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

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Date:   Mon, 10 May 2021 12:23:10 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Michael Walle <michael@...le.cc>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        "UNGLinuxDriver@...rochip.com" <UNGLinuxDriver@...rochip.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>
Subject: 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/
> 
> Fixes: 316bcffe4479 ("net: dsa: felix: disable always guard band bit for TAS config")
> Signed-off-by: Michael Walle <michael@...le.cc>
> ---

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