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>
18429c51c7