netdev - Re: [PATCH net-next] stmmac: align RX buffers

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Date:   Thu, 12 Aug 2021 12:05:38 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Matteo Croce <mcroce@...ux.microsoft.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Drew Fustini <drew@...gleboard.org>,
        Emil Renner Berthing <kernel@...il.dk>,
        Jon Hunter <jonathanh@...dia.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH net-next] stmmac: align RX buffers

On Thu, 12 Aug 2021 11:18:35 +0100,
Matteo Croce <mcroce@...ux.microsoft.com> wrote:
> 
> On Thu, 12 Aug 2021 10:48:03 +0200
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> > 
> > 
> > On 8/11/21 4:16 PM, Marc Zyngier wrote:
> > > On Wed, 11 Aug 2021 13:53:59 +0100,
> > > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > >
> > >> Are you sure you do not need to adjust stmmac_set_bfsize(), 
> > >> stmmac_rx_buf1_len() and stmmac_rx_buf2_len() ?
> > >>
> > >> Presumably DEFAULT_BUFSIZE also want to be increased by NET_SKB_PAD
> > >>
> > >> Patch for stmmac_rx_buf1_len() :
> > >>
> > >> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> > >> b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c index
> > >> 7b8404a21544cf29668e8a14240c3971e6bce0c3..041a74e7efca3436bfe3e17f972dd156173957a9
> > >> 100644 --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c +++
> > >> b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c @@ -4508,12
> > >> +4508,12 @@ static unsigned int stmmac_rx_buf1_len(struct
> > >> stmmac_priv *priv, /* First descriptor, not last descriptor and
> > >> not split header */ if (status & rx_not_ls)
> > >> -               return priv->dma_buf_sz;
> > >> +               return priv->dma_buf_sz - NET_SKB_PAD -
> > >> NET_IP_ALIGN; 
> > >>         plen = stmmac_get_rx_frame_len(priv, p, coe);
> > >>  
> > >>         /* First descriptor and last descriptor and not split
> > >> header */
> > >> -       return min_t(unsigned int, priv->dma_buf_sz, plen);
> > >> +       return min_t(unsigned int, priv->dma_buf_sz - NET_SKB_PAD
> > >> - NET_IP_ALIGN, plen); }
> > >>  
> > >>  static unsigned int stmmac_rx_buf2_len(struct stmmac_priv *priv,
> > > 
> > > Feels like a major deficiency of the original patch. Happy to test a
> > > more complete patch if/when you have one.
> > 
> > I wont have time in the immediate future.
> > 
> > Matteo, if you do not work on a fix, I suggest we revert
> >  a955318fe67ec0d962760b5ee58e74bffaf649b8 stmmac: align RX buffers
> > 
> > before a more polished version can be submitted.
> > 
> 
> Better to use stmmac_rx_offset() so to have the correct length when
> using XDP. Also, when XDP is enabled, the offset was
> XDP_PACKET_HEADROOM (i.e. 256 bytes) even before the change, so it
> could be already broken. Mark, can you try on the Jetson TX2 by
> attaching an XDP program and see if it works without my change?

Sorry, you'll have to hold my hand here, as I know exactly nothing
about XDP....

> A possible fix, which takes in account also the XDP headroom for
> stmmac_rx_buf1_len() only could be (only compile tested, I don't have
> the hardware now):

However, this doesn't fix my issue. I still get all sort of
corruption. Probably stmmac_rx_buf2_len() also need adjusting (it has
a similar logic as its buf1 counterpart...)

Unless you can fix it very quickly, and given that we're towards the
end of the cycle, I'd be more comfortable if we reverted this patch.

Thanks,

	M.

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