Eric Dumazet [mailto:eric.duma...@gmail.com]
> Sent: Friday, August 16, 2019 4:20 PM
[...]
> Which callback ?
The USB device has two endpoints for Tx and Rx.
If I submit tx or rx URB to the USB host controller,
the relative callback functions would be called, when
they are finished. For rx, it is read_bulk_callback.
For tx, it is write_bulk_callback.
> After an idle period (no activity, no prior packets being tx-completed ...),
> a packet is sent by the upper stack, enters the ndo_start_xmit() of a network
> driver.
>
> This driver ndo_start_xmit() simply adds an skb to a local list, and returns.
Base on the current method (without tasklet), when
ndo_start_xmit() is called, napi_schedule is called only
if there is at least one free buffer (!list_empty(&tp->tx_free))
to transmit the packet. Then, the flow would be as following.
- Call r8152_poll
-- Call bottom_half
--- Call tx_bottom
---- Call r8152_tx_agg_fill
----- submit tx urb
- Call write_bulk_callback if tx is completed
When the tx transfer is completed, write_bulk_callback would
be called. And it would check if there is any tx packet
in &tp->tx_queue and determine whether it is necessary to
schedule the napi again or not.
> Where/how is scheduled this callback ?
For tx, you could find the following code in r8152_tx_agg_fill().
usb_fill_bulk_urb(agg->urb, tp->udev, usb_sndbulkpipe(tp->udev, 2),
agg->head, (int)(tx_data - (u8 *)agg->head),
(usb_complete_t)write_bulk_callback, agg);
ret = usb_submit_urb(agg->urb, GFP_ATOMIC);
For rx you could find the following code in r8152_submit_rx().
usb_fill_bulk_urb(agg->urb, tp->udev, usb_rcvbulkpipe(tp->udev, 1),
agg->buffer, tp->rx_buf_sz,
(usb_complete_t)read_bulk_callback, agg);
ret = usb_submit_urb(agg->urb, mem_flags);
> Some kind of timer ?
> An (unrelated) incoming packet ?
When the rx or tx is completed, a interrupt of USB
host controller would be triggered. Then, the callback
functions would be called.
Best Regards,
Hayes
