netdev - Re: [PATCH V2 net-next] net: fec: add CBS offload support

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Message-ID: <Y/QrlM011Jw2hsnG@lunn.ch>
Date:   Tue, 21 Feb 2023 03:25:24 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Wei Fang <wei.fang@....com>
Cc:     Alexander Lobakin <alexandr.lobakin@...el.com>,
        Shenwei Wang <shenwei.wang@....com>,
        Clark Wang <xiaoning.wang@....com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "simon.horman@...igine.com" <simon.horman@...igine.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        dl-linux-imx <linux-imx@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 net-next] net: fec: add CBS offload support

> Unfortunately, FEC IP itself not follows the standard 802.1Qav spec completely.
> We only can program DMAnCFG[IDLE_SLOPE] field to calculate bandwidth fraction.
> And IDLE_SLOPE is restricted to certain values. It's far away from CBS QDisc implemented
> in Linux TC framework. It is more difficult to guarantee similar software behavior when
> the link speed changes.
> If the method of keeping the bandwidth ratio unchanged is not sensible, I can only give up
> CBS offload and use pure software CBS. :(

I know the hardware supports less parameters. But if you restrict the
CBS configuration such that the parameters you don't support are 0,
can you accurately implement what you do support?

If the user configures TC such that it matches the hardware
capabilities, you can accept the offload. If not return -EOPNOTSUPP,
and the software CBS should take over. And then you need to clearly
document what parameters can be set for hardware offload to work.

	 Andrew