netdev - Re: Adding Support for SG,GSO,GRO

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Message-ID: <AANLkTi=8zWMYWG0DU147B08nN7xcvwaB9Bpq9fE8_fBe@mail.gmail.com>
Date:	Thu, 9 Dec 2010 19:47:57 +0100
From:	Michał Mirosław <mirqus@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	"Govindarajan, Sriramakrishnan" <srk@...com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: Adding Support for SG,GSO,GRO

2010/12/9 Ben Hutchings <bhutchings@...arflare.com>:
> On Thu, 2010-12-09 at 16:03 +0530, Govindarajan, Sriramakrishnan wrote:
>> Hi
>> We have a NAPI compliant driver(net/drivers/davinci_emac.c), that does
>> well at 10/100Mbps loads. Now the same controller/driver is used for
>> 1000Mbps
>> mode as well, where the CPU gets saturated easily
>>
>> Internally the module supports scatter gather DMA(which is currently not
>> exercised) but there is no HW checksum support.
>>
>> To specifically implement GRO, GSO support would it be sufficient to add
>> SG support to the driver? Are there other means of increasing the throughput
>> and decreasing the CPU loading?
[...]
> On the TX side, NETIF_F_SG means that the stack may include data in the
> skb by reference to arbitrary pages *even if their contents are still
> being changed* (think sendfile()), which means it depends on hardware
> checksum generation.

Isn't that condition too broad? If the data could change after packet
is submitted to the driver then results would be unpredictable and
allow sending wrong data with correct (because hw-calculated)
checksum.

Right now NETIF_F_SG is removed from dev->features by
netdev_fix_features() if no checksum offloads are enabled.

Just an idea: would driver with NETIF_F_SG|NETIF_F_HW_CSUM using
skb_checksum_help() in xmit path work? This would allow to use DMA
scatter-gather without hardware checksumming (and avoid copying the
packet's data before sending).

Best Regards,
Michał Mirosław
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