netdev - Re: [RFC PATCH net-next] net: bridge: switchdev: expose the port hwdom as a netlink attribute

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Message-ID: <20210812161648.ncxtaftsoq5txnui@skbuf>
Date:   Thu, 12 Aug 2021 16:16:48 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Ido Schimmel <idosch@...sch.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <nikolay@...dia.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vadym Kochan <vkochan@...vell.com>,
        Taras Chornyi <tchornyi@...vell.com>,
        Jiri Pirko <jiri@...dia.com>, Ido Schimmel <idosch@...dia.com>,
        "UNGLinuxDriver@...rochip.com" <UNGLinuxDriver@...rochip.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Tobias Waldekranz <tobias@...dekranz.com>,
        Marek Behun <kabel@...ckhole.sk>,
        DENG Qingfang <dqfext@...il.com>
Subject: Re: [RFC PATCH net-next] net: bridge: switchdev: expose the port
 hwdom as a netlink attribute

On Thu, Aug 12, 2021 at 06:35:15PM +0300, Ido Schimmel wrote:
> Makes sense to me. Gives us further insight into the offload process. I
> vaguely remember discussing this with Nik in the past. The
> hwdom/fwd_mark is in the tree for long enough to be considered a stable
> and useful concept.
> 
> You are saying that it is useful to expose despite already having
> "switchid" exposed because you can have interfaces with the same
> "switchid" that are not member in the same hardware domain? E.g., the
> LAG example. If so, might be worth explicitly spelling it out in the
> commit message.

Indeed, the "switchid" is static, whereas the "hwdom" depends upon the
current configuration. So it is useful as a debug feature for the
reasons you mention, but I am also a bit worried whether we should
expose this now, since I am not sure if it will impact future redesigns
of the bridge driver or switchdev (the hwdom is a pretty detailed bit of
information). Basically the only guarantee we're giving user space is
that a hwdom of zero means unoffloaded, and two non-zero and equal
integer values can forward between each other without involving the CPU.
The numbers themselves are arbitrary, mean nothing and can vary even
depending on the port join order into the bridge. That shouldn't impose
any restrictions going further, should it?