Re: [RFC V2 2/3] nl80211: add bss selection attribute to CONNECT command

[Arend van Spriel <arend@broadcom.com>]

On 19-1-2016 14:20, Johannes Berg wrote:
> On Mon, 2016-01-18 at 10:34 +0100, Arend van Spriel wrote:
> 
>>> I'd prefer nl80211_bss_select_attr in this name and the constants
>>> too,
>>> so it's more obvious that it doesn't belong to the top-level
>>> namespace.
>>
>> Ok. Did not know that convention so it was not that obvious to me ;-)
>> Will change it.
> 
> I don't think we really follow it everywhere, and - hindsight being
> 20/20 - I think that we should perhaps have chosen shorter prefixes :)
> 
>>>> + * @__NL80211_ATTR_BSS_SELECT_INVALID: reserved.
>>>> + * @NL80211_ATTR_BSS_SELECT_PRIMITIVE: Indicates what criteria
>>>> are to
>>>> + *> 	> be used for bss selection. Value according
>>>> + *	%enum nl80211_bss_select_primitive.
>>>
>>> This I don't understand now. Wouldn't the given attributes just be
>>> used?
>>
>> The primitive just indicates the requested bss selection criteria and
>> determines what other attributes are to be expected. Could determine
>> it by looking at the other attributes, but that would make it harder
>> to validate the request. This way it also makes them mutually
>> exclusive.
> 
> I still don't really understand - if a given attribute just gives data
> about the remaining attributes, how does it make a difference? You get
> all the attributes at once, after all, and aren't really forced to
> parse them as they trickle in.

True. Ok. let me try without this.

>>> I was thinking you'd keep the NLA_FLAG "RSSI" preference, and use
>>> the attribute values for the bitmap ...
>>
>> You lost me here.
> 
> I meant: use BIT(NL80211_BSS_SELECT_ATTR_BAND_PREF) instead of the
> separate NL80211_BSS_SELECT_BAND_PREF.
> 
> Keeping the NLA_FLAG and all that was just a reference to the
> discussion above.
> 
>>>     RSSI (priority 100)
>>>     BAND_PREF (priority 1)
>>>     RSSI_ADJUST (priority 1) [since it's mutually exclusive with
>>>     BAND_PREF]
>>
>> Not sure about the priority. What I should document is that BAND_PREF
>> and RSSI_ADJUST also do RSSI based selection as a second step. As a
>> (possibly important) side note our firmware api allows multiple
>> primitives, but RSSI must be one of them as it will reject the
>> configuration otherwise. As such I could combine RSSI and RSSI_ADJUST
>> as RSSI would be RSSI_ADJUST(band=unspec, delta=0).
> 
> Ok, that's a different way of thinking about it.
> 
> I was thinking about it as a kind of small programmable state-machine
> or pipeline in the BSS selection pipeline, so if you have
> 
>  band_pref, rssi
> 
> you basically have a first "element" in the pipeline that throws away
> the BSSes that don't match the right band, and a second one that picks
> the one with the highest RSSI.

I looked at our firmware and it has a kinda pipeline, but it uses a
weighted score. So for "band_pref, rssi" the band_pref score would have
more weight than the rssi score and bss-es are sorted based on the
score. So not really throwing things away.

> If I understand you correctly, you're thinking about it more in terms
> of the overall behaviour. That's perfectly fine with me, but then we
> should document that more clearly.

Agree.

> Just to make sure I understand - you're basically saying that
> 
>  band_pref
> 
> would mean
>  * throw away BSSes not matching the right band
>  * pick the one with the highest RSSI
> 
> which basically makes it mutually exclusive with any of the other
> attributes you suggested, where I was thinking that you'd pretty much
> always have to specify multiple attributes to get a proper "pipeline".
> 
> 
> Your way definitely has advantages too, particularly that you don't
> need that whole discussion about priorities/ordering, which is good.
> 
> But then I understand the whole point of the "primitive" even less,
> since it should be trivial to check that of multiple attributes only a
> single one is specified?

Not really a single one as rssi_adjust needs two attributes, ie. band
and rssi_delta. Still you are right and we can probably drop the primitive.

Regards,
Arend