On 11/26/2012 02:45 AM, Johannes Berg wrote:
On Tue, 2012-11-20 at 10:11 -0800, Marco Porsch wrote:
This is ... strange? Can a single station really own *two* num_psp
refcounts?
Yes it can. A station can be both owner and recipient. And it would just
be overhead to distinguish between num_psp_owner and num_psp_recipient,
when in the end we only want to know if there is any PSP ongoing at all.
I'll change the comment to:
/* number of active PSPs (owner and recipient counted independently) */
atomic_t num_psp;
Ok.
+ nullfunc = (struct ieee80211_hdr *) skb->data;
+ if (!eosp)
+ nullfunc->frame_control |=
+ cpu_to_le16(IEEE80211_FCTL_MOREDATA);
This seems wrong -- EOSP and moredata are orthogonal (with the
restriction that "!EOSP => moredata") -- but if you just have that in
the code the moredata bit won't always be set correctly.
Imho, in the context of PSP trigger frames it does.
Sending a trigger frame to a mesh PS STA with no EOSP implies the start
of a PSP with the sender as owner -> following data. The other two
combinations imply that there is no more data following in that direction.
The EOSP bit in a trigger frame should always be 0 unless the frame is
also a PSP response, no?
No. That is one weird thing about the standard: the combination
RSPI:EOSP 0:0 is seen as a MPSP trigger frame when sent towards a
station in PS mode towards the sender. So whenever a frame should be
sent in a non-PSP context it has to set the EOSP flag.
Another situation that sets the EOSP flag is a kind of MPSP poll: if one
STA indicates buffered frames, the other STA can poll it with RSPI:EOSP 1:1.
What you seem to be missing though is the case when there _is_ more
data, but the service period has to end nonetheless, say because it was
limited to a few packets? Nothing here seems to indicate that an MPSP
ends only after all queued packets are transmitted, which would be a
requirement if this was supposed to be correct.
MPSPs themselves are not defined in length by the standard.
My current mechanism always sends all packets that are buffered at the
start of the MPSP. At the time the MPSP starts, the last
to-be-transmitted frame is the last buffered frame.
Of course, by the time the MPSP ends, new frames may have been buffered
in the meantime, which are not taken into account here. That would
require additional feedback from ieee80211_tx_status which always seems
a bit racy to me.
(In an earlier version I had something like a feedback loop with
ieee80211_tx_status, where the 'last' frame of a MPSP would always
trigger a re-check of the PS buffers. That allowed to have infinite
length MPSPs as long as data is enqueued for transmission. But that
seemed to be overly complex when something like dynamic PS state
switching is possible and advised by the standard.)
But I see one situation where the More Data flag is indeed not set
correctly. That is the case where I only send a single frame to a PS STA
in a non-MPSP context when no peering is established yet. I'll fix that.
(Btw, maybe it would be worthwhile to call all of this "MPSP" like the
spec, not just "PSP"?)
You are right, the standard always says MPSP. Ok, I'll adopt that.
But now that you mention it... is there any interest in having that
function used for uAPSD? Because ieee80211_sta_ps_deliver_response sets
the EOSP flag during uAPSD, but does not enforce a QoS Data frame to
carry it. But maybe uAPSD just permits transmitting anything else than
QoS Data frames...
Well, not really, but non-QoS frames won't happen in that case, because
the peer will have QoS enabled. Similarly here I think, why would there
ever be a non-QoS frame? But maybe this can happen with forwarding,
which can't happen in the non-mesh case.
For mesh also the HWMP routing (Management) frames are transmitted in
MPSPs. So a valid PSP would look like that:
1) QoS Null (RSPI:EOSP 0:0)
2) Mgmt frame (e.g. HWMP Path Reply)
3) QoS Null (RSPI:EOSP 0:1)
here the additional trailing Null is needed to end the MPSP. That
scenario happens regularly when no path has been set up previously.
+ ieee80211_sta_ps_deliver_response(sta, 1, 0,
+ IEEE80211_FRAME_RELEASE_UAPSD);
uAPSD?
The standard *explicitly* states that ASPD is *not* supported in mesh.
Absolutely correct. The PSP mechanism is just very similar to uAPSD,
though. So once the PSP is set up, the mechanisms are the same actually.
What do you advise? Renaming the release reason? Creating a different
one that is handled equally?
Well so far the more-data bit seems to be handled different, although I
argue above that you're actually not doing that correctly ;-)
But I think doing different reasons could be helpful, if only to
understand the code better.
In the last iteration of my RFC I created an own function similar to
ieee80211_sta_ps_deliver_response. That spares adding even more
complexity to ieee80211_sta_ps_deliver_response and allows me to keep
the mesh PA code separated in mesh_ps.c. I don't seem to need the
release reason then.
--Marco
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