Re: [PATCH net] net: dsa: felix: fix oversize frame dropping for always closed tc-taprio gates

[Simon Horman]

On Thu, Aug 17, 2023 at 03:01:11PM +0300, Vladimir Oltean wrote:
> The blamed commit resolved a bug where frames would still get stuck at
> egress, even though they're smaller than the maxSDU[tc], because the
> driver did not take into account the extra 33 ns that the queue system
> needs for scheduling the frame.
> 
> It now takes that into account, but the arithmetic that we perform in
> vsc9959_tas_remaining_gate_len_ps() is buggy, because we operate on
> 64-bit unsigned integers, so gate_len_ns - VSC9959_TAS_MIN_GATE_LEN_NS
> may become a very large integer if gate_len_ns < 33 ns.
> 
> In practice, this means that we've introduced a regression where all
> traffic class gates which are permanently closed will not get detected
> by the driver, and we won't enable oversize frame dropping for them.
> 
> Before:
> mscc_felix 0000:00:00.5: port 0: max frame size 1526 needs 12400000 ps, 1152000 ps for mPackets at speed 1000
> mscc_felix 0000:00:00.5: port 0 tc 0 min gate len 1000000, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 1 min gate len 0, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 2 min gate len 0, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 3 min gate len 0, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 4 min gate len 0, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 5 min gate len 0, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 6 min gate len 0, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 7 min gate length 5120 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 615 octets including FCS
> 
> After:
> mscc_felix 0000:00:00.5: port 0: max frame size 1526 needs 12400000 ps, 1152000 ps for mPackets at speed 1000
> mscc_felix 0000:00:00.5: port 0 tc 0 min gate len 1000000, sending all frames
> mscc_felix 0000:00:00.5: port 0 tc 1 min gate length 0 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 1 octets including FCS
> mscc_felix 0000:00:00.5: port 0 tc 2 min gate length 0 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 1 octets including FCS
> mscc_felix 0000:00:00.5: port 0 tc 3 min gate length 0 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 1 octets including FCS
> mscc_felix 0000:00:00.5: port 0 tc 4 min gate length 0 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 1 octets including FCS
> mscc_felix 0000:00:00.5: port 0 tc 5 min gate length 0 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 1 octets including FCS
> mscc_felix 0000:00:00.5: port 0 tc 6 min gate length 0 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 1 octets including FCS
> mscc_felix 0000:00:00.5: port 0 tc 7 min gate length 5120 ns not enough for max frame size 1526 at 1000 Mbps, dropping frames over 615 octets including FCS
> 
> Fixes: 11afdc6526de ("net: dsa: felix: tc-taprio intervals smaller than MTU should send at least one packet")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@xxxxxxx>

Reviewed-by: Simon Horman <horms@xxxxxxxxxx>