Re: [PATCH 2/7] sh_eth: RX checksum offload support

[Simon Horman <horms@verge.net.au>]

Hi Sergei,

On Mon, Jan 28, 2019 at 06:45:26PM +0300, Sergei Shtylyov wrote:
> Hello!
> 
> On 01/28/2019 03:18 PM, Simon Horman wrote:
> 
> >> Add support for the RX checksum offload. This is enabled by default and
> >> may be disabled and re-enabled using 'ethtool':
> >>
> >> # ethtool -K eth0 rx {on|off}
> >>
> >> Some Ether MACs provide a simple checksumming scheme which appears to be
> >> completely compatible with CHECKSUM_COMPLETE: sum of 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. The same checksumming
> >> scheme is implemented in the EtherAVB MACs and now supported by tha 'ravb'
> >> driver.
> >>
> >> In terms of performance, throughput is close to gigabit line rate with the
> >> RX checksum offload both enabled and disabled.  The 'perf' output, however,
> >> appears to indicate that significantly less time is spent in do_csum() --
> >> this is as expected.
> > 
> > Nice.
> > 
> > FYI, this seems similar to what I observed for RAVB, perhaps on H3 I don't
> > exactly recall. On E3, which has less CPU power, I recently observed that
> > with rx-csum enabled I can achieve gigabit line rate, but with rx-csum
> > disabled throughput is significantly lower. I.e. on that system throughput
> > is CPU bound with 1500 byte packets unless rx-csum enabled.
> 
>    Unfortunately, we can't teset these patches on the other gen3 boards. ISTR
> you have RZ/A1H board... if it's still with you, I'd appreciate testing.

Unfortunately, as of a few weeks ago, I no longer have that board.

> > Next point:
> > 
> > 2da64300fbc ("ravb: expand rx descriptor data to accommodate hw checksum")
> > is fresh in my mind and I wonder if mdp->rx_buf_sz needs to grow to ensure
> > that there is always enough space for the csum.
> 
>    Well, if you look at sh_eth_ring_init(), you'll see that the driver reserves
> plenty of space at the end the RX buffers.

Yes, I see that. And I assume that was enough space before this patch.
But is it still enough space now that 2 bytes are needed for the hardware csum?
2 bytes that might have previously been used as packet data in some
circumstances.

> > In particular, have you
> > tested this with MTU-size frames with VLANs. (My test is to run iperf3 over
> > a VLAN netdev, netperf over a VLAN netdev would likely work just as well.)
> 
>    Could you refresh me on how to bring up a VLAN on a given interface?

You will need a kernel with CONFIG_VLAN_8021Q enabled.

Then you can do something like this:

	ip link add link eth0 name eth0.1 type vlan id 1
	ip addr add 10.1.1.100/24 dev eth0.1
	ip link set dev eth0.1 up


> [...]
> >> The above results collected on the R-Car V3H Starter Kit board.
> >>
> >> Based on the commit 4d86d3818627 ("ravb: RX checksum offload")...
> >>
> >> Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
> [...]
> 
> MBR, Sergei
>