Subject: Re: [PATCH v1 1/2] ath9k: fix ath_get_rate_txpower() to respect the rate list end tag

From: Peter Seiderer <[email-masked]>
Date: Mon, 4 Apr 2022 22:52:12 +0200

Hello Toke,

On Mon, 04 Apr 2022 20:19:39 +0200, Toke Høiland-Jørgensen <[email-masked]> wrote:

> Peter Seiderer <[email-masked]> writes:
> 
> > Stop reading (and copying) from ieee80211_tx_rate to ath_tx_info.rates
> > after list end tag (count == 0, idx < 0), prevents copying of garbage
> > to card registers.  
> 
> In the normal case I don't think this patch does anything, since any
> invalid rate entries will already be skipped (just one at a time instead
> of all at once). So this comment is a bit misleading.

Save some (minimal) compute time? Found it something misleading while
debugging to see random values written out to the card and found this
comment in net/mac80211/rate.c:

 648                 /*
 649                  * make sure there's no valid rate following
 650                  * an invalid one, just in case drivers don't
 651                  * take the API seriously to stop at -1.
 652                  */

and multiple places doing the same check (count == 0, idx < 0) for validation
e.g.:

 723                 if (i < ARRAY_SIZE(info->control.rates) &&
 724                     info->control.rates[i].idx >= 0 &&
 725                     info->control.rates[i].count) {

or 

 742                 if (rates[i].idx < 0 || !rates[i].count)
 743                         break;

> 
> Also, Minstrel could in principle produce a rate sequence where the
> indexes are all positive, but there's one in the middle with a count of
> 0, couldn't it? With this patch, the last entries of such a sequence
> would now be skipped...

According to net/mac80211/rc80211_minstrel_ht.c:

1128 static bool
1129 minstrel_ht_txstat_valid(struct minstrel_priv *mp, struct minstrel_ht_sta *     mi,
1130                          struct ieee80211_tx_rate *rate)
1131 {
1132         int i;
1133 
1134         if (rate->idx < 0)
1135                 return false;
1136 
1137         if (!rate->count)
1138                 return false;
1139 

minstrel although evaluates a rate count of zero as invalid...

Regards,
Peter

> -Toke