Re: [PATCH v2] ftgmac100: Disable HW checksum generation on AST2500

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Thu, 2019-10-17 at 22:01 +0000, Vijay Khemka wrote:
> 
> On 10/16/19, 6:29 PM, "Benjamin Herrenschmidt" <benh@kernel.crashing.org> wrote:
> 
>     On Fri, 2019-10-11 at 14:30 -0700, Vijay Khemka wrote:
>     > HW checksum generation is not working for AST2500, specially with
>     > IPV6
>     > over NCSI. All TCP packets with IPv6 get dropped. By disabling this
>     > it works perfectly fine with IPV6. As it works for IPV4 so enabled
>     > hw checksum back for IPV4.
>     > 
>     > Verified with IPV6 enabled and can do ssh.
>     
>     So while this probably works, I don't think this is the right
>     approach, at least according to the comments in skbuff.h
> 
> 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.

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 */
>     
>     
>