[rt2x00-users] [RFC/RFT 00/15] Beaconing updates and tasklet conversion

Johannes Stezenbach js at sig21.net
Fri Jan 7 03:14:32 AEDT 2011

Hi Helmut,

On Tue, Dec 28, 2010 at 05:03:55PM +0100, Helmut Schaa wrote:
> Am Dienstag, 28. Dezember 2010 schrieb Johannes Stezenbach:
> > 
> > - PRE_TBTT irq
> > - call ieee80211_beacon_get() to get updated TIM for next beacon
> > - write updated beacon to hw in time before TBTT
> >
> > Can someone confirm my understanding is correct?
> It is.
> > If it is correct, how does it work on USB where we
> > don't get a PRE_TBTT irq?
> On USB we will never be able to generate correct beacons. We only push a new
> beacon to the hw when
> a) the beacon template changes
> b) a tim bit gets set (==STA wake/sleep transition)
> > And what would be the consequence of sending
> > a beacon with wrong TIM?
> The TIM should be correct on USB through the usage of set_tim but the DTIM
> count won't be correct (for DTIM periods > 1). And we don't have a chance to
> send out buffered mc and bc traffic out just after the DTIM beacon as
> described in 802.11.

Thank you for the info, and apologies for the slow reply.
I've read a bit more of the 802.11 spec and now I know a bit
more but I've also got some new questions.
(I looked at current rt2x00/experimental git)

For rt2800pci, on TBTT irq rt2x00lib_beacondone() is called
which eventually calls ieee80211_get_buffered_bc() to get the
bc and mc frames and queues them with rt2x00mac_tx().

Q1: IEEE80211_HW_HOST_BROADCAST_PS_BUFFERING is set for rt2800pci,
    which means mac80211 will not queue the frame but send it
    to the driver immediately.  Thus ieee80211_get_buffered_bc()
    will not return any frames.  Right?

Q2: DRIVER_REQUIRE_ATIM_QUEUE is not set for rt2800pci, which means
    bc + mc frames will be put on normal queue together with uc frames
    by rt2x00mac_tx()

Q3: Even if IEEE80211_HW_HOST_BROADCAST_PS_BUFFERING were not set,
    without ATIM queue frames are queued behind other uc frames
    and won't be sent immediately after DTIM as required by 802.11, right?

So I guess a seperate ATIM queue is a requirement to get it right?

Q4: For rt2800usb, IEEE80211_HW_HOST_BROADCAST_PS_BUFFERING is not set,
    thus bc/mc frames are queued seperately by mac80211; but since
    ieee80211_get_buffered_bc() is not called for rt2800usb,
    these frames are never sent.  Right?

I guess a possible way out for rt2800usb would be to read
the TBTT_TIMER register and then set a hrtimer to fire
at the time when PRE_TBTT irq would fire for rt2800pci.
Any opinions on this?
("TBTT_TIMER: TImer remains till next TBTT, read-only" is the
comment from rt2800.h)


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