opinion. Thank you for your time.
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xhci Control Transfer requiring a Status TRB before starting transfer
Status in QEMU:
This may not necessarily be a bug, but more of a change.
A little background may need to be in order.
With all USB Control transfers, there is a SETUP transfer, zero or
more DATA transfers, and if successful, a STATUS transfer. This
STATUS transfer is used to indicate to the recipient that the previous
transfers were successful. For example, in a CONTROL IN transfer, the
host sends a SETUP packet to the device, receives zero or more DATA
packets, and then on successful transfer, the HOST sends the STATUS
packet indicating to the device that all was received.
If no DATA packets are received, the HOST is not to send a STATUS
packet. This could be due to a STALL or other error.
With this in mind, the STATUS transfer, in this case an xHCI STATUS
TRB, may not even be on the transfer ring yet. The HOST software may
be waiting for a successful transfer before it submits the STATUS
However, if you look at the test at line 1701
the current code will not start the CONTROL transfer at all if it
doesn't see that STATUS TRB on the ring.
In my opinion, this is in error. It is not required that a STATUS TRB
be on the ring to start the CONTROL transfer. This STATUS TRB can be
placed on the ring after a successful SETUP and zero or more DATA
transfers followed by a ring to the door bell. Then after a
successful transfer to this point, placing this STATUS TRB on the ring
and another ring to the door bell.
In my opinion, the check at line 1701 should be removed.
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