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    Subject[PATCH 5.19 1153/1157] net: dsa: felix: fix min gate len calculation for tc when its first gate is closed
    Date
    From: Vladimir Oltean <vladimir.oltean@nxp.com>

    commit 7e4babffa6f340a74c820d44d44d16511e666424 upstream.

    min_gate_len[tc] is supposed to track the shortest interval of
    continuously open gates for a traffic class. For example, in the
    following case:

    TC 76543210

    t0 00000001b 200000 ns
    t1 00000010b 200000 ns

    min_gate_len[0] and min_gate_len[1] should be 200000, while
    min_gate_len[2-7] should be 0.

    However what happens is that min_gate_len[0] is 200000, but
    min_gate_len[1] ends up being 0 (despite gate_len[1] being 200000 at the
    point where the logic detects the gate close event for TC 1).

    The problem is that the code considers a "gate close" event whenever it
    sees that there is a 0 for that TC (essentially it's level rather than
    edge triggered). By doing that, any time a gate is seen as closed
    without having been open prior, gate_len, which is 0, will be written
    into min_gate_len. Once min_gate_len becomes 0, it's impossible for it
    to track anything higher than that (the length of actually open
    intervals).

    To fix this, we make the writing to min_gate_len[tc] be edge-triggered,
    which avoids writes for gates that are closed in consecutive intervals.
    However what this does is it makes us need to special-case the
    permanently closed gates at the end.

    Fixes: 55a515b1f5a9 ("net: dsa: felix: drop oversized frames with tc-taprio instead of hanging the port")
    Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
    Link: https://lore.kernel.org/r/20220804202817.1677572-1-vladimir.oltean@nxp.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/net/dsa/ocelot/felix_vsc9959.c | 15 ++++++++++++++-
    1 file changed, 14 insertions(+), 1 deletion(-)

    --- a/drivers/net/dsa/ocelot/felix_vsc9959.c
    +++ b/drivers/net/dsa/ocelot/felix_vsc9959.c
    @@ -1136,6 +1136,7 @@ static void vsc9959_tas_min_gate_lengths
    {
    struct tc_taprio_sched_entry *entry;
    u64 gate_len[OCELOT_NUM_TC];
    + u8 gates_ever_opened = 0;
    int tc, i, n;

    /* Initialize arrays */
    @@ -1163,16 +1164,28 @@ static void vsc9959_tas_min_gate_lengths
    for (tc = 0; tc < OCELOT_NUM_TC; tc++) {
    if (entry->gate_mask & BIT(tc)) {
    gate_len[tc] += entry->interval;
    + gates_ever_opened |= BIT(tc);
    } else {
    /* Gate closes now, record a potential new
    * minimum and reinitialize length
    */
    - if (min_gate_len[tc] > gate_len[tc])
    + if (min_gate_len[tc] > gate_len[tc] &&
    + gate_len[tc])
    min_gate_len[tc] = gate_len[tc];
    gate_len[tc] = 0;
    }
    }
    }
    +
    + /* min_gate_len[tc] actually tracks minimum *open* gate time, so for
    + * permanently closed gates, min_gate_len[tc] will still be U64_MAX.
    + * Therefore they are currently indistinguishable from permanently
    + * open gates. Overwrite the gate len with 0 when we know they're
    + * actually permanently closed, i.e. after the loop above.
    + */
    + for (tc = 0; tc < OCELOT_NUM_TC; tc++)
    + if (!(gates_ever_opened & BIT(tc)))
    + min_gate_len[tc] = 0;
    }

    /* Update QSYS_PORT_MAX_SDU to make sure the static guard bands added by the

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