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-HISTORY:
-February 16/2002 -- revision 0.2.1:
-COR typo corrected
-February 10/2002 -- revision 0.2:
-some spell checking ;->
-January 12/2002 -- revision 0.1
-This is still work in progress so may change.
-To keep up to date please watch this space.
-
-Introduction to NAPI
-====================
-
-NAPI is a proven (www.cyberus.ca/~hadi/usenix-paper.tgz) technique
-to improve network performance on Linux. For more details please
-read that paper.
-NAPI provides a "inherent mitigation" which is bound by system capacity
-as can be seen from the following data collected by Robert on Gigabit
-ethernet (e1000):
-
- Psize Ipps Tput Rxint Txint Done Ndone
- ---------------------------------------------------------------
- 60 890000 409362 17 27622 7 6823
- 128 758150 464364 21 9301 10 7738
- 256 445632 774646 42 15507 21 12906
- 512 232666 994445 241292 19147 241192 1062
- 1024 119061 1000003 872519 19258 872511 0
- 1440 85193 1000003 946576 19505 946569 0
-
-
-Legend:
-"Ipps" stands for input packets per second.
-"Tput" == packets out of total 1M that made it out.
-"txint" == transmit completion interrupts seen
-"Done" == The number of times that the poll() managed to pull all
-packets out of the rx ring. Note from this that the lower the
-load the more we could clean up the rxring
-"Ndone" == is the converse of "Done". Note again, that the higher
-the load the more times we couldn't clean up the rxring.
-
-Observe that:
-when the NIC receives 890Kpackets/sec only 17 rx interrupts are generated.
-The system cant handle the processing at 1 interrupt/packet at that load level.
-At lower rates on the other hand, rx interrupts go up and therefore the
-interrupt/packet ratio goes up (as observable from that table). So there is
-possibility that under low enough input, you get one poll call for each
-input packet caused by a single interrupt each time. And if the system
-cant handle interrupt per packet ratio of 1, then it will just have to
-chug along ....
-
-
-0) Prerequisites:
-==================
-A driver MAY continue using the old 2.4 technique for interfacing
-to the network stack and not benefit from the NAPI changes.
-NAPI additions to the kernel do not break backward compatibility.
-NAPI, however, requires the following features to be available:
-
-A) DMA ring or enough RAM to store packets in software devices.
-
-B) Ability to turn off interrupts or maybe events that send packets up
-the stack.
-
-NAPI processes packet events in what is known as dev->poll() method.
-Typically, only packet receive events are processed in dev->poll().
-The rest of the events MAY be processed by the regular interrupt handler
-to reduce processing latency (justified also because there are not that
-many of them).
-Note, however, NAPI does not enforce that dev->poll() only processes
-receive events.
-Tests with the tulip driver indicated slightly increased latency if
-all of the interrupt handler is moved to dev->poll(). Also MII handling
-gets a little trickier.
-The example used in this document is to move the receive processing only
-to dev->poll(); this is shown with the patch for the tulip driver.
-For an example of code that moves all the interrupt driver to
-dev->poll() look at the ported e1000 code.
-
-There are caveats that might force you to go with moving everything to
-dev->poll(). Different NICs work differently depending on their status/event
-acknowledgement setup.
-There are two types of event register ACK mechanisms.
- I) what is known as Clear-on-read (COR).
- when you read the status/event register, it clears everything!
- The natsemi and sunbmac NICs are known to do this.
- In this case your only choice is to move all to dev->poll()
-
- II) Clear-on-write (COW)
- i) you clear the status by writing a 1 in the bit-location you want.
- These are the majority of the NICs and work the best with NAPI.
- Put only receive events in dev->poll(); leave the rest in
- the old interrupt handler.
- ii) whatever you write in the status register clears every thing ;->
- Cant seem to find any supported by Linux which do this. If
- someone knows such a chip email us please.
- Move all to dev->poll()
-
-C) Ability to detect new work correctly.
-NAPI works by shutting down event interrupts when there's work and
-turning them on when there's none.
-New packets might show up in the small window while interrupts were being
-re-enabled (refer to appendix 2). A packet might sneak in during the period
-we are enabling interrupts. We only get to know about such a packet when the
-next new packet arrives and generates an interrupt.
-Essentially, there is a small window of opportunity for a race condition
-which for clarity we'll refer to as the "rotting packet".
-
-This is a very important topic and appendix 2 is dedicated for more
-discussion.
-
-Locking rules and environmental guarantees
-==========================================
-
--Guarantee: Only one CPU at any time can call dev->poll(); this is because
-only one CPU can pick the initial interrupt and hence the initial
-netif_rx_schedule(dev);
-- The core layer invokes devices to send packets in a round robin format.
-This implies receive is totally lockless because of the guarantee that only
-one CPU is executing it.
-- contention can only be the result of some other CPU accessing the rx
-ring. This happens only in close() and suspend() (when these methods
-try to clean the rx ring);
-****guarantee: driver authors need not worry about this; synchronization
-is taken care for them by the top net layer.
--local interrupts are enabled (if you dont move all to dev->poll()). For
-example link/MII and txcomplete continue functioning just same old way.
-This improves the latency of processing these events. It is also assumed that
-the receive interrupt is the largest cause of noise. Note this might not
-always be true.
-[according to Manfred Spraul, the winbond insists on sending one
-txmitcomplete interrupt for each packet (although this can be mitigated)].
-For these broken drivers, move all to dev->poll().
-
-For the rest of this text, we'll assume that dev->poll() only
-processes receive events.
-
-new methods introduce by NAPI
-=============================
-
-a) netif_rx_schedule(dev)
-Called by an IRQ handler to schedule a poll for device
-
-b) netif_rx_schedule_prep(dev)
-puts the device in a state which allows for it to be added to the
-CPU polling list if it is up and running. You can look at this as
-the first half of netif_rx_schedule(dev) above; the second half
-being c) below.
-
-c) __netif_rx_schedule(dev)
-Add device to the poll list for this CPU; assuming that _prep above
-has already been called and returned 1.
-
-d) netif_rx_reschedule(dev, undo)
-Called to reschedule polling for device specifically for some
-deficient hardware. Read Appendix 2 for more details.
-
-e) netif_rx_complete(dev)
-
-Remove interface from the CPU poll list: it must be in the poll list
-on current cpu. This primitive is called by dev->poll(), when
-it completes its work. The device cannot be out of poll list at this
-call, if it is then clearly it is a BUG(). You'll know ;->
-
-All of the above methods are used below, so keep reading for clarity.
-
-Device driver changes to be made when porting NAPI
-==================================================
-
-Below we describe what kind of changes are required for NAPI to work.
-
-1) introduction of dev->poll() method
-=====================================
-
-This is the method that is invoked by the network core when it requests
-for new packets from the driver. A driver is allowed to send upto
-dev->quota packets by the current CPU before yielding to the network
-subsystem (so other devices can also get opportunity to send to the stack).
-
-dev->poll() prototype looks as follows:
-int my_poll(struct net_device *dev, int *budget)
-
-budget is the remaining number of packets the network subsystem on the
-current CPU can send up the stack before yielding to other system tasks.
-*Each driver is responsible for decrementing budget by the total number of
-packets sent.
- Total number of packets cannot exceed dev->quota.
-
-dev->poll() method is invoked by the top layer, the driver just sends if it
-can to the stack the packet quantity requested.
-
-more on dev->poll() below after the interrupt changes are explained.
-
-2) registering dev->poll() method
-===================================
-
-dev->poll should be set in the dev->probe() method.
-e.g:
-dev->open = my_open;
-.
-.
-/* two new additions */
-/* first register my poll method */
-dev->poll = my_poll;
-/* next register my weight/quanta; can be overridden in /proc */
-dev->weight = 16;
-.
-.
-dev->stop = my_close;
-
-
-
-3) scheduling dev->poll()
-=============================
-This involves modifying the interrupt handler and the code
-path which takes the packet off the NIC and sends them to the
-stack.
-
-it's important at this point to introduce the classical D Becker
-interrupt processor:
-
-------------------
-static irqreturn_t
-netdevice_interrupt(int irq, void *dev_id, struct pt_regs *regs)
-{
-
- struct net_device *dev = (struct net_device *)dev_instance;
- struct my_private *tp = (struct my_private *)dev->priv;
-
- int work_count = my_work_count;
- status = read_interrupt_status_reg();
- if (status == 0)
- return IRQ_NONE; /* Shared IRQ: not us */
- if (status == 0xffff)
- return IRQ_HANDLED; /* Hot unplug */
- if (status & error)
- do_some_error_handling()
-
- do {
- acknowledge_ints_ASAP();
-
- if (status & link_interrupt) {
- spin_lock(&tp->link_lock);
- do_some_link_stat_stuff();
- spin_lock(&tp->link_lock);
- }
-
- if (status & rx_interrupt) {
- receive_packets(dev);
- }
-
- if (status & rx_nobufs) {
- make_rx_buffs_avail();
- }
-
- if (status & tx_related) {
- spin_lock(&tp->lock);
- tx_ring_free(dev);
- if (tx_died)
- restart_tx();
- spin_unlock(&tp->lock);
- }
-
- status = read_interrupt_status_reg();
-
- } while (!(status & error) || more_work_to_be_done);
- return IRQ_HANDLED;
-}
-
-----------------------------------------------------------------------
-
-We now change this to what is shown below to NAPI-enable it:
-
-----------------------------------------------------------------------
-static irqreturn_t
-netdevice_interrupt(int irq, void *dev_id, struct pt_regs *regs)
-{
- struct net_device *dev = (struct net_device *)dev_instance;
- struct my_private *tp = (struct my_private *)dev->priv;
-
- status = read_interrupt_status_reg();
- if (status == 0)
- return IRQ_NONE; /* Shared IRQ: not us */
- if (status == 0xffff)
- return IRQ_HANDLED; /* Hot unplug */
- if (status & error)
- do_some_error_handling();
-
- do {
-/************************ start note *********************************/
- acknowledge_ints_ASAP(); // dont ack rx and rxnobuff here
-/************************ end note *********************************/
-
- if (status & link_interrupt) {
- spin_lock(&tp->link_lock);
- do_some_link_stat_stuff();
- spin_unlock(&tp->link_lock);
- }
-/************************ start note *********************************/
- if (status & rx_interrupt || (status & rx_nobuffs)) {
- if (netif_rx_schedule_prep(dev)) {
-
- /* disable interrupts caused
- * by arriving packets */
- disable_rx_and_rxnobuff_ints();
- /* tell system we have work to be done. */
- __netif_rx_schedule(dev);
- } else {
- printk("driver bug! interrupt while in poll\n");
- /* FIX by disabling interrupts */
- disable_rx_and_rxnobuff_ints();
- }
- }
-/************************ end note note *********************************/
-
- if (status & tx_related) {
- spin_lock(&tp->lock);
- tx_ring_free(dev);
-
- if (tx_died)
- restart_tx();
- spin_unlock(&tp->lock);
- }
-
- status = read_interrupt_status_reg();
-
-/************************ start note *********************************/
- } while (!(status & error) || more_work_to_be_done(status));
-/************************ end note note *********************************/
- return IRQ_HANDLED;
-}
-
----------------------------------------------------------------------
-
-
-We note several things from above:
-
-I) Any interrupt source which is caused by arriving packets is now
-turned off when it occurs. Depending on the hardware, there could be
-several reasons that arriving packets would cause interrupts; these are the
-interrupt sources we wish to avoid. The two common ones are a) a packet
-arriving (rxint) b) a packet arriving and finding no DMA buffers available
-(rxnobuff) .
-This means also acknowledge_ints_ASAP() will not clear the status
-register for those two items above; clearing is done in the place where
-proper work is done within NAPI; at the poll() and refill_rx_ring()
-discussed further below.
-netif_rx_schedule_prep() returns 1 if device is in running state and
-gets successfully added to the core poll list. If we get a zero value
-we can _almost_ assume are already added to the list (instead of not running.
-Logic based on the fact that you shouldn't get interrupt if not running)
-We rectify this by disabling rx and rxnobuf interrupts.
-
-II) that receive_packets(dev) and make_rx_buffs_avail() may have disappeared.
-These functionalities are still around actually......
-
-infact, receive_packets(dev) is very close to my_poll() and
-make_rx_buffs_avail() is invoked from my_poll()
-
-4) converting receive_packets() to dev->poll()
-===============================================
-
-We need to convert the classical D Becker receive_packets(dev) to my_poll()
-
-First the typical receive_packets() below:
--------------------------------------------------------------------
-
-/* this is called by interrupt handler */
-static void receive_packets (struct net_device *dev)
-{
-
- struct my_private *tp = (struct my_private *)dev->priv;
- rx_ring = tp->rx_ring;
- cur_rx = tp->cur_rx;
- int entry = cur_rx % RX_RING_SIZE;
- int received = 0;
- int rx_work_limit = tp->dirty_rx + RX_RING_SIZE - tp->cur_rx;
-
- while (rx_ring_not_empty) {
- u32 rx_status;
- unsigned int rx_size;
- unsigned int pkt_size;
- struct sk_buff *skb;
- /* read size+status of next frame from DMA ring buffer */
- /* the number 16 and 4 are just examples */
- rx_status = le32_to_cpu (*(u32 *) (rx_ring + ring_offset));
- rx_size = rx_status >> 16;
- pkt_size = rx_size - 4;
-
- /* process errors */
- if ((rx_size > (MAX_ETH_FRAME_SIZE+4)) ||
- (!(rx_status & RxStatusOK))) {
- netdrv_rx_err (rx_status, dev, tp, ioaddr);
- return;
- }
-
- if (--rx_work_limit < 0)
- break;
-
- /* grab a skb */
- skb = dev_alloc_skb (pkt_size + 2);
- if (skb) {
- .
- .
- netif_rx (skb);
- .
- .
- } else { /* OOM */
- /*seems very driver specific ... some just pass
- whatever is on the ring already. */
- }
-
- /* move to the next skb on the ring */
- entry = (++tp->cur_rx) % RX_RING_SIZE;
- received++ ;
-
- }
-
- /* store current ring pointer state */
- tp->cur_rx = cur_rx;
-
- /* Refill the Rx ring buffers if they are needed */
- refill_rx_ring();
- .
- .
-
-}
--------------------------------------------------------------------
-We change it to a new one below; note the additional parameter in
-the call.
-
--------------------------------------------------------------------
-
-/* this is called by the network core */
-static int my_poll (struct net_device *dev, int *budget)
-{
-
- struct my_private *tp = (struct my_private *)dev->priv;
- rx_ring = tp->rx_ring;
- cur_rx = tp->cur_rx;
- int entry = cur_rx % RX_BUF_LEN;
- /* maximum packets to send to the stack */
-/************************ note note *********************************/
- int rx_work_limit = dev->quota;
-
-/************************ end note note *********************************/
- do { // outer beginning loop starts here
-
- clear_rx_status_register_bit();
-
- while (rx_ring_not_empty) {
- u32 rx_status;
- unsigned int rx_size;
- unsigned int pkt_size;
- struct sk_buff *skb;
- /* read size+status of next frame from DMA ring buffer */
- /* the number 16 and 4 are just examples */
- rx_status = le32_to_cpu (*(u32 *) (rx_ring + ring_offset));
- rx_size = rx_status >> 16;
- pkt_size = rx_size - 4;
-
- /* process errors */
- if ((rx_size > (MAX_ETH_FRAME_SIZE+4)) ||
- (!(rx_status & RxStatusOK))) {
- netdrv_rx_err (rx_status, dev, tp, ioaddr);
- return 1;
- }
-
-/************************ note note *********************************/
- if (--rx_work_limit < 0) { /* we got packets, but no quota */
- /* store current ring pointer state */
- tp->cur_rx = cur_rx;
-
- /* Refill the Rx ring buffers if they are needed */
- refill_rx_ring(dev);
- goto not_done;
- }
-/********************** end note **********************************/
-
- /* grab a skb */
- skb = dev_alloc_skb (pkt_size + 2);
- if (skb) {
- .
- .
-/************************ note note *********************************/
- netif_receive_skb (skb);
-/********************** end note **********************************/
- .
- .
- } else { /* OOM */
- /*seems very driver specific ... common is just pass
- whatever is on the ring already. */
- }
-
- /* move to the next skb on the ring */
- entry = (++tp->cur_rx) % RX_RING_SIZE;
- received++ ;
-
- }
-
- /* store current ring pointer state */
- tp->cur_rx = cur_rx;
-
- /* Refill the Rx ring buffers if they are needed */
- refill_rx_ring(dev);
-
- /* no packets on ring; but new ones can arrive since we last
- checked */
- status = read_interrupt_status_reg();
- if (rx status is not set) {
- /* If something arrives in this narrow window,
- an interrupt will be generated */
- goto done;
- }
- /* done! at least that's what it looks like ;->
- if new packets came in after our last check on status bits
- they'll be caught by the while check and we go back and clear them
- since we havent exceeded our quota */
- } while (rx_status_is_set);
-
-done:
-
-/************************ note note *********************************/
- dev->quota -= received;
- *budget -= received;
-
- /* If RX ring is not full we are out of memory. */
- if (tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
- goto oom;
-
- /* we are happy/done, no more packets on ring; put us back
- to where we can start processing interrupts again */
- netif_rx_complete(dev);
- enable_rx_and_rxnobuf_ints();
-
- /* The last op happens after poll completion. Which means the following:
- * 1. it can race with disabling irqs in irq handler (which are done to
- * schedule polls)
- * 2. it can race with dis/enabling irqs in other poll threads
- * 3. if an irq raised after the beginning of the outer beginning
- * loop (marked in the code above), it will be immediately
- * triggered here.
- *
- * Summarizing: the logic may result in some redundant irqs both
- * due to races in masking and due to too late acking of already
- * processed irqs. The good news: no events are ever lost.
- */
-
- return 0; /* done */
-
-not_done:
- if (tp->cur_rx - tp->dirty_rx > RX_RING_SIZE/2 ||
- tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
- refill_rx_ring(dev);
-
- if (!received) {
- printk("received==0\n");
- received = 1;
- }
- dev->quota -= received;
- *budget -= received;
- return 1; /* not_done */
-
-oom:
- /* Start timer, stop polling, but do not enable rx interrupts. */
- start_poll_timer(dev);
- return 0; /* we'll take it from here so tell core "done"*/
-
-/************************ End note note *********************************/
-}
--------------------------------------------------------------------
-
-From above we note that:
-0) rx_work_limit = dev->quota
-1) refill_rx_ring() is in charge of clearing the bit for rxnobuff when
-it does the work.
-2) We have a done and not_done state.
-3) instead of netif_rx() we call netif_receive_skb() to pass the skb.
-4) we have a new way of handling oom condition
-5) A new outer for (;;) loop has been added. This serves the purpose of
-ensuring that if a new packet has come in, after we are all set and done,
-and we have not exceeded our quota that we continue sending packets up.
-
-
------------------------------------------------------------
-Poll timer code will need to do the following:
-
-a)
-
- if (tp->cur_rx - tp->dirty_rx > RX_RING_SIZE/2 ||
- tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
- refill_rx_ring(dev);
-
- /* If RX ring is not full we are still out of memory.
- Restart the timer again. Else we re-add ourselves
- to the master poll list.
- */
-
- if (tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
- restart_timer();
-
- else netif_rx_schedule(dev); /* we are back on the poll list */
-
-5) dev->close() and dev->suspend() issues
-==========================================
-The driver writer needn't worry about this; the top net layer takes
-care of it.
-
-6) Adding new Stats to /proc
-=============================
-In order to debug some of the new features, we introduce new stats
-that need to be collected.
-TODO: Fill this later.
-
-APPENDIX 1: discussion on using ethernet HW FC
-==============================================
-Most chips with FC only send a pause packet when they run out of Rx buffers.
-Since packets are pulled off the DMA ring by a softirq in NAPI,
-if the system is slow in grabbing them and we have a high input
-rate (faster than the system's capacity to remove packets), then theoretically
-there will only be one rx interrupt for all packets during a given packetstorm.
-Under low load, we might have a single interrupt per packet.
-FC should be programmed to apply in the case when the system cant pull out
-packets fast enough i.e send a pause only when you run out of rx buffers.
-Note FC in itself is a good solution but we have found it to not be
-much of a commodity feature (both in NICs and switches) and hence falls
-under the same category as using NIC based mitigation. Also, experiments
-indicate that it's much harder to resolve the resource allocation
-issue (aka lazy receiving that NAPI offers) and hence quantify its usefulness
-proved harder. In any case, FC works even better with NAPI but is not
-necessary.
-
-
-APPENDIX 2: the "rotting packet" race-window avoidance scheme
-=============================================================
-
-There are two types of associations seen here
-
-1) status/int which honors level triggered IRQ
-
-If a status bit for receive or rxnobuff is set and the corresponding
-interrupt-enable bit is not on, then no interrupts will be generated. However,
-as soon as the "interrupt-enable" bit is unmasked, an immediate interrupt is
-generated. [assuming the status bit was not turned off].
-Generally the concept of level triggered IRQs in association with a status and
-interrupt-enable CSR register set is used to avoid the race.
-
-If we take the example of the tulip:
-"pending work" is indicated by the status bit(CSR5 in tulip).
-the corresponding interrupt bit (CSR7 in tulip) might be turned off (but
-the CSR5 will continue to be turned on with new packet arrivals even if
-we clear it the first time)
-Very important is the fact that if we turn on the interrupt bit on when
-status is set that an immediate irq is triggered.
-
-If we cleared the rx ring and proclaimed there was "no more work
-to be done" and then went on to do a few other things; then when we enable
-interrupts, there is a possibility that a new packet might sneak in during
-this phase. It helps to look at the pseudo code for the tulip poll
-routine:
-
---------------------------
- do {
- ACK;
- while (ring_is_not_empty()) {
- work-work-work
- if quota is exceeded: exit, no touching irq status/mask
- }
- /* No packets, but new can arrive while we are doing this*/
- CSR5 := read
- if (CSR5 is not set) {
- /* If something arrives in this narrow window here,
- * where the comments are ;-> irq will be generated */
- unmask irqs;
- exit poll;
- }
- } while (rx_status_is_set);
-------------------------
-
-CSR5 bit of interest is only the rx status.
-If you look at the last if statement:
-you just finished grabbing all the packets from the rx ring .. you check if
-status bit says there are more packets just in ... it says none; you then
-enable rx interrupts again; if a new packet just came in during this check,
-we are counting that CSR5 will be set in that small window of opportunity
-and that by re-enabling interrupts, we would actually trigger an interrupt
-to register the new packet for processing.
-
-[The above description nay be very verbose, if you have better wording
-that will make this more understandable, please suggest it.]
-
-2) non-capable hardware
-
-These do not generally respect level triggered IRQs. Normally,
-irqs may be lost while being masked and the only way to leave poll is to do
-a double check for new input after netif_rx_complete() is invoked
-and re-enable polling (after seeing this new input).
-
-Sample code:
-
----------
- .
- .
-restart_poll:
- while (ring_is_not_empty()) {
- work-work-work
- if quota is exceeded: exit, not touching irq status/mask
- }
- .
- .
- .
- enable_rx_interrupts()
- netif_rx_complete(dev);
- if (ring_has_new_packet() && netif_rx_reschedule(dev, received)) {
- disable_rx_and_rxnobufs()
- goto restart_poll
- } while (rx_status_is_set);
----------
-
-Basically netif_rx_complete() removes us from the poll list, but because a
-new packet which will never be caught due to the possibility of a race
-might come in, we attempt to re-add ourselves to the poll list.
-
-
-
-
-APPENDIX 3: Scheduling issues.
-==============================
-As seen NAPI moves processing to softirq level. Linux uses the ksoftirqd as the
-general solution to schedule softirq's to run before next interrupt and by putting
-them under scheduler control. Also this prevents consecutive softirq's from
-monopolize the CPU. This also have the effect that the priority of ksoftirq needs
-to be considered when running very CPU-intensive applications and networking to
-get the proper balance of softirq/user balance. Increasing ksoftirq priority to 0
-(eventually more) is reported cure problems with low network performance at high
-CPU load.
-
-Most used processes in a GIGE router:
-USER PID %CPU %MEM SIZE RSS TTY STAT START TIME COMMAND
-root 3 0.2 0.0 0 0 ? RWN Aug 15 602:00 (ksoftirqd_CPU0)
-root 232 0.0 7.9 41400 40884 ? S Aug 15 74:12 gated
-
---------------------------------------------------------------------
-
-relevant sites:
-==================
-ftp://robur.slu.se/pub/Linux/net-development/NAPI/
-
-
---------------------------------------------------------------------
-TODO: Write net-skeleton.c driver.
--------------------------------------------------------------
-
-Authors:
-========
-Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
-Jamal Hadi Salim <hadi@cyberus.ca>
-Robert Olsson <Robert.Olsson@data.slu.se>
-
-Acknowledgements:
-================
-People who made this document better:
-
-Lennert Buytenhek <buytenh@gnu.org>
-Andrew Morton <akpm@zip.com.au>
-Manfred Spraul <manfred@colorfullife.com>
-Donald Becker <becker@scyld.com>
-Jeff Garzik <jgarzik@pobox.com>