netdev - Re: [PATCH v2 net-next 3/6] sfc: optional logging of TC offload errors

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Date:   Wed, 28 Sep 2022 19:17:51 +0100
From:   Edward Cree <ecree.xilinx@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, linux-net-drivers@....com,
        davem@...emloft.net, pabeni@...hat.com, edumazet@...gle.com,
        habetsm.xilinx@...il.com
Subject: Re: [PATCH v2 net-next 3/6] sfc: optional logging of TC offload
 errors

On 28/09/2022 18:44, Jakub Kicinski wrote:
> On Mon, 26 Sep 2022 19:57:33 +0100 ecree@...inx.com wrote:
>> TC offload support will involve complex limitations on what matches and
>>  actions a rule can do, in some cases potentially depending on rules
>>  already offloaded.  So add an ethtool private flag "log-tc-errors" which
>>  controls reporting the reasons for un-offloadable TC rules at NETIF_INFO.
> 
> Because extack does not work somehow?

Last I checked, flow rules coming from an indirect binding to a tunnel
 netdev did not report the hw driver's extack (or even rc) back to the
 user.
Also, extack can only contain fixed strings (netlink.h: "/* Currently
 string formatting is not supported (due to the lack of an output
 buffer.) */") which was a real problem for us.

> Somehow you limitations are harder to debug that everyone else's so you
> need a private flag? :/

It's not about debugging the driver, it's about communicating the
 limitations to the end user.  Having TC rules mysteriously fail to be
 offloaded with no indication of why is not a great UX :(
I couldn't see a way to handle this without vendor-specific ugliness,
 but if you have a proposal I don't mind putting in some work to
 implement it.

-ed