From: Bruno Randolf <br1@einfach.org>
To: "Luis R. Rodriguez" <lrodriguez@atheros.com>
Subject: Re: [ath5k-devel] [PATCH v2 13/20] cfg80211: Add nl80211 antenna configuration
Date: Fri, 21 May 2010 10:59:33 +0900
Cc: David Quan <David.Quan@atheros.com>, Sam Ng <Sam.Ng@atheros.com>,
	"ath5k-devel@lists.ath5k.org" <ath5k-devel@lists.ath5k.org>,
	"linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
	"linville@tuxdriver.com" <linville@tuxdriver.com>
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On Friday 21 May 2010 07:05:48 Luis R. Rodriguez wrote:
> For legacy, keep it simple, use 3 settings, fixed_a, fixed_b,
> diversity, for all devices.

did you not understand my examples why i think it makes sense to use a bitmask 
for "legacy"? i think they are perfectly valid use-cases. do i need to re-
iterate them a third time?
 
> Lets use a different API for 802.11n. Reason being that even the case
> I mentioned of an 802.11n device connecting on a legacy network needs
> to be treated differently actually.
> 
> For 802.11n you have a few more considerations. You can actually TX at
> the same time on two or more different antennas at the same time. The
> data you transmit will be the contents on both chains on a dual stream
> device. So both antenna 0 and antenna 1 will both be transmitting the
> data for both stream 0 and stream 1. As it turns out the combination
> of TX'ing on two antennas at the same time at a certain dBm power will
> yield a higher received frame on the RX side. This is why when you use
> multiple chains you have to take regulatory rules into considerations
> as well, since adding more chains will increase the overall output
> power. Today ath9k handles this itself since this data is calibrated
> but the max EIRP is passed out from cfg80211. Devices which do not
> deal with these regulatory considerations likely won't support
> changing chainmasks unless they use an API to respect regulatory
> internally somehow. Perhaps the iwlagn firmware does this, beats me.
> 
> The right terminology for antenna control for both TX and RX is
> chainmask and a bitmap of 8 will suffice for existing hardware and up
> to the not-yet-existant 600mbps 4 stream devices. Supporting 8 bits
> will support up to 8 streams and we do not envision using beyond that
> at this point. There is some considerations in the future for
> supporting something other than HT40, like maybe HT80 and so forth but
> those things won't be using more streams it seems.
> 
> Then, some devices won't support all possible chainmask settings. This
> will vary depending on the chipset. I work for Atheros so I can only
> tell you what we can support, we'll have to check with the Intel folks
> about their chipset limitations and settings.
> 
> AR5416, AR5418 can only support chainmask settings which always keep
> the first chain on. The AR9001 family and beyond cannot support the
> 0b110 chaimask (David, you had pointed out some other restrictions,
> what were they again?), the details are complex and I did not get a
> chance to review them.
> 
> I would not be surprised if other vendors had similar restrictions so
> I'm thinking maybe we can express this as a requirement mask, or a set
> of requirement masks. This way userspace utilities for debugging would
> only expose certain chainmask settings.
> 
> Now technically then you can incorporate the legacy API with the
> 802.11n API here somehow but it just seems cleaner to keep them
> separate.
> 
> Also, David indicated that when we change the chainmask when are are
> associated we have to do an actual chip reset, this is different than
> the antenna diversity settings which an be done on the fly. We likely
> will need to reassociate for a chainmask setting, not sure.

so from my point of view this is not very different from what we can support 
with the API i suggested. for RX it seems to be 100% equivalent.

the main difference as i see it is that with 802.11n you transmit on more than 
one antenna, while with 'legacy' we can only transmit on one antenna at a 
time.

actually i have to admit that on legacy "antenna set tx 3 (b11)" (select two 
antennas for transmit) does not make much sense. i have defined it before as 
"use diversity" but what about a different definition: like "bitmap of 
antennas/chains to TRANSMIT". so for 802.11n that would allow you to select 
multiple trasmit chains. on legacy you are only allowed to select one antenna 
in the bitmap. if it is set to "0" (or a separate flag) this could enable 
"follow RX antenna diversity" on legacy.

most of the other things you mention (need a reset/reassociate, regulatory 
concerns...) are driver implementation issues, which can be dealt with in the 
driver. i would just suggest to let the driver reject antenna/chainmask 
configurations which it cannot support.

bruno