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wifi: cfg80211: Fix bitrates overflow issue

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When invoking function cfg80211_calculate_bitrate_eht about
(320 MHz, EHT-MCS 13, EHT-NSS 2, EHT-GI 0), which means the
parameters as flags: 0x80, bw: 7, mcs: 13, eht_gi: 0, nss: 2,
this formula (result * rate->nss) will overflow and causes
the returned bitrate to be 3959 when it should be 57646.

Here is the explanation:
 u64 tmp;
 u32 result;
 …
 /* tmp = result = 4 * rates_996[0]
  *     = 4 * 480388888 = 0x72889c60
  */
 tmp = result;

 /* tmp = 0x72889c60 * 6144 = 0xabccea90000 */
 tmp *= SCALE;

 /* tmp = 0xabccea90000 / mcs_divisors[13]
  *     = 0xabccea90000 / 5120 = 0x8970bba6
  */
 do_div(tmp, mcs_divisors[rate->mcs]);

 /* result = 0x8970bba6 */
 result = tmp;

 /* normally (result * rate->nss) = 0x8970bba6 * 2 = 0x112e1774c,
  * but since result is u32, (result * rate->nss) = 0x12e1774c,
  * overflow happens and it loses the highest bit.
  * Then result =  0x12e1774c / 8 = 39595753,
  */
 result = (result * rate->nss) / 8;

Signed-off-by: Paul Zhang <quic_paulz@quicinc.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
This commit is contained in:
Paul Zhang 2022-10-11 21:04:28 +08:00 committed by Johannes Berg
parent 57b962e627
commit 18429c51c7
1 changed files with 4 additions and 2 deletions
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6
net/wireless/util.c
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@ -1557,10 +1557,12 @@ static u32 cfg80211_calculate_bitrate_eht(struct rate_info *rate)
tmp = result;
tmp *= SCALE;
do_div(tmp, mcs_divisors[rate->mcs]);
result = tmp;
/* and take NSS */
result = (result * rate->nss) / 8;
tmp *= rate->nss;
do_div(tmp, 8);
result = tmp;
return result / 10000;
}