On Sat, Jan 17, 2026 at 03:59:22PM +0800, Tao Wang wrote:

quoted

Rather than using tx_q->tx_skbuff_dma[].last_segment to determine
whether the first descriptor entry is the only segment, calculate the
number of descriptor entries used. If there is only one descriptor,
then the first is also the last, so mark it as such.

This is a good idea. tx_q->tx_skbuff_dma[].last_segment no longer carries
 much meaning and can indeed be removed altogether.

quoted

+       is_last_segment = ((tx_q->cur_tx - first_entry) &
+                          (priv->dma_conf.dma_tx_size - 1)) == 1;

Since tx_q->cur_tx may wrap around and become smaller than first_entry,
the following statement is more concise:
is_last_segment = (tx_q->cur_tx == first_entry);

That's incorrect. We advance tx_q->cur_tx by at least one by this
point:

        first_entry = tx_q->cur_tx;

... fill descriptors ...

        /* We've used all descriptors we need for this skb, however,
         * advance cur_tx so that it references a fresh descriptor.
         * ndo_start_xmit will fill this descriptor the next time it's
         * called and stmmac_tx_clean may clean up to this descriptor.
         */
        tx_q->cur_tx = STMMAC_GET_ENTRY(tx_q->cur_tx, priv->dma_conf.dma_tx_size);

...

        /* If we only have one entry used, then the first entry is the last
         * segment.
         */
        is_last_segment = ((tx_q->cur_tx - first_entry) &
                           (priv->dma_conf.dma_tx_size - 1)) == 1;

So, replacing this with a check for tx_q->cur_tx == first_entry
would always be false here, unless we completely filled the ring
with a single TSO.

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