netdev - Re: [RFC PATCH v2 net-next 04/10] net: bridge: switchdev: allow the data plane forwarding to be offloaded

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Date:   Fri, 9 Jul 2021 16:16:13 +0300
From:   Grygorii Strashko <grygorii.strashko@...com>
To:     Vladimir Oltean <vladimir.oltean@....com>,
        <netdev@...r.kernel.org>, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>
CC:     Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Ido Schimmel <idosch@...sch.org>,
        Tobias Waldekranz <tobias@...dekranz.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <nikolay@...dia.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        <bridge@...ts.linux-foundation.org>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [RFC PATCH v2 net-next 04/10] net: bridge: switchdev: allow the
 data plane forwarding to be offloaded



On 03/07/2021 14:56, Vladimir Oltean wrote:
> From: Tobias Waldekranz <tobias@...dekranz.com>
> 
> Allow switchdevs to forward frames from the CPU in accordance with the
> bridge configuration in the same way as is done between bridge
> ports. This means that the bridge will only send a single skb towards
> one of the ports under the switchdev's control, and expects the driver
> to deliver the packet to all eligible ports in its domain.
> 
> Primarily this improves the performance of multicast flows with
> multiple subscribers, as it allows the hardware to perform the frame
> replication.
> 
> The basic flow between the driver and the bridge is as follows:
> 
> - The switchdev accepts the offload by returning a non-null pointer
>    from .ndo_dfwd_add_station when the port is added to the bridge.
> 
> - The bridge sends offloadable skbs to one of the ports under the
>    switchdev's control using dev_queue_xmit_accel.
> 
> - The switchdev notices the offload by checking for a non-NULL
>    "sb_dev" in the core's call to .ndo_select_queue.

Sry, I could be missing smth.

Is there any possibility to just mark skb itself as "fwd_offload" (or smth), so driver can
just check it and decide what to do. Following you series:
- BR itself will send packet only once to one port if fwd offload possible and supported
- switchdev driver can check/negotiate BR_FWD_OFFLOAD flag

In our case, TI CPSW can send directed packet (default now), by specifying port_id if DMA desc
or keep port_id == 0 which will allow HW to process packet internally, including MC duplication.

Sry, again, but necessity to add 3 callbacks and manipulate with "virtual" queue to achieve
MC offload (seems like one of the primary goals) from BR itself looks a bit over-complicated :(

> 
> v1->v2:
> - convert br_input_skb_cb::fwd_hwdoms to a plain unsigned long
> - introduce a static key "br_switchdev_fwd_offload_used" to minimize the
>    impact of the newly introduced feature on all the setups which don't
>    have hardware that can make use of it
> - introduce a check for nbp->flags & BR_FWD_OFFLOAD to optimize cache
>    line access
> - reorder nbp_switchdev_frame_mark_accel() and br_handle_vlan() in
>    __br_forward()
> - do not strip VLAN on egress if forwarding offload on VLAN-aware bridge
>    is being used
> - propagate errors from .ndo_dfwd_add_station() if not EOPNOTSUPP
> 
> Signed-off-by: Tobias Waldekranz <tobias@...dekranz.com>
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>
> ---
>   include/linux/if_bridge.h |  1 +
>   net/bridge/br_forward.c   | 18 +++++++-
>   net/bridge/br_private.h   | 24 +++++++++++
>   net/bridge/br_switchdev.c | 87 +++++++++++++++++++++++++++++++++++++--

[...]

-- 
Best regards,
grygorii