Re: [PATCH net] net: dsa: felix: re-enable TAS guard band mode

On Mon, May 10, 2021 at 01:07:08PM +0200, Michael Walle wrote:

Commit 316bcffe4479 ("net: dsa: felix: disable always guard band bit for
TAS config") disabled the guard band and broke 802.3Qbv compliance.

There are two issues here:
 (1) Without the guard band the end of the scheduling window could be
     overrun by a frame in transit.
 (2) Frames that don't fit into a configured window will still be sent.

The reason for both issues is that the switch will schedule the _start_
of a frame transmission inside the predefined window without taking the
length of the frame into account. Thus, we'll need the guard band which
will close the gate early, so that a complete frame can still be sent.
Revert the commit and add a note.

For a lengthy discussion see [1].

[1] https://lore.kernel.org/netdev/c7618025da6723418c56a54fe4683bd7@walle.cc/ (local)

Fixes: 316bcffe4479 ("net: dsa: felix: disable always guard band bit for TAS config")
Signed-off-by: Michael Walle <redacted>
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Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>