Re: [wireless-next PATCH 3/5] wifi: Allow overriding some HT information.

[Ben Greear <greearb@candelatech.com>]

On 11/02/2011 10:49 AM, Johannes Berg wrote:
> On Wed, 2011-11-02 at 09:59 -0700, Ben Greear wrote:
>
>>> No. Neither of these can ever be increased so it's not that simple. And
>>> making it smaller is always possible since it's just advertising.
>>> Presumably you do understand the reasons for this advertising/these
>>> restrictions?
>>
>> It seems that a driver might default to a mid-range value for the MPDU values
>> because it is somehow 'better' for whatever reason, and yet it might still
>> support larger values if the user desires, perhaps because in specific
>> scenarios larger values are better.  Ath9k does set to the max value,
>> so if we do a per-driver capabilities flag as I did in v2 then we
>> are safe.
>
> Then the proper way to do that would be to not have a flag, but a max it
> can be set to. However, I see no reason it should default to a mid-range
> value?! iwlwifi for example needs to allocate enough space but ... I
> don't get it. What's wrong with simply not allowing to increase, only
> decrease?

Ok, I'll work on allowing the value to only be decreased.
I should be able to compare against whatever the hardware set in
the channel ht-info I think.

>>>>           /* add attributes here, update the policy in nl80211.c */
>>
>> I copied some of that code from nl80211_set_station, which appears to
>> also forget to check the length for the NL80211_ATTR_HT_CAPABILITY
>> object.  Is there some reason why it doesn't need to check, or does
>> that code need fixing as well?
>
> NL80211_ATTR_HT_CAPABILITY in particular *has* a policy entry.

Ahh, I didn't realize that's what was meant by policy.  Mind if
I change that comment to something like what is below?

/* add attributes here, update the nl80211_policy array in nl80211.c */


>>    From 20.1.1 of the 802.11n spec:
>>
>> "An HT non-AP STA shall support all equal modulation (EQM) rates for one spatial stream (MCSs 0 through
>> 7) using 20 MHz channel width. An HT AP shall support all EQM rates for one and two spatial streams
>> (MCSs 0 through 15) using 20 MHz channel width."
>>
>> That is why I wrote that code as I did, but perhaps I misunderstand that section of
>> the docs.
>
> No, that makes some sense, I wasn't aware of that restriction.

Well, personally it seems like a lame restriction, and at least hostapd and ath9k will
deal fine with a station advertising less than that, but probably best to
stick with the spec if possible.

Thanks,
Ben


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com