taskqueue_enqueue() was changed to support both fast and non-fast
taskqueues 10 years ago in r154167. It has been a compat shim ever
since. It's time for the compat shim to go.
Submitted by: Howard Su <howard0su@gmail.com>
Reviewed by: sephe
Differential Revision: https://reviews.freebsd.org/D5131
In particular, don't check the value of the bus_dma map against NULL
to determine if either bus_dmamem_alloc() or bus_dmamap_load() succeeded.
Instead, assume that bus_dmamap_load() succeeeded (and thus that
bus_dmamap_unload() should be called) if the bus address for a resource
is non-zero, and assume that bus_dmamem_alloc() succeeded (and thus
that bus_dmamem_free() should be called) if the virtual address for a
resource is not NULL.
In many cases these bugs could result in leaks when a driver was detached.
Reviewed by: yongari
MFC after: 2 weeks
to this event, adding if_var.h to files that do need it. Also, include
all includes that now are included due to implicit pollution via if_var.h
Sponsored by: Netflix
Sponsored by: Nginx, Inc.
them, please let me know if not). Most of these are of the form:
static const struct bzzt_type {
[...list of members...]
} const bzzt_devs[] = {
[...list of initializers...]
};
The second const is unnecessary, as arrays cannot be modified anyway,
and if the elements are const, the whole thing is const automatically
(e.g. it is placed in .rodata).
I have verified this does not change the binary output of a full kernel
build (except for build timestamps embedded in the object files).
Reviewed by: yongari, marius
MFC after: 1 week
a taskqueue.
This gives a 16% performance improvement under high load on slow systems,
especially when vr shares an interrupt with another device, which is
common with the Alix x86 boards.
Contrary to the other devices, I left the interrupt processing for loop
in because there was no significant difference in performance and this
should avoid enqueuing more taskqueues unnecessarily.
We also decided to move the vr_start_locked() call inside the for loop
because we found out that it helps performance since TCP ACKs now have a
chance to go out quicker.
Reviewed by: yongari (older version, same idea)
Discussed with: yongari, jhb
control for all vr(4) controllers that support it. It's known that
old vr(4) controllers(Rhine II) does not support TX pause but Rhine
III supports both TX and RX pause.
Make TX pause really work on Rhine III by letting controller know
available RX buffers.
While here, adjust XON/XOFF parameters to get better performance
with flow control.
using member variables in softc.
While I'm here change media after setting IFF_DRV_RUNNING. This
will remove unnecessary link state handling in vr_tick() if
controller established a link immediately.
one. Interestingly, these are actually the default for quite some time
(bus_generic_driver_added(9) since r52045 and bus_generic_print_child(9)
since r52045) but even recently added device drivers do this unnecessarily.
Discussed with: jhb, marcel
- While at it, use DEVMETHOD_END.
Discussed with: jhb
- Also while at it, use __FBSDID.
Because driver is accessing a common MII structure in
mii_pollstat(), updating user supplied structure should be done
before dropping a driver lock.
Reported by: Karim (fodillemlinkarimi <> gmail dot com)
register both status change and link state change callbacks.
Implement checking valid link in state change callback and poll
active link state in vr_tick(). This allows immediate detection of
lost link as well as protecting driver from frequent link flips during
link renegotiation. taskq implementation was removed because driver
now needs to poll link state in vr_tick().
While I'm here do not report current link state if interface is not
running.
Tested by: n_hibma
MFC after: 1 week
(reporting IFM_LOOP based on BMCR_LOOP is left in place though as
it might provide useful for debugging). For most mii(4) drivers it
was unclear whether the PHYs driven by them actually support
loopback or not. Moreover, typically loopback mode also needs to
be activated on the MAC, which none of the Ethernet drivers using
mii(4) implements. Given that loopback media has no real use (and
obviously hardly had a chance to actually work) besides for driver
development (which just loopback mode should be sufficient for
though, i.e one doesn't necessary need support for loopback media)
support for it is just dropped as both NetBSD and OpenBSD already
did quite some time ago.
- Let mii_phy_add_media() also announce the support of IFM_NONE.
- Restructure the PHY entry points to use a structure of entry points
instead of discrete function pointers, and extend this to include
a "reset" entry point. Make sure any PHY-specific reset routine is
always used, and provide one for lxtphy(4) which disables MII
interrupts (as is done for a few other PHYs we have drivers for).
This includes changing NIC drivers which previously just called the
generic mii_phy_reset() to now actually call the PHY-specific reset
routine, which might be crucial in some cases. While at it, the
redundant checks in these NIC drivers for mii->mii_instance not being
zero before calling the reset routines were removed because as soon
as one PHY driver attaches mii->mii_instance is incremented and we
hardly can end up in their media change callbacks etc if no PHY driver
has attached as mii_attach() would have failed in that case and not
attach a miibus(4) instance.
Consequently, NIC drivers now no longer should call mii_phy_reset()
directly, so it was removed from EXPORT_SYMS.
- Add a mii_phy_dev_attach() as a companion helper to mii_phy_dev_probe().
The purpose of that function is to perform the common steps to attach
a PHY driver instance and to hook it up to the miibus(4) instance and to
optionally also handle the probing, addition and initialization of the
supported media. So all a PHY driver without any special requirements
has to do in its bus attach method is to call mii_phy_dev_attach()
along with PHY-specific MIIF_* flags, a pointer to its PHY functions
and the add_media set to one. All PHY drivers were updated to take
advantage of mii_phy_dev_attach() as appropriate. Along with these
changes the capability mask was added to the mii_softc structure so
PHY drivers taking advantage of mii_phy_dev_attach() but still
handling media on their own do not need to fiddle with the MII attach
arguments anyway.
- Keep track of the PHY offset in the mii_softc structure. This is done
for compatibility with NetBSD/OpenBSD.
- Keep track of the PHY's OUI, model and revision in the mii_softc
structure. Several PHY drivers require this information also after
attaching and previously had to wrap their own softc around mii_softc.
NetBSD/OpenBSD also keep track of the model and revision on their
mii_softc structure. All PHY drivers were updated to take advantage
as appropriate.
- Convert the mebers of the MII data structure to unsigned where
appropriate. This is partly inspired by NetBSD/OpenBSD.
- According to IEEE 802.3-2002 the bits actually have to be reversed
when mapping an OUI to the MII ID registers. All PHY drivers and
miidevs where changed as necessary. Actually this now again allows to
largely share miidevs with NetBSD, which fixed this problem already
9 years ago. Consequently miidevs was synced as far as possible.
- Add MIIF_NOMANPAUSE and mii_phy_flowstatus() calls to drivers that
weren't explicitly converted to support flow control before. It's
unclear whether flow control actually works with these but typically
it should and their net behavior should be more correct with these
changes in place than without if the MAC driver sets MIIF_DOPAUSE.
Obtained from: NetBSD (partially)
Reviewed by: yongari (earlier version), silence on arch@ and net@
the NIC drivers as well as the PHY drivers to take advantage of the
mii_attach() introduced in r213878 to get rid of certain hacks. For
the most part these were:
- Artificially limiting miibus_{read,write}reg methods to certain PHY
addresses; we now let mii_attach() only probe the PHY at the desired
address(es) instead.
- PHY drivers setting MIIF_* flags based on the NIC driver they hang
off from, partly even based on grabbing and using the softc of the
parent; we now pass these flags down from the NIC to the PHY drivers
via mii_attach(). This got us rid of all such hacks except those of
brgphy() in combination with bce(4) and bge(4), which is way beyond
what can be expressed with simple flags.
While at it, I took the opportunity to change the NIC drivers to pass
up the error returned by mii_attach() (previously by mii_phy_probe())
and unify the error message used in this case where and as appropriate
as mii_attach() actually can fail for a number of reasons, not just
because of no PHY(s) being present at the expected address(es).
Reviewed by: jhb, yongari
IF_ADDR_UNLOCK() across network device drivers when accessing the
per-interface multicast address list, if_multiaddrs. This will
allow us to change the locking strategy without affecting our driver
programming interface or binary interface.
For two wireless drivers, remove unnecessary locking, since they
don't actually access the multicast address list.
Approved by: re (kib)
MFC after: 6 weeks
CPU for too long period than necessary. Additively, interfaces are kept
polled (in the tick) even if no more packets are available.
In order to avoid such situations a new generic mechanism can be
implemented in proactive way, keeping track of the time spent on any
packet and fragmenting the time for any tick, stopping the processing
as soon as possible.
In order to implement such mechanism, the polling handler needs to
change, returning the number of packets processed.
While the intended logic is not part of this patch, the polling KPI is
broken by this commit, adding an int return value and the new flag
IFCAP_POLLING_NOCOUNT (which will signal that the return value is
meaningless for the installed handler and checking should be skipped).
Bump __FreeBSD_version in order to signal such situation.
Reviewed by: emaste
Sponsored by: Sandvine Incorporated
vr(4) overhauling(r177050).
It seems that filtering multicast addresses with multicast CAM
entries require accessing 'CAM enable bit' for each CAM entry.
Subsequent accessing multicast CAM control register without
toggling the 'CAM enable bit' seem to no effects.
In order to fix that separate CAM setup from CAM mask configuration
and CAM entry modification. While I'm here add VLAN CAM filtering
feature which will be enabled in future(FreeBSD now can receive
VLAN id insertion/removal event from vlan(4) on the fly).
For VT6105M hardware, explicitly disable VLAN hardware tag
insertion/stripping and enable VLAN CAM filtering for VLAN id 0.
This shall make non-VLAN frames set VR_RXSTAT_VIDHIT bit in Rx
status word.
Added multicast/VLAN CAM address definition to header file.
PR: kern/125010, kern/125024
MFC after: 1 week
years. All datasheet I have indicates the bit 15 is the
VR_RXSTAT_RX_OK. The bit 14 is reserved for all Rhine family
except VT6105M. VT6105M uses that bit to indicate a VLAN frame
with matching CAM VLAN id.
Use the VR_RXSTAT_RX_OK instead of VR_RXSTAT_RXERR when vr(4)
checks the validity of received frame.
This should fix occasional dropping frames on VT6105M.
Tested by: Goran Lowkrantz ( goran.lowkrantz at ismobile dot com )
MFC after: 1 week
state change and reliable error recovery.
o Moved vr_softc structure and relevant macros to header file.
o Use PCIR_BAR macro to get BARs.
o Implemented suspend/resume methods.
o Implemented automatic Tx threshold configuration which will be
activated when it suffers from Tx underrun. Also Tx underrun
will try to restart only Tx path and resort to previous
full-reset(both Rx/Tx) operation if restarting Tx path have failed.
o Removed old bit-banging MII interface. Rhine provides simple and
efficient MII interface. While I'm here show PHY address and PHY
register number when its read/write operation was failed.
o Define VR_MII_TIMEOUT constant and use it in MII access routines.
o Always honor link up/down state reported by mii layers. The link
state information is used in vr_start() to determine whether we
got a valid link.
o Removed vr_setcfg() which is now handled in vr_link_task(), link
state taskqueue handler. When mii layer reports link state changes
the taskqueue handler reprograms MAC to reflect negotiated duplex
settings. Flow-control changes are not handled yet and it should
be revisited when mii layer knows the notion of flow-control.
o Added a new sysctl interface to get statistics of an instance of
the driver.(sysctl dev.vr.0.stats=1)
o Chip name was renamed to reflect the official name of the chips
described in VIA Rhine I/II/III datasheet.
REV_ID_3065_A -> REV_ID_VT6102_A
REV_ID_3065_B -> REV_ID_VT6102_B
REV_ID_3065_C -> REV_ID_VT6102_C
REV_ID_3106_J -> REV_ID_VT6105_A0
REV_ID_3106_S -> REV_ID_VT6105M_A0
The following chip revisions were added.
#define REV_ID_VT6105_B0 0x83
#define REV_ID_VT6105_LOM 0x8A
#define REV_ID_VT6107_A0 0x8C
#define REV_ID_VT6107_A1 0x8D
#define REV_ID_VT6105M_B1 0x94
o Always show chip revision number in device attach. This shall help
identifying revision specific issues.
o Check whether EEPROM reloading is complete by inspecting the state
of VR_EECSR_LOAD bit. This bit is self-cleared after the EEPROM
reloading. Previously vr(4) blindly spins for 200us which may/may
not enough to complete the EEPROM reload.
o Removed if_mtu setup. It's done in ether_ifattach().
o Use our own callout to drive watchdog timer.
o In vr_attach disable further interrupts after reset. For VT6102 or
newer hardwares, diable MII state change interrupt as well because
mii state handling is done by mii layer.
o Add more sane register initialization for VT6102 or newer chips.
- Have NIC report error instead of retrying forever.
- Let hardware detect MII coding error.
- Enable MODE10T mode.
- Enable memory-read-multiple for VT6107.
o PHY address for VT6105 or newer chips is located at fixed address 1.
For older chips the PHY address is stored in VR_PHYADDR register.
Armed with these information, there is no need to re-read
VR_PHYADDR register in miibus handler to get PHY address. This
saves one register access cycle for each MII access.
o Don't reprogram VR_PHYADDR register whenever access to a register
located at a PHY address is made. Rhine fmaily allows reprogramming
PHY address location via VR_PHYADDR register depending on
VR_MIISTAT_PHYOPT bit of VR_MIISTAT register. This used to lead
numerous phantom PHYs attached to miibus during phy probe phase and
driver used to limit allowable PHY address in mii register accessors
for certain chip revisions. This removes one more register access
cycle for each MII access.
o Correctly set VLAN header length.
o bus_dma(9) conversion.
- Limit DMA access to be in range of 32bit address space. Hardware
doesn't support DAC.
- Apply descriptor ring alignment requirements(16 bytes alignment)
- Apply Rx buffer address alignment requirements(4 bytes alignment)
- Apply Tx buffer address alignment requirements(4 bytes alignment)
for Rhine I chip. Rhine II or III has no Tx buffer address
alignment restrictions, though.
- Reduce number of allowable number of DMA segments to 8.
- Removed the atomic(9) used in descriptor ownership managements
as it's job of bus_dmamap_sync(9).
With these change vr(4) should work on all platforms.
o Rhine uses two separated 8bits command registers to control Tx/Rx
MAC. So don't access it as a single 16bit register.
o For non-strict alignment architectures vr(4) no longer require
time-consuming copy operation for received frames to align IP
header. This greatly improves Rx performance on i386/amd64
platforms. However the alignment is still necessary for
strict-alignment platforms(e.g. sparc64). The alignment is handled
in new fuction vr_fixup_rx().
o vr_rxeof() now rejects multiple-segmented(fragmented) frames as
vr(4) is not ready to handle this situation. Datasheet said nothing
about the reason when/why it happens.
o In vr_newbuf() don't set VR_RXSTAT_FIRSTFRAG/VR_RXSTAT_LASTFRAG
bits as it's set by hardware.
o Don't pass checksum offload information to upper layer for
fragmented frames. The hardware assisted checksum is valid only
when the frame is non-fragmented IP frames. Also mark the checksum
is valid for corrupted frames such that upper layers doesn't need
to recompute the checksum with software routine.
o Removed vr_rxeoc(). RxDMA doesn't seem to need to be idle before
sending VR_CMD_RX_GO command. Previously it used to stop RxDMA
first which in turn resulted in long delays in Rx error recovery.
o Rewrote Tx completion handler.
- Always check VR_TXSTAT_OWN bit in status word prior to
inspecting other status bits in the status word.
- Collision counter updates were corrected as VT3071 or newer
ones use different bits to notify collisions.
- Unlike other chip revisions, VT86C100A uses different bit to
indicate Tx underrun. For VT3071 or newer ones, check both
VR_TXSTAT_TBUFF and VR_TXSTAT_UDF bits to see whether Tx
underrun was happend. In case of Tx underrun requeue the failed
frame and restart stalled Tx SM. Also double Tx DMA threshold
size on each failure to mitigate future Tx underruns.
- Disarm watchdog timer only if we have no queued packets,
otherwise don't touch watchdog timer.
o Rewrote interrupt handler.
- status word in Tx/Rx descriptors indicates more detailed error
state required to recover from the specific error. There is no
need to rely on interrupt status word to recover from Tx/Rx
error except PCI bus error. Other event notifications like
statistics counter overflows or link state events will be
handled in main interrupt handler.
- Don't touch VR_IMR register if we are in suspend mode. Touching
the register may hang the hardware if we are in suspended state.
Previously it seems that touching VR_IMR register in interrupt
handler was to work-around panic occurred in system shutdown
stage on SMP systems. I think that work-around would hide
root-cause of the panic and I couldn't reproduce the panic
with multiple attempts on my box.
o While padding space to meet minimum frame size, zero the pad data
in order to avoid possibly leaking sensitive data.
o Rewrote vr_start_locked().
- Don't try to queue packets if number of available Tx descriptors
are short than that of required one.
o Don't reinitialize hardware whenever media configuration is
changed. Media/link state changes are reported from mii layer if
this happens and vr_link_task() will perform necessary changes.
o Don't reinitialize hardware if only PROMISC bit was changed. Just
toggle the PROMISC bit in hardware is sufficient to reflect the
request.
o Rearrganed the IFCAP_POLLING/IFCAP_HWCSUM handling in vr_ioctl().
o Generate Tx completion interrupts for every VR_TX_INTR_THRESH-th
frames. This reduces Tx completion interrupts under heavy network
loads.
o Since vr(4) doesn't request Tx interrupts for every queued frames,
reclaim any pending descriptors not handled in Tx completion
handler before actually firing up watchdog timeouts.
o Added vr_tx_stop()/vr_rx_stop() to wait for the end of active
TxDMA/RxDMA cycles(draining). These routines are used in vr_stop()
to ensure sane state of MAC before releasing allocated Tx/Rx
buffers. vr_link_task() also takes advantage of these functions to
get to idle state prior to restarting Tx/Rx.
o Added vr_tx_start()/vr_rx_start() to restart Rx/Tx. By separating
Rx operation from Tx operation vr(4) no longer need to full-reset
the hardware in case of Tx/Rx error recovery.
o Implemented WOL.
o Added VT6105M specific register definitions. VT6105M has the
following hardware capabilities.
- Tx/Rx IP/TCP/UDP checksum offload.
- VLAN hardware tag insertion/extraction. Due to lack of information
for getting extracted VLAN tag in Rx path, VLAN hardware support
was not implemented yet.
- CAM(Content Addressable Memory) based 32 entry perfect multicast/
VLAN filtering.
- 8 priority queues.
o Implemented CAM based 32 entry perfect multicast filtering for
VT6105M. If number of multicast entry is greater than 32, vr(4)
uses traditional hash based filtering.
o Reflect real Tx/Rx descriptor structure. Previously vr(4) used to
embed other driver (private) data into these structure. This type
of embedding make it hard to work on LP64 systems.
o Removed unused vr_mii_frame structure and MII bit-baning
definitions.
o Added new PCI configuration registers that controls mii operation
and mode selection.
o Reduced number of Tx/Rx descriptors to 128 from 256. From my
testing, increasing number of descriptors above than 64 didn't help
increasing performance at all. Experimentations show 128 Rx
descriptors seems to help a lot reducing Rx FIFO overruns under
high system loads. It seems the poor Tx performance of Rhine
hardwares comes from the limitation of hardware. You wouldn't
satuarte the link with vr(4) no matter how fast CPU/large number of
descriptors are used.
o Added vr_statistics structure to hold various counter values.
No regression was reported but one variant of Rhine III(VT6105M)
found on RouterBOARD 44 does not work yet(Reported by Milan Obuch).
I hope this would be resolved in near future.
I'd like to say big thanks to Mike Tancsa who kindly donated a Rhine
hardware to me. Without his enthusiastic testing and feedbacks
overhauling vr(4) never have been possible. Also thanks to Masayuki
Murayama who provided some good comments on the hardware's internals.
This driver is result of combined effort of many users who provided
many feedbacks so I'd like to say special thanks to them.
Hardware donated by: Mike Tancsa (mike AT sentex dot net)
Reviewed by: remko (initial version)
Tested by: Mike Tancsa(x86), JoaoBR ( joao AT matik DOT com DOT br )
Marcin Wisnicki ( mwisnicki+freebsd AT gmail DOT com )
Stefan Ehmann ( shoesoft AT gmx DOT net )
Florian Smeets ( flo AT kasimir DOT com )
Phil Oleson ( oz AT nixil DOT net )
Larry Baird ( lab AT gta DOT com )
Milan Obuch ( freebsd-current AT dino DOT sk )
remko (initial version)
VR_STICKHW register would result in unexpected results on these
hardwares. wpaul said the following for the issue.
The vr_attach() routine unconditionally does this for all supported
chips:
/*
* Windows may put the chip in suspend mode when it
* shuts down. Be sure to kick it in the head to wake it
* up again.
*/
VR_CLRBIT(sc, VR_STICKHW, (VR_STICKHW_DS0|VR_STICKHW_DS1));
The problem is, the VR_STICKHW register is not valid on all Rhine
devices. The VT86C100A chip, which is present on the D-Link DFE-530TX
boards, doesn't support power management, and its register space is
only 128 bytes wide. The VR_STICKHW register offset falls outside this
range. This may go unnoticed in most scenarios, but if you happen to have
another PCI device in your system which is assigned the register
space immediately after that of the Rhine, the vr(4) driver will
incorrectly stomp it. In my case, the BIOS on my test board decided
to put the register space for my PRO/100 ethernet board right next
to the Rhine, and the Rhine driver ended up clobbering the IMR register
of the PRO/100 device. (Long story short: the board kept locking up on
boot. Took me the better part of the morning suss out why.)
The strictly correct thing to do would be to check the PCI config space
to make sure the device supports the power management capability and only
write to the VR_STICKHW register if it does.
Instead of inspecting chip revision numbers for the availability of
VR_STICKHW register, check the existence of power management capability
of the hardware as wpaul suggested.
Reported by: wpaul
Suggested by: wpaul
OK'ed by: jhb
The 6105M and 6102 does not have the DWORD alignment problem, so
don't m_defrag() every packet in the transmit path for those.
More stringent usage of tx-descriptor ring and its flags.
Tested on 6102 and 6105M, other chips may also be able to run
without the m_defrag() but I have neither hardware nor docs to
find out.
Sponsored by: Soekris Engineering
if_ioctl, if_watchdog, etc, or in functions that are used by
these methods only. In all other cases use device_printf().
This also fixes several panics, when if_printf() is called before
softc->ifp was initialized.
Submitted by: Alex Lyashkov <umka sevcity.net>
rather than in ifindex_table[]; all (except one) accesses are
through ifp anyway. IF_LLADDR() works faster, and all (except
one) ifaddr_byindex() users were converted to use ifp->if_addr.
- Stop storing a (pointer to) Ethernet address in "struct arpcom",
and drop the IFP2ENADDR() macro; all users have been converted
to use IF_LLADDR() instead.
I had to initialize the ifnet a bit earlier in attach so that the
if_printf()'s in vr_reset() didn't explode with a page fault.
- Use M_ZERO with contigmalloc() rather than an explicit bzero.
opt_device_polling.h
- Include opt_device_polling.h into appropriate files.
- Embrace with HAVE_KERNEL_OPTION_HEADERS the include in the files that
can be compiled as loadable modules.
Reviewed by: bde
o Axe poll in trap.
o Axe IFF_POLLING flag from if_flags.
o Rework revision 1.21 (Giant removal), in such a way that
poll_mtx is not dropped during call to polling handler.
This fixes problem with idle polling.
o Make registration and deregistration from polling in a
functional way, insted of next tick/interrupt.
o Obsolete kern.polling.enable. Polling is turned on/off
with ifconfig.
Detailed kern_poll.c changes:
- Remove polling handler flags, introduced in 1.21. The are not
needed now.
- Forget and do not check if_flags, if_capenable and if_drv_flags.
- Call all registered polling handlers unconditionally.
- Do not drop poll_mtx, when entering polling handlers.
- In ether_poll() NET_LOCK_GIANT prior to locking poll_mtx.
- In netisr_poll() axe the block, where polling code asks drivers
to unregister.
- In netisr_poll() and ether_poll() do polling always, if any
handlers are present.
- In ether_poll_[de]register() remove a lot of error hiding code. Assert
that arguments are correct, instead.
- In ether_poll_[de]register() use standard return values in case of
error or success.
- Introduce poll_switch() that is a sysctl handler for kern.polling.enable.
poll_switch() goes through interface list and enabled/disables polling.
A message that kern.polling.enable is deprecated is printed.
Detailed driver changes:
- On attach driver announces IFCAP_POLLING in if_capabilities, but
not in if_capenable.
- On detach driver calls ether_poll_deregister() if polling is enabled.
- In polling handler driver obtains its lock and checks IFF_DRV_RUNNING
flag. If there is no, then unlocks and returns.
- In ioctl handler driver checks for IFCAP_POLLING flag requested to
be set or cleared. Driver first calls ether_poll_[de]register(), then
obtains driver lock and [dis/en]ables interrupts.
- In interrupt handler driver checks IFCAP_POLLING flag in if_capenable.
If present, then returns.This is important to protect from spurious
interrupts.
Reviewed by: ru, sam, jhb
