usb: f_ncm: only first packet of aggregate needs to start timer

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>

diff --git a/drivers/usb/gadget/function/f_ncm.c b/drivers/usb/gadget/function/f_ncm.c
index 0d23c6c..8551272 100644 (file)
--- 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(struct gether *port,
                        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);