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Date:   Fri, 20 Aug 2021 18:09:48 +0100
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From:   Marc Zyngier <maz@kernel.org>
To:     Matteo Croce <mcroce@linux.microsoft.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        Thierry Reding <thierry.reding@gmail.com>,
        netdev@vger.kernel.org,
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        Alexandre Torgue <alexandre.torgue@foss.st.com>,
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        Drew Fustini <drew@beagleboard.org>,
        Emil Renner Berthing <kernel@esmil.dk>,
        Jon Hunter <jonathanh@nvidia.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [PATCH net-next] stmmac: align RX buffers
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On Fri, 20 Aug 2021 17:38:14 +0100,
Matteo Croce <mcroce@linux.microsoft.com> wrote:
> 
> On Fri, Aug 20, 2021 at 6:26 PM Marc Zyngier <maz@kernel.org> wrote:
> >
> > On Fri, 20 Aug 2021 11:37:03 +0100,
> > Matteo Croce <mcroce@linux.microsoft.com> wrote:
> > >
> > > On Thu, Aug 19, 2021 at 6:29 PM Marc Zyngier <maz@kernel.org> wrote:
> >
> > [...]
> >
> > > > diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac.h b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > > > index fcdb1d20389b..244aa6579ef4 100644
> > > > --- a/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > > > +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac.h
> > > > @@ -341,7 +341,7 @@ static inline unsigned int stmmac_rx_offset(struct stmmac_priv *priv)
> > > >         if (stmmac_xdp_is_enabled(priv))
> > > >                 return XDP_PACKET_HEADROOM + NET_IP_ALIGN;
> > > >
> > > > -       return NET_SKB_PAD + NET_IP_ALIGN;
> > > > +       return 8 + NET_IP_ALIGN;
> > > >  }
> > > >
> > > >  void stmmac_disable_rx_queue(struct stmmac_priv *priv, u32 queue);
> > > >
> > > > I don't see the system corrupting packets anymore. Is that exactly
> > > > what you had in mind? This really seems to point to a basic buffer
> > > > overflow.
> >
> > [...]
> >
> > > Sorry, I meant something like:
> > >
> > > -       return NET_SKB_PAD + NET_IP_ALIGN;
> > > +       return 8;
> > >
> > > I had some hardware which DMA fails if the receive buffer was not word
> > > aligned, but this seems not the case, as 8 + NET_IP_ALIGN = 10, and
> > > it's not aligned too.
> >
> > No error in that case either, as expected. Given that NET_SKB_PAD is
> > likely to expand to 64, it is likely a DMA buffer overflow which
> > probably only triggers for large-ish packets.
> >
> > Now, we're almost at -rc7, and we don't have a solution in sight.
> >
> > Can we please revert this until we have an understanding of what is
> > happening? I'll hopefully have more cycles to work on the issue once
> > 5.14 is out, and hopefully the maintainers of this driver can chime in
> > (they have been pretty quiet so far).
> >
> > Thanks,
> >
> >         M.
> >
> > --
> > Without deviation from the norm, progress is not possible.
> 
> Last try, what about adding only NET_IP_ALIGN and leaving NET_SKB_PAD?
> 
> -       return NET_SKB_PAD + NET_IP_ALIGN;
> +       return NET_IP_ALIGN;
> 
> I think that alloc_skb adds another NET_SKB_PAD anyway.

I don't see any packet corruption with this. However, this doesn't
prove that this is correct either. What was the rational for adding
NET_SKB_PAD the first place?

	M.

-- 
Without deviation from the norm, progress is not possible.