netdev - Re: [PATCH v2 0/3] Add NETIF_F_HW_BR

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Date:   Mon, 26 Aug 2019 10:01:33 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Andrew Lunn <andrew@...n.ch>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>
Cc:     roopa@...ulusnetworks.com, nikolay@...ulusnetworks.com,
        davem@...emloft.net, UNGLinuxDriver@...rochip.com,
        alexandre.belloni@...tlin.com, allan.nielsen@...rochip.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bridge@...ts.linux-foundation.org
Subject: Re: [PATCH v2 0/3] Add NETIF_F_HW_BR_CAP feature

On 8/26/19 5:38 AM, Andrew Lunn wrote:
> On Mon, Aug 26, 2019 at 10:11:12AM +0200, Horatiu Vultur wrote:
>> When a network port is added to a bridge then the port is added in
>> promisc mode. Some HW that has bridge capabilities(can learn, forward,
>> flood etc the frames) they are disabling promisc mode in the network
>> driver when the port is added to the SW bridge.
>>
>> This patch adds the feature NETIF_F_HW_BR_CAP so that the network ports
>> that have this feature will not be set in promisc mode when they are
>> added to a SW bridge.
>>
>> In this way the HW that has bridge capabilities don't need to send all the
>> traffic to the CPU and can also implement the promisc mode and toggle it
>> using the command 'ip link set dev swp promisc on'
> 
> Hi Horatiu
> 
> I'm still not convinced this is needed. The model is, the hardware is
> there to accelerate what Linux can do in software. Any peculiarities
> of the accelerator should be hidden in the driver.  If the accelerator
> can do its job without needing promisc mode, do that in the driver.
> 
> So you are trying to differentiate between promisc mode because the
> interface is a member of a bridge, and promisc mode because some
> application, like pcap, has asked for promisc mode.
> 
> dev->promiscuity is a counter. So what you can do it look at its
> value, and how the interface is being used. If the interface is not a
> member of a bridge, and the count > 0, enable promisc mode in the
> accelerator. If the interface is a member of a bridge, and the count >
> 1, enable promisc mode in the accelerator.

That is an excellent suggestion actually.

Horatiu, the other issue with your approach here is that the features
don't propagate to/from lower/upper/real devices, so if e.g.: you have a
VLAN interface enslaved as a part of the bridge, or a bond, or a tunnel
interface, the logic won't make us check NETIF_F_HW_BR_CAP because those
virtual network devices won't inherit it from their real device. I am
not suggesting you fix this with your patch series, but rather, seek a
driver local solution.
-- 
Florian