[RFC 1/3] nl/cfg80211: add chan_time for scan request

[Michal Kazior]

On 2 August 2013 13:25, Johannes Berg <johannes at sipsolutions.net> wrote:
> On Thu, 2013-08-01 at 11:14 +0200, Michal Kazior wrote:
>
>> > This seems a bit iffy - you don't differentiate between active/passive
>> > scans?
>>
>> Is there a reason this should be differentiated?
>
> Well they don't usually have the same timing, passive channel dwell time
> usually needs to be longer than on active channels.
>
>> > This also interferes a bit with some other scan optimisations that could
>> > be done at a low firmware level, so I think we should be careful and
>> > actually say that this is really more intended for measurement use cases
>> > and not for normal scans?
>> >
>> > Or maybe we should have a separate measurement command with similar
>> > semantics? This all doesn't seem very clear to me yet :)
>>
>> Motivation behind this patchset is to simplify ACS implementation and
>> depend on scan. This also allows easier fallback from survey-based ACS
>> to BSS-based one.
>
> Sure, I understand.
>
>> Other use cases are a side-effect so perhaps clarifying the intent in
>> the docs is enough.
>
> I'm just saying that this is a fundamentally different usage than actual
> scanning. Let's say for the sake of the argument I find a way to scan
> channels quicker and implement that in my device (hw_scan). I don't
> think this is too far-fetched, imagine I have an 80MHz capable device
> and can send probe request 4x transmissions on all the subchannels, if
> somehow I manage to separate the receive then I could scan 4x as
> quickly. I don't think this particular thing is possible but I could
> imagine scan optimisations like it.
>
> Now this comes along - and suddenly the API requires that you stay on
> each specified channel for the given period of time. Any such
> "non-traditional" scan optimisations would have to be given up, so in a
> sense this fundamentally alters the semantics of the scan primitive,
> where before it was "give me the list of networks (with these SSIDs) on
> these channels" it now becomes "go to each channel, stay there for this
> amount of time and ..."
>
> That's what I'm worried about - the implicit semantic change here.

I think I understand your point now.

I'm thinking of using series of passive scans for ACS now, so perhaps
the whole chan_time stuff is unnecessary. What are your thoughts about
it?


Pozdrawiam / Best regards,
Michał Kazior.