Re: Power save mode in 2.6.32-22

[Christian Lamparter <chunkeey@googlemail.com>]

On Friday 04 June 2010 01:27:38 Gordon, Charles wrote:
> Hi,
> 
> I'm trying to get power save mode to work in a wireless driver on Linux
> 2.6.32.  What I'm seeing is that the wireless stack is sending null-data
> frames with the power management bit on or off from time to time.  It does
> this regardless of whether flags IEEE80211_HW_SUPPORTS_PS and
> IEEE80211_HW_SUPPORTS_DYNAMIC_PS are set.  This is not how I would expect
> it to work.  If IEEE80211_HW_SUPPORTS_PS is set, I would expect it to let
> the driver handle the sending of these frames.  If IEEE80211_HW_SUPPORTS_PS
> is not set, then this would indicate that the driver cannot power down, in
> which case I still would not expect the stack to be sending these frames. 
> The documentation for power save is very thin and it seems the
> implementation has changed recently. 
> 
> Why is the stack sending these frames when the driver says the H/W does not
> support powering down?

well, could this be a periodic scan / site survey? (perhaps
NM/wicd/wpa_supplicant is running in the background?).

The device signalize the AP that it goes "off-channel" to scan on other
frequencies. Therefore the AP should be prepared to buffer any traffic,
until the STA resumes normal operation.

This can be easily tested with the help of a sniffer,
because you should then get probe requests from the interface.

> Why is the stack sending these frames when it would seem that the driver
> could better handle them?
> 
> The reason why I think the driver can better handle them is that if the
> stack sends a frame with the power-mgmt bit set, and then tells the driver
> to power down, there will be a race condition where the interface could
> receive a frame before the hardware is powered down.  The AP may send such
> a frame after it receives the null-data frame because some could be in a
> H/W queue.  I have seen this happen.  In this case, the interface H/W on
> the station will send an ACK and since the H/W does not know that it is
> about to be powered down, that ACK will be sent with the power-mgmt bit
> clear, which may confuse the access point.

> In short, sending these frames
> is something the driver should do since it will know that it should adjust
> the H/W so that any ACKs or other control frames will be sent with the
> power mgmt bit correctly set until such time as the H/W is actually powered
> down.

quote from: 802.11-2007 "11.2.1 Power management in an infrastructure network"

"A STA shall remain in its current Power Management mode until it informs the AP of
 a Power Management mode change via a frame exchange that includes an acknowledgment
 from the AP. Power Management mode shall not change during any single frame exchange
 sequence as described in 9.12"

The way I read it, it means two things:

Firstly, the STA has to initiate a "frame exchange" in order to
notify the AP about its new power state. (And importantly, that
frame exchange has to succeed, before the STA can actually
go off-channel/sleep! - This is important, because not all
drivers/HW do support an accurate *frame success* report.)

Secondly: The AP should only consider the PSM bit setting,
if the "exchange" was actually initiated by the STA and not
by the AP. (Furthermore, for most control frames timing is
critical. Therefore they have to be implemented in the
hardware/firmware and this makes it nearly impossible to
do what you are describing in your last _quoted_ sentence) 

BTW: have you tried to reproduce your "timing" problem with
a recent compat-wireless?

Regards,
	Chr