net: dsa: felix: fix oversize frame dropping for always closed tc-taprio gates

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: 11afdc6526 ("net: dsa: felix: tc-taprio intervals smaller than MTU should send at least one packet")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230817120111.3522827-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/ocelot/felix_vsc9959.c

@@ -1069,6 +1069,9 @@ static u64 vsc9959_tas_remaining_gate_len_ps(u64 gate_len_ns)
 	if (gate_len_ns == U64_MAX)
 		return U64_MAX;
 
+	if (gate_len_ns < VSC9959_TAS_MIN_GATE_LEN_NS)
+		return 0;
+
 	return (gate_len_ns - VSC9959_TAS_MIN_GATE_LEN_NS) * PSEC_PER_NSEC;
 }