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    Subject[PATCH 5.12 086/173] usb: f_ncm: only first packet of aggregate needs to start timer
    Date
    From: Maciej Żenczykowski <maze@google.com>

    commit 1958ff5ad2d4908b44a72bcf564dfe67c981e7fe upstream.

    The reasoning for this change is that if we already had
    a packet pending, then we also already had a pending timer,
    and as such there is no need to reschedule it.

    This also prevents packets getting delayed 60 ms worst case
    under a tiny packet every 290us transmit load, by keeping the
    timeout always relative to the first queued up packet.
    (300us delay * 16KB max aggregation / 80 byte packet =~ 60 ms)

    As such the first packet is now at most delayed by 300us.

    Under low transmit load, this will simply result in us sending
    a shorter aggregate, as originally intended.

    This patch has the benefit of greatly reducing (by ~10 factor
    with 1500 byte frames aggregated into 16 kiB) the number of
    (potentially pretty costly) updates to the hrtimer.

    Cc: Brooke Basile <brookebasile@gmail.com>
    Cc: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
    Cc: Felipe Balbi <balbi@kernel.org>
    Cc: Lorenzo Colitti <lorenzo@google.com>
    Signed-off-by: Maciej Żenczykowski <maze@google.com>
    Link: https://lore.kernel.org/r/20210608085438.813960-1-zenczykowski@gmail.com
    Cc: stable <stable@vger.kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/usb/gadget/function/f_ncm.c | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    --- a/drivers/usb/gadget/function/f_ncm.c
    +++ b/drivers/usb/gadget/function/f_ncm.c
    @@ -1101,11 +1101,11 @@ static struct sk_buff *ncm_wrap_ntb(stru
    ncm->ndp_dgram_count = 1;

    /* Note: we skip opts->next_ndp_index */
    - }

    - /* Delay the timer. */
    - hrtimer_start(&ncm->task_timer, TX_TIMEOUT_NSECS,
    - HRTIMER_MODE_REL_SOFT);
    + /* Start the timer. */
    + hrtimer_start(&ncm->task_timer, TX_TIMEOUT_NSECS,
    + HRTIMER_MODE_REL_SOFT);
    + }

    /* Add the datagram position entries */
    ntb_ndp = skb_put_zero(ncm->skb_tx_ndp, dgram_idx_len);

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