[PATCH 5.4 47/84] usb: f_ncm: only first packet of aggregate needs to start timer

Greg Kroah-Hartman
2021-06-14T10:58:46Z

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);