netdev - Re: [PATCH/RFC net-next] ravb: RX checksum offload

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Message-ID: <20170928104918.GA11212@verge.net.au>
Date:   Thu, 28 Sep 2017 12:49:18 +0200
From:   Simon Horman <horms@...ge.net.au>
To:     Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Cc:     David Miller <davem@...emloft.net>,
        Magnus Damm <magnus.damm@...il.com>, netdev@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH/RFC net-next] ravb: RX checksum offload

On Wed, Sep 13, 2017 at 08:54:00PM +0300, Sergei Shtylyov wrote:
> Hello!
> 
> On 09/12/2017 04:04 PM, Simon Horman wrote:
> 
> >Add support for RX checksum offload. This is enabled by default and
> >may be disabled and re-enabled using ethtool:
> >
> >  # ethtool -K eth0 rx off
> >  # ethtool -K eth0 rx on
> >
> >The RAVB provides a simple checksumming scheme which appears to be
> >completely compatible with CHECKSUM_COMPLETE: a 1's complement sum of
> 
>    Hm, the gen2/3 manuals say calculation doesn't involve bit inversion...

Yes, I believe that matches my observation of the values supplied by
the hardware. Empirically this appears to be what the kernel expects.

> >all packet data after the L2 header is appended to packet data; this may
> >be trivially read by the driver and used to update the skb accordingly.
> >
> >In terms of performance throughput is close to gigabit line-rate both with
> >and without RX checksum offload enabled. Perf output, however, appears to
> >indicate that significantly less time is spent in do_csum(). This is as
> >expected.
> 
> [...]
> 
> >By inspection this also appears to be compatible with the ravb found
> >on R-Car Gen 2 SoCs, however, this patch is currently untested on such
> >hardware.
> 
>    I probably won't be able to test it on gen2 too...
> 
> >Signed-off-by: Simon Horman <horms+renesas@...ge.net.au>
> 
>    I'm generally OK with the patch but have some questions/comments below...

Thanks, I will try to address them.

> >---
> >  drivers/net/ethernet/renesas/ravb_main.c | 58 +++++++++++++++++++++++++++++++-
> >  1 file changed, 57 insertions(+), 1 deletion(-)
> >
> >diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/ethernet/renesas/ravb_main.c
> >index fdf30bfa403b..7c6438cd7de7 100644
> >--- a/drivers/net/ethernet/renesas/ravb_main.c
> >+++ b/drivers/net/ethernet/renesas/ravb_main.c
> [...]
> >@@ -1842,6 +1859,41 @@ static int ravb_do_ioctl(struct net_device *ndev, struct ifreq *req, int cmd)
> >  	return phy_mii_ioctl(phydev, req, cmd);
> >  }
> >+static void ravb_set_rx_csum(struct net_device *ndev, bool enable)
> >+{
> >+	struct ravb_private *priv = netdev_priv(ndev);
> >+	unsigned long flags;
> >+
> >+	spin_lock_irqsave(&priv->lock, flags);
> >+
> >+	/* Disable TX and RX */
> >+	ravb_rcv_snd_disable(ndev);
> >+
> >+	/* Modify RX Checksum setting */
> >+	if (enable)
> >+		ravb_modify(ndev, ECMR, 0, ECMR_RCSC);
> 
>    Please use ECMR_RCSC as the 3rd argument too to conform the common driver
> style.
> 
> >+	else
> >+		ravb_modify(ndev, ECMR, ECMR_RCSC, 0);
> 
>    This *if* can easily be folded into a single ravb_modify() call...

Thanks, something like this?

	ravb_modify(ndev, ECMR, ECMR_RCSC, enable ? ECMR_RCSC : 0);

> [...]
> >@@ -2004,6 +2057,9 @@ static int ravb_probe(struct platform_device *pdev)
> >  	if (!ndev)
> >  		return -ENOMEM;
> >+	ndev->features |= NETIF_F_RXCSUM;
> >+	ndev->hw_features |= ndev->features;
> 
>    Hum, both fields are 0 before this? Then why not use '=' instead of '|='?
> Even if not, why not just use the same value as both the rvalues?

I don't feel strongly about this, how about?

	ndev->features = NETIF_F_RXCSUM;
	ndev->hw_features = NETIF_F_RXCSUM;