netdev - Re: [PATCH 1/5] send-crc: Add framework to allow sending packets with customized CRC.

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Message-ID: <CAHXqBFKbv13jL_RqbOTZp6P-TOO7iQ1211SXmMso_3LK27LkPw@mail.gmail.com>
Date:	Wed, 8 Feb 2012 00:59:17 +0100
From:	Michał Mirosław <mirqus@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 1/5] send-crc: Add framework to allow sending packets with
 customized CRC.

W dniu 7 lutego 2012 21:43 użytkownik Ben Greear
<greearb@...delatech.com> napisał:
> On 02/07/2012 12:33 PM, Michał Mirosław wrote:
>>
>> 2012/2/7<greearb@...delatech.com>:
>>>
>>> From: Ben Greear<greearb@...delatech.com>
>>>
>>> This is useful for testing RX handling of frames with bad
>>> CRCs.
>>>
>>> Requires driver support to actually put the packet on the
>>> wire properly.
>>
>> [...]
>>
>> I like this idea. I think that this is not a feature to have in
>> dev->features to toggle, though. Driver either supports this or not,
>> so it's better to put it in dev->priv_flags instead. Disabling of FCS
>> inserting is a per-packet option in the drivers, isn't it?
>>
>> For one problem with toggling: if you create a socket using it and
>> later turn off the feature, the socket will still send skbs with FCS
>> appended.
> For the priv-flags, is there a way to query that from user-space to
> see if the NIC supports it?
>
> The pkt-socket xmit logic will throw away pkts if the NIC doesn't
> have the feature enabled, but a few already queued might get
> through with extra CRC appended.
>
> So, I'm fine with your suggestion, as long as there is some way to
> query whether the NIC supports this feature or not...

There's a way, as you implemented -EINVAL when trying to send via
unsupporting device. :-) Would be even better if setting SO_NO_FCS
would fail at setsockopt() time.

priv_flags are not exposed to userspace. It should be easy to export
them using ETHTOOL_GFEATURES as non-changeable features, if that's
useful.

Best Regards,
Michał Mirosław
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