[Qemu-devel] NetClientInfo::can_receive change notification and its purpose

[Max Filippov]

Hello,

is there any way to notify networking core that adapter's
NetClientInfo::can_receive return value is about to change from 0 to 1?
I'm trying to talk to DHCP server from the recent (2014.01) u-boot running
in the xtensa system emulation over the tap interface. On the guest I see
the following picture (lines marked **...** are printed from the qemu):

U-Boot> dhcp
**open_eth_can_receive: 0**
**open_eth_can_receive: 0**
BOOTP broadcast 1
BOOTP broadcast 2
BOOTP broadcast 3
BOOTP broadcast 4
BOOTP broadcast 5

Retry count exceeded; starting again
ethoc
BOOTP broadcast 1
BOOTP broadcast 2
BOOTP broadcast 3
BOOTP broadcast 4
BOOTP broadcast 5

Retry count exceeded; starting again
^C
**open_eth_can_receive: 1**
**open_eth_can_receive: 1**

The u-boot driver enables packet reception on the network adapter just
before it starts sending packets, but on the networking core doesn't check
that NetClientInfo::can_receive has changed until I press ^C to interrupt
it. Then it receives some of the pending packets, but throws them away.
On the host side I see that DHCP server replies immediately.

When I remove NetClientInfo::can_receive callback from my network
adapter and throw away packets that cannot be received in the
NetClientInfo::receive callback it all works well. So I've got two questions:
- is there any way to push NetClientInfo::can_receive update to the
  network core?
- wouldn't it be more correct to emulate real hardware more closely:
  always be ready to receive a packet, but throw it away if there's e.g.
  no buffers or the receiver is disabled?

-- 
Thanks.
-- Max