[PATCH net] net: stmmac: raise TX completion interrupt at the end of an xmit burst

[Johan Alvarado]

On Mon, Jul 06, 2026 at 01:04:44PM +0200, Maciej Fijalkowski wrote:
> Very messy thread. To reiterate - does tx coalescing do any good in this
> driver?

It does, but today only for traffic that queues deeply enough. Within an
xmit_more batch the frame counter reduces the IC density to one descriptor
per tx_coal_frames, and that keeps working with this patch applied: in the
447 Mbit/s test from the changelog the NIC raised ~3.3k IRQs/s for ~40 kpps
on the wire, i.e. roughly 12 packets per interrupt, because qdisc bulk
dequeue, TSO and the intra-batch frame counter still coalesce. What was
broken is only the case where a flow never accumulates tx_coal_frames
in-flight frames; there the timer was the sole completion mechanism.

> I did some digging and seems there was a rework of coalescing in 2018 and
> then some more polishing happened in 2023 (net: stmmac: improve TX timer
> arm logic).

The 2023 change only avoids re-arming the timer while NAPI is already
scheduled, which helps the loaded case. For a lightly queued flow NAPI is
idle, so the timer still gets armed and the completion still waits out the
full tx_coal_timer. That is the path this patch addresses.

> Wouldn't your change imply that tx coalescing could be dropped altogether?
> I do agree that each single batch of tx descs should be signalled with ic
> bit at the end.

I don't think it can be dropped, for one correctness reason and two
practical ones.

The correctness one: the frame counter enforces an upper bound of
tx_coal_frames descriptors on the distance between IC bits in the TX ring.
If coalescing were dropped and the IC bit only set on batch tails, that
distance becomes unbounded, and a batch larger than the ring can fill it
without a single IC descriptor: every frame queued had xmit_more set, and
the tail frame that would have carried the IC bit is never queued because
stmmac_xmit stops the queue first. The hardware drains the entire ring
without raising a completion interrupt, and only the coalesce timer can
restart the queue - reintroducing the same timer-bound stall this patch
removes, now on the bulk path and once per ring drain. With the bound in
place a full ring always contains at least dma_tx_size / tx_coal_frames
completion points; even the minimum 64-descriptor ring (DMA_MIN_TX_SIZE)
still holds two.

The practical ones: the frame counter is what keeps IC density low inside
large batches (one IC per 25 descriptors rather than a single one at the
tail with nothing in between), and tx-frames / tx-usecs are user-visible
ethtool knobs on this driver that users can rely on.

Dropping the frame/timer machinery would also be a rework rather than a
fix. If there is appetite for that I would rather keep this patch minimal
for net and discuss a rework separately for net-next.

Best regards,
Johan