mac80211: minstrels: spare numerous useless calls to get_random_bytes

ATM, only the first array value returned by get_random_bytes is used.
This change moves the call to get_random_bytes from the nested loop it
is in to its parent.
While at it, replace get_random_bytes with prandom_bytes since PRNs are
way enough for the selection process.
After this, minstrel_ht reclaims 80 PR-bytes instead of 640 R-bytes.

minstrels use sample tables to probe different rates in a randomized
manner.
minstrel_ht inits one single sample table upon registration (during
subsys_initcalls) and minstrel uses one per STA addition in minstrel.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel.c

@@ -422,10 +422,9 @@ init_sample_table(struct minstrel_sta_info *mi)
 	memset(mi->sample_table, 0xff, SAMPLE_COLUMNS * mi->n_rates);
 
 	for (col = 0; col < SAMPLE_COLUMNS; col++) {
+		prandom_bytes(rnd, sizeof(rnd));
 		for (i = 0; i < mi->n_rates; i++) {
-			get_random_bytes(rnd, sizeof(rnd));
 			new_idx = (i + rnd[i & 7]) % mi->n_rates;
-
 			while (SAMPLE_TBL(mi, new_idx, col) != 0xff)
 				new_idx = (new_idx + 1) % mi->n_rates;
 

net/mac80211/rc80211_minstrel_ht.c

@@ -1051,10 +1051,9 @@ init_sample_table(void)
 
 	memset(sample_table, 0xff, sizeof(sample_table));
 	for (col = 0; col < SAMPLE_COLUMNS; col++) {
+		prandom_bytes(rnd, sizeof(rnd));
 		for (i = 0; i < MCS_GROUP_RATES; i++) {
-			get_random_bytes(rnd, sizeof(rnd));
 			new_idx = (i + rnd[i]) % MCS_GROUP_RATES;
-
 			while (sample_table[col][new_idx] != 0xff)
 				new_idx = (new_idx + 1) % MCS_GROUP_RATES;