[report] wifi: mt76: mt7996: fix FCS error flag check in RX descriptor

[Peter Chiu]

On Mon, 2025-10-13 at 11:11 -0700, Ben Greear wrote:

Hi Ben,

The incorrect bit is MT_RXD1_NORMAL_CLM.
It is only set when there is a decryption error so most of the time
this bit is zero.

Thanks,
Peter Chiu

> 
> External email : Please do not click links or open attachments until
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> 
> Hello Peter,
> 
> To help understand the severity of this problem, can you let us know
> when
> the incorrect bit is set?  In other words, would looking at the wrong
> bit
> erroneously cause good frames to be dropped, or was the bit always
> zero and
> it just allowed bad frames to be accepted?
> 
> Thanks,
> Ben
> 
> On 10/12/25 20:19, Peter Chiu wrote:
> > On Sun, 2025-10-12 at 22:02 +0530, ALOK TIWARI wrote:
> > 
> > Hi Alok,
> > 
> > Thanks for your mail.
> > Your understanding is correct.
> > There is a typo in mt7996.
> > 
> > Thanks,
> > Peter Chiu
> > 
> > > 
> > > External email : Please do not click links or open attachments
> > > until
> > > you have verified the sender or the content.
> > > 
> > > 
> > > Hi,
> > > 
> > > While looking through the FCS error handling in the mt76 drivers,
> > > I noticed that mt7996 uses rxd1 to check the
> > > MT_RXD3_NORMAL_FCS_ERR
> > > bit,
> > > whereas other Connac3-based drivers (such as mt7925) use rxd3.
> > > 
> > > According to the definitions, MT_RXD3_NORMAL_FCS_ERR is located
> > > in
> > > the
> > > fourth
> > > RX descriptor word (rxd3), so I prepared a small patch to switch
> > > the
> > > check from
> > > rxd1 to rxd3 for consistency and correctness.
> > > 
> > > Before sending it formally, I wanted to confirm whether this
> > > understanding is correct.
> > > is mt7996 indeed supposed to use rxd3 for this flag, like mt7925?
> > > 
> > > Fixes: 98686cd21624 ("wifi: mt76: mt7996: add driver for MediaTek
> > > Wi-
> > > Fi
> > > 7 (802.11be) devices")
> > > 
> > > Thanks,
> > > Alok
> > > 
> > > ---
> > >   drivers/net/wireless/mediatek/mt76/mt7996/mac.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > > b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > > index 9501def3e0e3..59744e5593b6 100644
> > > --- a/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > > +++ b/drivers/net/wireless/mediatek/mt76/mt7996/mac.c
> > > @@ -527,7 +527,7 @@ mt7996_mac_fill_rx(struct mt7996_dev *dev,
> > > enum
> > > mt76_rxq_id q,
> > >              !(csum_status & (BIT(0) | BIT(2) | BIT(3))))
> > >                  skb->ip_summed = CHECKSUM_UNNECESSARY;
> > > 
> > > -       if (rxd1 & MT_RXD3_NORMAL_FCS_ERR)
> > > +       if (rxd3 & MT_RXD3_NORMAL_FCS_ERR)
> > >                  status->flag |= RX_FLAG_FAILED_FCS_CRC;
> > > 
> > >          if (rxd1 & MT_RXD1_NORMAL_TKIP_MIC_ERR)
> > > --
> > > 2.50.1
> > 
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> --
> Ben Greear <greearb at candelatech.com>
> Candela Technologies Inc 
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