On 10-02-26 04:38 PM, Anton Vorontsov wrote:

OK, I think I found what's happening in gianfar.

Some background...

start_xmit() prepares new skb for transmitting, generally it does
three things:

1. sets up all BDs (marks them ready to send), except the first one.
2. stores skb into tx_queue->tx_skbuff so that clean_tx_ring()
    would cleanup it later.
3. sets up the first BD, i.e. marks it ready.

Here is what clean_tx_ring() does:

1. reads skbs from tx_queue->tx_skbuff
2. Checks if the *last* BD is ready. If it's still ready [to send]
    then it it isn't transmitted, so clean_tx_ring() returns.
    Otherwise it actually cleanups BDs. All is OK.

Now, if there is just one BD, code flow:

- start_xmit(): stores skb into tx_skbuff. Note that the first BD
   (which is also the last one) isn't marked as ready, yet.
- clean_tx_ring(): sees that skb is not null, *and* its lstatus
   says that it is NOT ready (like if BD was sent), so it cleans
   it up (bad!)
- start_xmit(): marks BD as ready [to send], but it's too late.

We can fix this simply by reordering lstatus/tx_skbuff writes.

It works flawlessly on my p2020, please try it.

I've skipped right to the test part (I'll think about the description
more later) and it passed 5 out of 5 boot tests on NFSroot sbc8641d.
Looks like you've got a solution.

Paul.