Message-ID: <871reumudf.fsf@kurt>
Date:   Sat, 09 Jan 2021 08:39:40 +0100
From:   Kurt Kanzenbach <kurt@...utronix.de>
To:     Vladimir Oltean <olteanv@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Hauke Mehrtens <hauke@...ke-m.de>,
        Woojung Huh <woojung.huh@...rochip.com>,
        Microchip Linux Driver Support <UNGLinuxDriver@...rochip.com>,
        Sean Wang <sean.wang@...iatek.com>,
        Landen Chao <Landen.Chao@...iatek.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Vadym Kochan <vkochan@...vell.com>,
        Taras Chornyi <tchornyi@...vell.com>,
        Jiri Pirko <jiri@...dia.com>, Ido Schimmel <idosch@...dia.com>,
        Grygorii Strashko <grygorii.strashko@...com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Ivan Vecera <ivecera@...hat.com>,
        Petr Machata <petrm@...dia.com>
Subject: Re: [PATCH v4 net-next 01/11] net: switchdev: remove vid_begin -> vid_end range from VLAN objects

On Sat Jan 09 2021, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
>
> The call path of a switchdev VLAN addition to the bridge looks something
> like this today:
>
>         nbp_vlan_init
>         |  __br_vlan_set_default_pvid
>         |  |                       |
>         |  |    br_afspec          |
>         |  |        |              |
>         |  |        v              |
>         |  | br_process_vlan_info  |
>         |  |        |              |
>         |  |        v              |
>         |  |   br_vlan_info        |
>         |  |       / \            /
>         |  |      /   \          /
>         |  |     /     \        /
>         |  |    /       \      /
>         v  v   v         v    v
>       nbp_vlan_add   br_vlan_add ------+
>        |              ^      ^ |       |
>        |             /       | |       |
>        |            /       /  /       |
>        \ br_vlan_get_master/  /        v
>         \        ^        /  /  br_vlan_add_existing
>          \       |       /  /          |
>           \      |      /  /          /
>            \     |     /  /          /
>             \    |    /  /          /
>              \   |   /  /          /
>               v  |   | v          /
>               __vlan_add         /
>                  / |            /
>                 /  |           /
>                v   |          /
>    __vlan_vid_add  |         /
>                \   |        /
>                 v  v        v
>       br_switchdev_port_vlan_add
>
> The ranges UAPI was introduced to the bridge in commit bdced7ef7838
> ("bridge: support for multiple vlans and vlan ranges in setlink and
> dellink requests") (Jan 10 2015). But the VLAN ranges (parsed in br_afspec)
> have always been passed one by one, through struct bridge_vlan_info
> tmp_vinfo, to br_vlan_info. So the range never went too far in depth.
>
> Then Scott Feldman introduced the switchdev_port_bridge_setlink function
> in commit 47f8328bb1a4 ("switchdev: add new switchdev bridge setlink").
> That marked the introduction of the SWITCHDEV_OBJ_PORT_VLAN, which made
> full use of the range. But switchdev_port_bridge_setlink was called like
> this:
>
> br_setlink
> -> br_afspec
> -> switchdev_port_bridge_setlink
>
> Basically, the switchdev and the bridge code were not tightly integrated.
> Then commit 41c498b9359e ("bridge: restore br_setlink back to original")
> came, and switchdev drivers were required to implement
> .ndo_bridge_setlink = switchdev_port_bridge_setlink for a while.
>
> In the meantime, commits such as 0944d6b5a2fa ("bridge: try switchdev op
> first in __vlan_vid_add/del") finally made switchdev penetrate the
> br_vlan_info() barrier and start to develop the call path we have today.
> But remember, br_vlan_info() still receives VLANs one by one.
>
> Then Arkadi Sharshevsky refactored the switchdev API in 2017 in commit
> 29ab586c3d83 ("net: switchdev: Remove bridge bypass support from
> switchdev") so that drivers would not implement .ndo_bridge_setlink any
> longer. The switchdev_port_bridge_setlink also got deleted.
> This refactoring removed the parallel bridge_setlink implementation from
> switchdev, and left the only switchdev VLAN objects to be the ones
> offloaded from __vlan_vid_add (basically RX filtering) and  __vlan_add
> (the latter coming from commit 9c86ce2c1ae3 ("net: bridge: Notify about
> bridge VLANs")).
>
> That is to say, today the switchdev VLAN object ranges are not used in
> the kernel. Refactoring the above call path is a bit complicated, when
> the bridge VLAN call path is already a bit complicated.
>
> Let's go off and finish the job of commit 29ab586c3d83 by deleting the
> bogus iteration through the VLAN ranges from the drivers. Some aspects
> of this feature never made too much sense in the first place. For
> example, what is a range of VLANs all having the BRIDGE_VLAN_INFO_PVID
> flag supposed to mean, when a port can obviously have a single pvid?
> This particular configuration _is_ denied as of commit 6623c60dc28e
> ("bridge: vlan: enforce no pvid flag in vlan ranges"), but from an API
> perspective, the driver still has to play pretend, and only offload the
> vlan->vid_end as pvid. And the addition of a switchdev VLAN object can
> modify the flags of another, completely unrelated, switchdev VLAN
> object! (a VLAN that is PVID will invalidate the PVID flag from whatever
> other VLAN had previously been offloaded with switchdev and had that
> flag. Yet switchdev never notifies about that change, drivers are
> supposed to guess).
>
> Nonetheless, having a VLAN range in the API makes error handling look
> scarier than it really is - unwinding on errors and all of that.
> When in reality, no one really calls this API with more than one VLAN.
> It is all unnecessary complexity.
>
> And despite appearing pretentious (two-phase transactional model and
> all), the switchdev API is really sloppy because the VLAN addition and
> removal operations are not paired with one another (you can add a VLAN
> 100 times and delete it just once). The bridge notifies through
> switchdev of a VLAN addition not only when the flags of an existing VLAN
> change, but also when nothing changes. There are switchdev drivers out
> there who don't like adding a VLAN that has already been added, and
> those checks don't really belong at driver level. But the fact that the
> API contains ranges is yet another factor that prevents this from being
> addressed in the future.
>
> Of the existing switchdev pieces of hardware, it appears that only
> Mellanox Spectrum supports offloading more than one VLAN at a time,
> through mlxsw_sp_port_vlan_set. I have kept that code internal to the
> driver, because there is some more bookkeeping that makes use of it, but
> I deleted it from the switchdev API. But since the switchdev support for
> ranges has already been de facto deleted by a Mellanox employee and
> nobody noticed for 4 years, I'm going to assume it's not a biggie.
>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> Reviewed-by: Ido Schimmel <idosch@...dia.com> # switchdev and mlxsw

Reviewed-by: Kurt Kanzenbach <kurt@...utronix.de> # hellcreek

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