Date:	Fri, 05 Jun 2009 16:25:45 +0200
From:	Marcel Holtmann <marcel@...tmann.org>
To:	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org, slapin@...fans.org,
	davem@...emloft.net, Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH 4/5] ieee802154: add documentation about our stack

Hi Dmitry,

> >> Add MAINTAINERS entry and a small text describing our stack interfaces,
> >> how to hook the drivers, etc.
> >>
> >> Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@...il.com>
> >> Signed-off-by: Sergey Lapin <slapin@...fans.org>
> >> ---
> >>  Documentation/networking/ieee802154.txt |   76 +++++++++++++++++++++++++++++++
> >>  MAINTAINERS                             |   12 +++++
> >>  2 files changed, 88 insertions(+), 0 deletions(-)
> >>  create mode 100644 Documentation/networking/ieee802154.txt
> >>
> >> diff --git a/Documentation/networking/ieee802154.txt b/Documentation/networking/ieee802154.txt
> >> new file mode 100644
> >> index 0000000..a0280ad
> >> --- /dev/null
> >> +++ b/Documentation/networking/ieee802154.txt
> >> @@ -0,0 +1,76 @@
> >> +
> >> +             Linux IEEE 802.15.4 implementation
> >> +
> >> +
> >> +Introduction
> >> +============
> >> +
> >> +The Linux-ZigBee project goal is to provide complete implementation
> >> +of IEEE 802.15.4 / ZigBee / 6LoWPAN protocols. IEEE 802.15.4 is a stack
> >> +of protocols for organizing Low-Rate Wireless Personal Area Networks.
> >> +
> >> +Currently only IEEE 802.15.4 layer is implemented. We have choosen
> >> +to use plain Berkeley socket API, the generic Linux networking stack
> >> +to transfer IEEE 802.15.4 messages and a special protocol over genetlink
> >> +for configuration/management
> >> +
> >> +
> >> +Socket API
> >> +==========
> >> +
> >> +int sd = socket(PF_IEEE802154, SOCK_DGRAM, 0);
> >> +.....
> >> +
> >> +The address family, socket addresses etc. are defined in the
> >> +include/net/ieee802154/af_ieee802154.h header or in the special header
> >> +in our userspace package (see either linux-zigbee sourceforge download page
> >> +or git tree at git://linux-zigbee.git.sourceforge.net/gitroot/linux-zigbee).
> >> +
> >> +One can use SOCK_RAW for passing raw data towards device xmit function. YMMV.
> >> +
> >> +
> >> +MLME - MAC Level Management
> >> +============================
> >> +
> >> +Most of IEEE 802.15.4 MLME interfaces are directly mapped on netlink commands.
> >> +See the include/net/ieee802154/nl802154.h header. Our userspace tools package
> >> +(see above) provides CLI configuration utility for radio interfaces and simple
> >> +coordinator for IEEE 802.15.4 networks as an example users of MLME protocol.
> >> +
> >> +
> >> +Kernel side
> >> +=============
> >> +
> >> +Like with WiFi, there are several types of devices implementing IEEE 802.15.4.
> >> +1) 'HardMAC'. The MAC layer is implemented in the device itself, the device
> >> +   exports MLME and data API.
> >> +2) 'SoftMAC' or just radio. These types of devices are just radio transceivers
> >> +   possibly with some kinds of acceleration like automatic CRC computation and
> >> +   comparation, automagic ACK handling, address matching, etc.
> >> +
> >> +Those types of devices require different approach to be hooked into Linux kernel.
> >> +
> >> +
> >> +HardMAC
> >> +=======
> >> +
> >> +See the header include/net/ieee802154/netdevice.h. You have to implement Linux
> >> +net_device, with .type = ARPHRD_IEEE802154. Data is exchanged with socket family
> >> +code via plain sk_buffs. The control block of sk_buffs will contain additional
> >> +info as described in the struct ieee802154_mac_cb.
> >
> > are you sending IP packets over this ARPHRD_IEEE802154 network devices
> > or are you just abusing it as a control interface. If it is the later
> > then can we please stop doing this. I really dislike these irda0,
> > wmaster0 and alike stuff. If you can't configure it as real IP device,
> > you should not use struct net_device at all from my point of view. For
> > all your control details you have netlink so no need for an extra device
> > here unless it can actually talk IP (or similar like IPX etc.).
> 
> Hmm. Once I've thought about it, as net_device seemed a Really Big Thing to me
> to be used for our stack. However from the beginning we wanted to use plain BSD
> sockets API, we wanted to use sk_buffs (of course) for the whole
> message interface.
> And when prototyping lowpan_device, I ended duplicating lots of net_device
> functionality on my own.
> Either we have to develop some lightweight "net_device" abstracted from IP & Ko,
> but containing sk_buff queues, header ops, etc, or end up with functionality
> duplication and hacks like in bluetooth (where skb->dev is used to store hci_dev
> instead of net_device).

that skb->dev pointer is typed as net_device is just a cosmetic detail
from my point of view. And having Bluetooth use it as hci_dev is not
actually a hack. We just can't send Bluetooth HCI skb directly to the IP
layer, but we also don't do that ;)

> Hmm. AX.25, AppleTalk, CAN, DecNet, most of other procotols sitting inside
> Linux kernel do use net_device structure, why should we differ?

Don't know about CAN since I never looked deep enough into it. However
for the other, they do transport some sort of networking packets over
it. So my point is as long as I can use ifconfig of ip to set an address
on these interfaces and route them it makes sense to. Just to reuse
net_device because it is convenient for a control interface sounds wrong
to me. The IrDA stuff is one big example of this.

> Moreover, once we implement 6lowpan support (an encapsulation for IPv6 frames
> into IEEE 802.15.4 ones) it will be possible to send IPv6 frames directly over
> IEEE 802.15.4.

So using the socket interfaces has nothing do with the net_device you
are creating. So that argument doesn't work for me. Even mac80211
nowadays uses a proper hardware abstraction (ieee80211_hw and
ieee80211_ops) and you should do the same.

Once you have actually implemented 6lowpan then these are the ones that
should use net_device and show up within ifconfig.

Regards

Marcel


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