Re: [PATCH 2/2] wireless: return correct mandatory rates

[Johannes Berg <johannes@sipsolutions.net>]

On Fri, 2017-09-08 at 10:43 +0200, Richard Schütz wrote:
> Am 08.09.2017 um 08:55 schrieb Johannes Berg:
> > On Thu, 2017-09-07 at 17:47 +0200, Richard Schütz wrote:
> > > Use IEEE80211_RATE_MANDATORY_G instead of
> > > IEEE80211_RATE_MANDATORY_B
> > > for comparison to get all mandatory rates in 2.4 GHz band. It is
> > > safe
> > > to do so because ERP mandatory rates are a superset of HR/DSSS
> > > mandatory rates.
> > 
> > This I don't understand - what "comparison" are you talking about?
> 
> Sorry, I meant the condition that checks for the presence of 
> mandatory_flag at the bottom of the function.

Ah, sorry, I got confused with the other patch.

> Do we actually allow 10 MHz and 5 MHz operation in the 2.4 GHz band?
> As  far as I can tell that has only been specified for OFDM PHYs,
> which use the 5 GHz band and are covered by
> IEEE80211_RATE_MANDATORY_A, but I am not a hundred per cent sure
> about that. Cc'ing Simon Wunderlich who originally implemented
> checking of scan_width here.

Clearly we do allow that, since the existing check is:

        if (sband->band == NL80211_BAND_2GHZ) {
                if (scan_width == NL80211_BSS_CHAN_WIDTH_5 ||
                    scan_width == NL80211_BSS_CHAN_WIDTH_10)
                        mandatory_flag = IEEE80211_RATE_MANDATORY_G;

That wouldn't make any sense if we didn't have 5/10 MHz on 2.4 GHz.

> The main intention of this patch series is to fix mandatory rates 
> returned for normal operation in 2.4 GHz band. Currently only 1 Mb/s
> is returned here, which is wrong for both HR/DSSS and ERP PHYs.

The patch is still wrong wrt. 5/10 MHz though.

I think what you really wanted to do is the following:

 * rename RATE_MANDATORY_B to RATE_MANDATORY_HR_DSSS
 * combine RATE_MANDATORY_G/_A to RATE_MANDATORY_OFDM

Then, what you need to do, is change the checks in
ieee80211_mandatory_rates() to be

        if (sband->band == NL80211_BAND_2GHZ) {
                if (scan_width == NL80211_BSS_CHAN_WIDTH_5 ||
                    scan_width == NL80211_BSS_CHAN_WIDTH_10)
                        mandatory_flag = IEEE80211_RATE_MANDATORY_OFDM;
                else
                        mandatory_flag = IEEE80211_RATE_MANDATORY_HR_DSSS;
        } else {
                mandatory_flag = IEEE80211_RATE_MANDATORY_OFDM;
        }

That would actually fix a bug in a way because right now the code
treats HR/DSSS rates (1, 2, 5.5, 11) for 2.4 GHz narrow-band operation
as mandatory, which seems wrong.

johannes