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Subject: Re: [PATCH v2] ftgmac100: Disable HW checksum generation on AST2500
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     Vijay Khemka <vijaykhemka@fb.com>,
        "David S. Miller" <davem@davemloft.net>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Sven Van Asbroeck <TheSven73@gmail.com>,
        Mark Brown <broonie@kernel.org>,
        Bhupesh Sharma <bhsharma@redhat.com>,
        YueHaibing <yuehaibing@huawei.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     "openbmc @ lists . ozlabs . org" <openbmc@lists.ozlabs.org>,
        "joel@jms.id.au" <joel@jms.id.au>,
        "linux-aspeed@lists.ozlabs.org" <linux-aspeed@lists.ozlabs.org>,
        Sai Dasari <sdasari@fb.com>
Date:   Fri, 18 Oct 2019 11:32:57 +1100
In-Reply-To: <9AA81274-01F2-4803-8905-26F0521486CE@fb.com>
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On Fri, 2019-10-18 at 00:06 +0000, Vijay Khemka wrote:
> 
>     > This is not a matter of unsupported csum, it is broken hw csum. 
>     > That's why we disable hw checksum. My guess is once we disable
>     > Hw checksum, it will use sw checksum. So I am just disabling hw 
>     > Checksum.
>     
>     I don't understand what you are saying. You reported a problem with
>     IPV6 checksums generation. The HW doesn't support it. What's "not a
>     matter of unsupported csum" ?
>     
>     Your patch uses a *deprecated* bit to tell the network stack to only do
>     HW checksum generation on IPV4.
>     
>     This bit is deprecated for a reason, again, see skbuff.h. The right
>     approach, *which the driver already does*, is to tell the stack that we
>     support HW checksuming using NETIF_F_HW_CSUM, and then, in the transmit
>     handler, to call skb_checksum_help() to have the SW calculate the
>     checksum if it's not a supported type.
> 
> My understanding was when we enable NETIF_F_HW_CSUM means network 
> stack enables HW checksum and doesn't calculate SW checksum. But as per
> this supported types HW checksum are used only for IPV4 and not for IPV6 even
> though driver enabled NETIF_F_HW_CSUM. For IPV6 it is always a SW generated
> checksum, please correct me here.

Have you actually read the comments in skbuff.h that I pointed you to ?

And the rest of my email for that matter ?

>     This is exactly what ftgmac100_prep_tx_csum() does. It only enables HW
>     checksum generation on supported types and uses skb_checksum_help()
>     otherwise, supported types being protocol ETH_P_IP and IP protocol
>     being raw IP, TCP and UDP.
> 
>     
>     So this *should* have fallen back to SW for IPV6. So either something
>     in my code there is making an incorrect assumption, or something is
>     broken in skb_checksum_help() for IPV6 (which I somewhat doubt) or
>     something else I can't think of, but setting a *deprecated* flag is
>     definitely not the right answer, neither is completely disabling HW
>     checksumming.
>     
>     So can you investigate what's going on a bit more closely please ? I
>     can try myself, though I have very little experience with IPV6 and
>     probably won't have time before next week.
>     
>     Cheers,
>     Ben.
>     
>     >     The driver should have handled unsupported csum via SW fallback
>     >     already in ftgmac100_prep_tx_csum()
>     >     
>     >     Can you check why this didn't work for you ?
>     >     
>     >     Cheers,
>     >     Ben.
>     >     
>     >     > Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
>     >     > ---
>     >     > Changes since v1:
>     >     >  Enabled IPV4 hw checksum generation as it works for IPV4.
>     >     > 
>     >     >  drivers/net/ethernet/faraday/ftgmac100.c | 13 ++++++++++++-
>     >     >  1 file changed, 12 insertions(+), 1 deletion(-)
>     >     > 
>     >     > diff --git a/drivers/net/ethernet/faraday/ftgmac100.c
>     >     > b/drivers/net/ethernet/faraday/ftgmac100.c
>     >     > index 030fed65393e..0255a28d2958 100644
>     >     > --- a/drivers/net/ethernet/faraday/ftgmac100.c
>     >     > +++ b/drivers/net/ethernet/faraday/ftgmac100.c
>     >     > @@ -1842,8 +1842,19 @@ static int ftgmac100_probe(struct
>     >     > platform_device *pdev)
>     >     >  	/* AST2400  doesn't have working HW checksum generation */
>     >     >  	if (np && (of_device_is_compatible(np, "aspeed,ast2400-mac")))
>     >     >  		netdev->hw_features &= ~NETIF_F_HW_CSUM;
>     >     > +
>     >     > +	/* AST2500 doesn't have working HW checksum generation for IPV6
>     >     > +	 * but it works for IPV4, so disabling hw checksum and enabling
>     >     > +	 * it for only IPV4.
>     >     > +	 */
>     >     > +	if (np && (of_device_is_compatible(np, "aspeed,ast2500-mac")))
>     >     > {
>     >     > +		netdev->hw_features &= ~NETIF_F_HW_CSUM;
>     >     > +		netdev->hw_features |= NETIF_F_IP_CSUM;
>     >     > +	}
>     >     > +
>     >     >  	if (np && of_get_property(np, "no-hw-checksum", NULL))
>     >     > -		netdev->hw_features &= ~(NETIF_F_HW_CSUM |
>     >     > NETIF_F_RXCSUM);
>     >     > +		netdev->hw_features &= ~(NETIF_F_HW_CSUM |
>     >     > NETIF_F_RXCSUM
>     >     > +					 | NETIF_F_IP_CSUM);
>     >     >  	netdev->features |= netdev->hw_features;
>     >     >  
>     >     >  	/* register network device */
>     >     
>     >     
>     > 
>     
>     
>