netdev - Re: [PATCH net-next v2 0/8] net: Shift responsibility for FDB notifications to drivers

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Message-ID: <959af10c-8d51-4bc5-9a85-ec00ad74994d@redhat.com>
Date: Tue, 5 Nov 2024 11:12:01 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Petr Machata <petrm@...dia.com>
Cc: Jakub Kicinski <kuba@...nel.org>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 netdev@...r.kernel.org, Ido Schimmel <idosch@...dia.com>,
 Amit Cohen <amcohen@...dia.com>, Vladimir Oltean <vladimir.oltean@....com>,
 Andy Roulin <aroulin@...dia.com>, mlxsw@...dia.com
Subject: Re: [PATCH net-next v2 0/8] net: Shift responsibility for FDB
 notifications to drivers

On 11/5/24 10:45, Petr Machata wrote:
> Paolo Abeni <pabeni@...hat.com> writes:
>> On 11/4/24 12:43, Petr Machata wrote:
>>> Jakub Kicinski <kuba@...nel.org> writes:
>>>> On Thu, 24 Oct 2024 18:57:35 +0200 Petr Machata wrote:
>>>>> Besides this approach, we considered just passing a boolean back from the
>>>>> driver, which would indicate whether the notification was done. But the
>>>>> approach presented here seems cleaner.
>>>>
>>>> Oops, I missed the v2, same question:
>>>>
>>>>   What about adding a bit to the ops struct to indicate that 
>>>>   the driver will generate the notification? Seems smaller in 
>>>>   terms of LoC and shifts the responsibility of doing extra
>>>>   work towards more complex users.
>>>>
>>>> https://lore.kernel.org/all/20241029121619.1a710601@kernel.org/
>>>
>>> Sorry for only responding now, I was out of office last week.
>>>
>>> The reason I went with outright responsibility shift is that the
>>> alternatives are more complex.
>>>
>>> For the flag in particular, first there's no place to set the flag
>>> currently, we'd need a field in struct net_device_ops. But mainly, then
>>> you have a code that needs to corrently handle both states of the flag,
>>> and new-style drivers need to remember to set the flag, which is done in
>>> a different place from the fdb_add/del themselves. It might be fewer
>>> LOCs, but it's a harder to understand system.
>>>
>>> Responsibility shift is easy. "Thou shalt notify." Done, easy to
>>> understand, easy to document. When cut'n'pasting, you won't miss it.
>>>
>>> Let me know what you think.
>>
>> I think that keeping as much action/responsibilities as possible in the
>> core code is in general a better option - at very least to avoid
>> duplicate code.
>>
>> I don't think that the C&P is a very good argument, as I would argue
>> against C&P without understanding of the underlying code. Still I agree
>> that keeping all the relevant info together is better, and a separate
>> flag would be not so straight-forward.
>>
>> What about using the return value of fbd_add/fdb_del to tell the core
>> that the driver did the notification? a positive value means 'already
>> notified', a negative one error, zero 'please notify.
> 
> That would work.
> 
> How about passing an explicit bool* argument for the callee to set? I'm
> suspicious of these one-off errno protocols. Most of the time the return
> value is an errno, these aberrations feel easy to miss.

I would be ok with that - a large arguments list should not be something
concerning for the control path. Just to be clear: the caller init the
bool to false, only the callees doing the notification set it, right?

Thanks!

Paolo