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

[Vijay Khemka <vijaykhemka@fb.com>]

On 10/17/19, 5:33 PM, "Benjamin Herrenschmidt" <benh@kernel.crashing.org> wrote:

    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 ?

Yes, I went through comments in skbuff.h and your comments as well. I knew about
this deprecated bit that's why I have disabled NETIF_F_HW_CSUM completely in my
V1 patch. Then Florian gave a comment and asked me to disable only IPV6 not IPV4
as it is working for IPV4 and issue with only IPV6. That's why I sent patch V2 
accommodating his feedback.

I don't have much understanding of IP Stack but I went through code details and 
you are right and found that it should fallback to SW calculation for IPV6 but it doesn't
happen because ftgmac100_hard_start_xmit checks for CHECKSUM_PARTIAL before
setting HW checksum and calling ftgmac100_prep_tx_csum function. And in my 
understanding, this value is set CHECKSUM_PARTIAL in IP stack. I looked up IP stack for
IPV6, file net/ipv6/ip6_output.c, function __ip6_append_data: here it sets 
CHECKSUM_PARTIAL only for UDP packets not for TCP packets. Please look at line
 number 1880. This could be an issue we are seeing here as why
ftgmac100_prep_tx_csum is not getting triggered for IPV6 with TCP. Please correct
me if my understanding is wrong.
    
    >     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 */
    >     >     
    >     >     
    >     > 
    >     
    >     
    >