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

[Vladimir Oltean]

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@xxxxxxxx/
> 
> Fixes: 316bcffe4479 ("net: dsa: felix: disable always guard band bit for TAS config")
> Signed-off-by: Michael Walle <michael@xxxxxxxx>
> ---

Reviewed-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>