Re: [PATCH 1/4] ftgmac100: Fix race issue on TX descriptor[0]

[Joel Stanley]

On Mon, 19 Oct 2020 at 23:20, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 2020-10-19 at 16:57 +0800, Dylan Hung wrote:
> > These rules must be followed when accessing the TX descriptor:
> >
> > 1. A TX descriptor is "cleanable" only when its value is non-zero
> > and the owner bit is set to "software"
>
> Can you elaborate ? What is the point of that change ? The owner bit
> should be sufficient, why do we need to check other fields ?

I would like Dylan to clarify too. The datasheet has a footnote below
the descriptor layout:

 - TXDES#0: Bits 27 ~ 14 are valid only when FTS = 1
 - TXDES#1: Bits 31 ~ 0 are valid only when FTS = 1

So the ownership bit (31) is not valid unless FTS is set. However,
this isn't what his patch does. It adds checks for EDOTR.

>
> > 2. A TX descriptor is "writable" only when its value is zero
> > regardless the edotr mask.
>
> Again, why is that ? Can you elaborate ? What race are you trying to
> address here ?
>
> Cheers,
> Ben.
>
> > Fixes: 52c0cae87465 ("ftgmac100: Remove tx descriptor accessors")
> > Signed-off-by: Dylan Hung <dylan_hung@xxxxxxxxxxxxxx>
> > Signed-off-by: Joel Stanley <joel@xxxxxxxxx>
> > ---
> >  drivers/net/ethernet/faraday/ftgmac100.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/drivers/net/ethernet/faraday/ftgmac100.c
> > b/drivers/net/ethernet/faraday/ftgmac100.c
> > index 00024dd41147..7cacbe4aecb7 100644
> > --- a/drivers/net/ethernet/faraday/ftgmac100.c
> > +++ b/drivers/net/ethernet/faraday/ftgmac100.c
> > @@ -647,6 +647,9 @@ static bool ftgmac100_tx_complete_packet(struct
> > ftgmac100 *priv)
> >       if (ctl_stat & FTGMAC100_TXDES0_TXDMA_OWN)
> >               return false;
> >
> > +     if ((ctl_stat & ~(priv->txdes0_edotr_mask)) == 0)
> > +             return false;
> > +
> >       skb = priv->tx_skbs[pointer];
> >       netdev->stats.tx_packets++;
> >       netdev->stats.tx_bytes += skb->len;
> > @@ -756,6 +759,9 @@ static netdev_tx_t
> > ftgmac100_hard_start_xmit(struct sk_buff *skb,
> >       pointer = priv->tx_pointer;
> >       txdes = first = &priv->txdes[pointer];
> >
> > +     if (le32_to_cpu(txdes->txdes0) & ~priv->txdes0_edotr_mask)
> > +             goto drop;
> > +
> >       /* Setup it up with the packet head. Don't write the head to
> > the
> >        * ring just yet
> >        */
> > @@ -787,6 +793,10 @@ static netdev_tx_t
> > ftgmac100_hard_start_xmit(struct sk_buff *skb,
> >               /* Setup descriptor */
> >               priv->tx_skbs[pointer] = skb;
> >               txdes = &priv->txdes[pointer];
> > +
> > +             if (le32_to_cpu(txdes->txdes0) & ~priv-
> > >txdes0_edotr_mask)
> > +                     goto dma_err;
> > +
> >               ctl_stat = ftgmac100_base_tx_ctlstat(priv, pointer);
> >               ctl_stat |= FTGMAC100_TXDES0_TXDMA_OWN;
> >               ctl_stat |= FTGMAC100_TXDES0_TXBUF_SIZE(len);
>