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From: Hayes Wang <hayeswang@realtek.com>
To: Eric Dumazet <eric.dumazet@gmail.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Cc: nic_swsd <nic_swsd@realtek.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH net-next] r8152: divide the tx and rx bottom functions
Date: Fri, 16 Aug 2019 09:08:41 +0000
Message-ID: <0835B3720019904CB8F7AA43166CEEB2F18D47C8@RTITMBSVM03.realtek.com.tw> (raw)
In-Reply-To: <68015004-fb60-f6c6-05b0-610466223cf5@gmail.com>

Eric Dumazet [mailto:eric.dumazet@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



  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-14  8:30 Hayes Wang
2019-08-15 20:58 ` David Miller
2019-08-16  2:59   ` Hayes Wang
2019-08-16  5:17     ` David Miller
2019-08-16  6:39 ` Eric Dumazet
2019-08-16  8:10   ` Hayes Wang
2019-08-16  8:19     ` Eric Dumazet
2019-08-16  9:08       ` Hayes Wang [this message]
2019-08-16  9:27         ` Eric Dumazet
2019-08-16 10:04           ` Hayes Wang
2019-08-19  6:40 ` [PATCH net-next v2] " Hayes Wang
2019-08-20 19:19   ` David Miller

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