patch "usb: f_ncm: only first packet of aggregate needs to start timer" added to usb-linus

This is a note to let you know that I've just added the patch titled

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

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 1958ff5ad2d4908b44a72bcf564dfe67c981e7fe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Maciej=20=C5=BBenczykowski?= <[email protected]>
Date: Tue, 8 Jun 2021 01:54:38 -0700
Subject: usb: f_ncm: only first packet of aggregate needs to start timer
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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.

---
 drivers/usb/gadget/function/f_ncm.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index 0d23c6c11a13..855127249f24 100644
@@ -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);

2.32.0