[PATCH 5.12 086/173] usb: f_ncm: only first packet of aggregate needs to start timer

[Greg Kroah-Hartman]

From: Maciej Żenczykowski <maze@xxxxxxxxxx>

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@xxxxxxxxx>
Cc: Bryan O'Donoghue <bryan.odonoghue@xxxxxxxxxx>
Cc: Felipe Balbi <balbi@xxxxxxxxxx>
Cc: Lorenzo Colitti <lorenzo@xxxxxxxxxx>
Signed-off-by: Maciej Żenczykowski <maze@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20210608085438.813960-1-zenczykowski@xxxxxxxxx
Cc: stable <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 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);