netdev - Re: [PATCH net-next 2/2] net: dsa: mv88e6xxx: implement egress tbf qdisc for 6393x family

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Date: Fri, 9 Jun 2023 19:16:04 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Alexis Lothoré <alexis.lothore@...tlin.com>
Cc: Florian Fainelli <f.fainelli@...il.com>,
	Vladimir Oltean <olteanv@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
	paul.arola@...us.com, scott.roberts@...us.com
Subject: Re: [PATCH net-next 2/2] net: dsa: mv88e6xxx: implement egress tbf
 qdisc for 6393x family

> Yes, I can do that (or maybe -EINVAL to match Vladimir's comment ?). I think
> it's worth mentioning that I encountered an issue regarding those values during
> tests: I use tc program to set the tbf, and I observed that tc does not even
> reach kernel to set the qdisc if we pass no burst/latency value OR if we set it
> to 0. So tc enforces right on userspace side non-zero value for those
> parameters, and I have passed random values and ignored them on kernel side.

That is not good. Please take a look around and see if any other
driver offloads TBF, and what they do with burst.

> Checking available doc about tc-tbf makes me feel like that indeed a TBF qdisc
> command without burst or latency value makes no sense, except my use case can
> not have such values. That's what I struggled a bit to find a proper qdisc to
> match hardware cap. I may fallback to a custom netlink program to improve testing.

We don't really want a custom application, since we want users to use
TC to set this up.

Looking at the 6390 datasheet, Queue Counter Registers, mode 8 gives
the number of egress buffers for a port. You could validate that the
switch has at least the requested number of buffers assigned to the
port? There is quite a bit you can configure, so maybe there is a way
to influence the number of buffers, so you can actually implement the
burst parameter?

      Andrew