tree cb98505a1642c47c8b52ee555a9486420955f26e
parent 4fa4052c7ebb59e4d4aa396f1563c89118623ec7
author Michael Brown <mcb30@ipxe.org> 1687344593 +0100
committer Michael Brown <mcb30@ipxe.org> 1687344593 +0100

[efi] Always poll for TX completions

Polling for TX completions is arguably redundant when there are no
transmissions currently in progress.  Commit c6c7e78 ("[efi] Poll for
TX completions only when there is an outstanding TX buffer") switched
to setting the PXE_OPFLAGS_GET_TRANSMITTED_BUFFERS flag only when
there is an in-progress transmission awaiting completion, in order to
reduce reported TX errors and debug message noise from buggy NII
implementations that report spurious TX completions whenever the
transmit queue is empty.

Some other NII implementations (observed with the Realtek driver in a
Dell Latitude 3440) seem to have a bug in the transmit datapath
handling which results in the transmit ring freezing after sending a
few hundred packets under heavy load.  The symptoms are that the
TPPoll register's NPQ bit remains set and the 256-entry transmit ring
contains a large number of uncompleted descriptors (with the OWN bit
set), the first two of which have identical data buffer addresses.

Though iPXE will submit at most one in-progress transmission via NII,
the Dell/Realtek driver seems to make a page-aligned copy of each
transmit data buffer and to report TX completions immediately without
waiting for the packet to actually be transmitted.  These synthetic TX
completions continue even after the hardware transmit ring freezes.

Setting PXE_OPFLAGS_GET_TRANSMITTED_BUFFERS on every poll reduces the
probability of this Dell/Realtek driver bug being triggered by a
factor of around 500, which brings the failure rate down to the point
that it can sensibly be managed by external logic such as the
"--timeout" option for image downloads.  Closing and reopening the
interface (via "ifclose"/"ifopen") will clear the error condition and
allow transmissions to resume.

Revert to setting PXE_OPFLAGS_GET_TRANSMITTED_BUFFERS on every poll,
and silently ignore situations in which the hardware reports a
completion when no transmission is in progress.  This approximately
matches the behaviour of the SnpDxe driver, which will also generally
set PXE_OPFLAGS_GET_TRANSMITTED_BUFFERS on every poll.

Signed-off-by: Michael Brown <mcb30@ipxe.org>