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Subject: Re: [PATCH] mac80211: do not allow bigger VHT MPDUs than the hardware
 supports
To:     Felix Fietkau <nbd@nbd.name>, linux-wireless@vger.kernel.org
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On 17/09/2020 15:50, Felix Fietkau wrote:
> Limit maximum VHT MPDU size by local capability
>
> Signed-off-by: Felix Fietkau <nbd@nbd.name>
> ---
>   net/mac80211/vht.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/net/mac80211/vht.c b/net/mac80211/vht.c
> index 7e601d067d53..fb0e3a657d2d 100644
> --- a/net/mac80211/vht.c
> +++ b/net/mac80211/vht.c
> @@ -168,10 +168,7 @@ ieee80211_vht_cap_ie_to_sta_vht_cap(struct ieee80211_sub_if_data *sdata,
>          /* take some capabilities as-is */
>          cap_info = le32_to_cpu(vht_cap_ie->vht_cap_info);
>          vht_cap->cap = cap_info;
> -       vht_cap->cap &= IEEE80211_VHT_CAP_MAX_MPDU_LENGTH_3895 |
> -                       IEEE80211_VHT_CAP_MAX_MPDU_LENGTH_7991 |
> -                       IEEE80211_VHT_CAP_MAX_MPDU_LENGTH_11454 |
> -                       IEEE80211_VHT_CAP_RXLDPC |
> +       vht_cap->cap &= IEEE80211_VHT_CAP_RXLDPC |
>                          IEEE80211_VHT_CAP_VHT_TXOP_PS |
>                          IEEE80211_VHT_CAP_HTC_VHT |
>                          IEEE80211_VHT_CAP_MAX_A_MPDU_LENGTH_EXPONENT_MASK |
> @@ -180,6 +177,9 @@ ieee80211_vht_cap_ie_to_sta_vht_cap(struct ieee80211_sub_if_data *sdata,
>                          IEEE80211_VHT_CAP_RX_ANTENNA_PATTERN |
>                          IEEE80211_VHT_CAP_TX_ANTENNA_PATTERN;
>
> +       vht_cap->cap |= min_t(u32, cap_info & IEEE80211_VHT_CAP_MAX_MPDU_MASK,
> +                             own_cap.cap & IEEE80211_VHT_CAP_MAX_MPDU_MASK);
> +
>          /* and some based on our own capabilities */
>          switch (own_cap.cap & IEEE80211_VHT_CAP_SUPP_CHAN_WIDTH_MASK) {
>          case IEEE80211_VHT_CAP_SUPP_CHAN_WIDTH_160MHZ:
>

The vht_cap->cap IEEE80211_VHT_CAP_MAX_MPDU* reflect the associated STA 
_RX_ capabilities.

Why would you override the original STA _RX_ capabilities (vht_cap->cap) 
with your own HW _RX_ size limitation ?

e.g. if HW support 11454 in TX and 3895 in RX, due to this change, you 
limit your TX to your own _RX_ limitation (3895) while STA actually 
reported 11454 RX.